Front Cover
 Title Page
 Historic chronology
 Description of methods, past and...
 Ornamental architectural glass...
 The handout
 Back Cover

Title: Notes on the history, manufacture, and application of window and ornamental architectural glass
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Permanent Link: http://ufdc.ufl.edu/UF00098826/00001
 Material Information
Title: Notes on the history, manufacture, and application of window and ornamental architectural glass
Physical Description: Book
Language: English
Creator: Mills, Jerry W.
Publisher: Jerry W. Mills
Place of Publication: Gainesville, Fla.
Copyright Date: 1976
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Bibliographic ID: UF00098826
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Table of Contents
    Front Cover
        Front cover
    Title Page
        Title page
        Page 1
        Page 2
        Page 3
    Historic chronology
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
    Description of methods, past and present
        Page 19
        Page 20
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
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        Page 32
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        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
    Ornamental architectural glass (to 1940)
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
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        Page 72
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        Page 94
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        Page 97
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        Page 100
        Page 101
        Page 102
        Page 103
        Page 104
        Page 105
        Page 106
        Page 107
        Page 108
        Page 109
        Page 110
        Page 111
        Page 112
        Page 113
    The handout
        Page 114
        Page 115
        Page 116
        Page 117
        Page 118
        Page 119
    Back Cover
        Page 120
        Page 121
Full Text

Notes on the History, Manufacture, & Application of Window & Ornamental Architectural Glass
for AE 685
Architectural Technologies II Spring, 1976
Jerry W. Mills

Originally this project was intended for the purpose of seeking' information pertaining to the application and manufacture of ornamental architectural galss. During the course of research it was noted that there was a lack of pertinent information on this subject which resulted in the expansion of the original intent. The subject matter contained within this paper is now divided into three major topics, all of which are centered around either window glass or ornamental architectural glass.
The first section is a chronology of important events which occured throughout history and is more-or-less organized by country. The reader should note how the expansion of glass making appears to spread from the Near East in a westerly direction to Egypt and Phoenicia, to Rome and Venice, across Europe and up to Great Britain, and finally the big jump, still in a westerly direction, to the United States. Here, by the turn of the century, glass making appears to be most productive in terms of creativity, quality and speed with which it could be turned out.
The second section is concerned with manufacturing processes. This section relies heavily on Denis Diderot's A Diderot Pictorial Encyclopedia of Trades and Industry, from an encyclopedia which was originally published in France in the 18th century and translated, edited and reprinted in 1959.
His description of manufacturing processes of glass making is quite typical of the day and remained so until the late nineteenth century, when mass production and industrial mechanization greatly changed the scope of window and ornamental glass. Some of these late 19th and early

20th century processes will also be briefly described.
The third section studies ornamental architectural glass, attempting to discover the methods involved when producing architectural glass other than the common window glass, the purpose of which is not to be noticed.
In concluding the paper one small four page section has been added. This is a copy of the handout given to the students, prior to the lecture corresponding this paper.
Footnotes As in earlier papers the footnotes used are of my own method, which I feel is easier -to understand than the accepted standard
method. It is simply composed of two numbers, the first of which refers to a corresponding number in the bibliography, "the source," and the second, the page within that source from which the information was taken or quoted. If the lower case letter "p" follows the source number, then refer to the periodical section of the bibliography.

Note about glass
"This hard, inert, transparent material is made by heating together a mixture of materials such as sand, limestone and soda. At a sufficiently high temperature, a white heat of about 1400-1500" C just below the melting point of iron, these materials react to form a liquid. When this liquid is taken from the furnace it gets stiffer and stiffer as it cools until at about 500*C it has become as solid as the glass we are familar with in our windows. Glass articles are subjected to a controlled heat treatment after manufacture which releases the stresses in the glass, a process known as annealing

Historic Chronology
A. Egypt and Before
B. Phoenicia
C. Rome
D. Venice
E. England
F. France
G. United States

A. Egypt and Before
12,000 B.C. Egyptian mummies have been found
adorned with glass beads discovered to be of this period. However, prior to the 16th or 15th century B. C. little or no glass making occured in Egypt. The beads found had been produced in Mesopotamia.3-3
1,500 B.C. Glassmaking for the Egyptians was
primarily centered around producing precious stones more cheaply. Thus, their glass was never transparent, in an attempt to imitate these gems.
The consistency of their glass during^production was a paste. They never reached the extra 500 degrees necessary to liquify their raw ma-
terials .

Among the Egyptian glasses some of their small vases are specially attractive. It is said that some of them can be definitely attributed to as early as the eighteenth dynasty, about the sixteenth or fifteenth centuries b.c. The method of making was described by Sir Flinders Petrie in a report he published of his excavations at Tel-el-Amarna. He stated that a metal rod, of the size of the intended interior of the neck, and rather conical, was coated at the end with a ball of sand held together by cloth and string. This was covered with glass, probably by winding a thread of glass round it, as large beads of this age are so made. The vase could then be reheated as often as needed for working by holding it in a furnace, the metal rod forming a handle and the sand inside the vase preventing its collapse. Threads of coloured glass would then be wound round it and incorporated by rolling; the wavy pattern found so frequently was produced by dragging the surface in different directions. Then the foot was pressed into shape with pincers, the brim was formed, and the handles were put on. Lastly, on cooling, the metal rod would contract and come loose from the neck, and after it was withdrawn the sand could be rubbed out from the body of the vase.
The wavy decoration mentioned is of two types. In the first, which is the earlier kind, the design consists of a series of crescent-shaped curves, and in the second something of the effect of palm leaves is obtained by means of a double drag. These little vases often attain quite a high degree of artistic merit.

B. Phoenicia
50 B. C. Information pertaining to the Phoenicians on the making of glass is limited, however this section remains apart from the others because it is they, during this time period who are attributed with the discovery or invention of blowing glass.^"^

C. Rome
Their importance in glass lies not necessarily in their achievements in manufacturing or applications, but more directly in their learning of the art and spreading their knowledge around to conquered possesions, from Syria and Mesopotamia to Spain and Britain.''' ^
The first use of glass in architectural elements appears to be with the Romans for it has been found that they used glass as an imitation of marble in residences: on pillars in entrance halls and also on walls in bathrooms. x
As early as the 1st century A. D. glazed window slits were em-
ployed on a very limited basis and was either cast or rolled. Some bronze framed windows measuring 21" x 28" have been found at Pompeii (destroyed in A. D. 79 by the volcano Vesuvius). Also some frames measuring 40" x 28" with glass h" thick were found there in
/ 1 *5 1
the ruins of a bathhouse.
Although most openings were probably not glazed at all, some windows employed the use of thin sheets of alabaster, marble, or even parchment.

D. Venice
1100 A. D. Venice was the glass center of the to 1500 world, learning their skill from
the Near East and not directly from the Romans.1-18
1289 The fear of fire was mounting on
the island and with the major portion of their economy relying on glass-making the danger of total destruction was emminent.
1291 All glass factories were torn down
and the manufacture was removed to the island of Murano, an island suburb of Venice.1
By this time the glass workers had formed a guild, with apprentices working for their masters for eight years before becoming glass blowers.
So organized and jealous was the guild of their skills, no glass workers were allowed to move from the island, ship pieces of glass out, or teach the art to foreigners under the penalty of death.
Colbert, Louis the XIV's First Minister during the late 17th century commissioned two glassmakers from Murano. Within a week both had mysteriously died.
Althouth very little documented information is available which refers to Venetian window glass some Venetian Palaces can still be found with small panes of thick green crown glass.I"24

E. England
674 Abbot Benedict of Weremouth, Durham
requested glazing for his church and monastery. Since there was no glass manufacturing in England at this time the glass and glaziers were sent for from continental i'! Europe.1-133
Early glazing in English dwellings was either nonexistant or consisted of cloth or wood lattices.
1226 Chiddingford in Surrey was the center
of glassmaking in England during the Medieval" ^period. This information was discovered on a land ggant to Laurence the Glassmaker.
1567 The first Lorraine glassmakers (North-
western Europe) came to England and were granted a patent for making window glass, the license for which lasted 21 years. They settled in Sussex and quickly began to strip the forests.1"136
1635 R. Mansell obtained a patent for
making flint glass using coal instead of wood as fuel.3-9 The forests of Great Britain were being rapidly depleted by this time and search for a new source of fuel was necessary.
1690's To supply money for William Ill's wars, a tax was placed on windows since they were regarded as luxuries,
1773 The first English plate glass fac-
tory was begun; it was a Prescot.
1825 England enjoyed a building boom.
In conjunction with this, but not necessarily in relation to the boom, the tax on glass in windows was cut in half, thus making its use much more desireable. At this time Europeans commonly had twice as much window area.4~31

The Chance Brothers were awarded the contract for the glazing of the Crystal Palace for the Exposition of 1851.
200 tons of sheet glass, which were hand blown (broad glass) had to be produced within a few months. Employees worked day and night as production was increased and by January of 1851, 63,000 panes of 16 oz. were being made in a fortnight. They employed the hand cylinder process producing nearly one million square feet of sheet

Fig. 5. The Crystal Palace was built to house the Great Exhibition of J 851 and to display for the first time 'the Works of Industry of all Nations'. The building itself, designed by Paxton, contained nearly one million square feet of glass supplied by Chance Brothers of Birmingham.
Fig. 47. Chance Brothers supplied nearly one million square feet of sheet glass for the Crystal Palace. Work continued night and day for many weeks and 200 tons of glass over and above the normal output were produced.

Fig. 1. The tools of the glassmaker, taken from De Arte Vitraria, Neri-Mcrrctt, 1st Latin edition, Amsterdam, 1668. The simple tools used by the glassmaker have changed little over the centuries and implements like those shown in this seventeenth-century illustration are still in use today. Amongst the tools seen in the picture are the blowpipe for blowing the initial shape, pincers, tongs and shears for finishing the piece, and a holder for carrying away the glassware. -
The glassmaker took up the molten glass on the end of the blowpipe by rotating it while dipping its end just below the surface of the glass, a process known as gathering. When a sufficient weight of glass had been gathered, it could be marvered and blown to form a hollow ball or bulb which was pressed into shape by the shaping-tool. This implement resembled
a large pair of sugar tongs, made of iron, with long blades in place of spoons. The pontil rod was an iron rod used for gathering small pieces of molten glass or seals and for holding the vessel temporarily during manufacture.
Small bowls, cups and bottles could be made with diese tools and, later, bulbs of sufficient size were blown to be converted into flat window glass.

F. France
14th Century Attempts were made to establish
a glass industry.
16th Century Henry IV gave exclusive rights to
Italians in four major areas:
1. Paris and its surroundings
2. Rouen with Normandy
3. Orleans and Loire country
4. Never with the Nivernais.
Their rights lasted from 10 to 30
4-T9 years.H 17
1665 Louis XIV with Colbert expended
vast sums of money to create a flat glass industry in France. One of the most important influences of this movement were the mirrors placed in the Galerie de Glaces at Versailles.4~^
1668 The casting of plate glass was in-
vented by Thevart of Paris.
1667 Lucas de Nehou a Norman glassmaker
became head of the Royal Glassworks.

Plate 8. The Galerie des Glaees, Versailles. This great hall in the Palace of Versailles, built for Louis XIV in 1678-84, has a complete wall of mirrors which can be seen in the left hand part of the picture.

G. United States
With the seventy settlers who left England with Captain Chistopher Newport in 1608, his 2nd trip to the New World, were V 8 Dutchmen and Poles' some of whom were glassmakers."
Since the industry in England was doing well at this time, no
British glassmakers were among the new settlers.^
The glass factory, according to Smith, was located "in the woods neare a myle from James Towne," or, as William Strachey described it, "a little without the Island where Jamestown stands." There, as Strachey goes on to say, the glass workers and their helpers erected a glasshouse, which was "a goodly howse . with all offices-and furnaces thereto belonging."
These newcomers must have set themselves to this task with greater diligence than most of the colonists had previously approached their work, for, when Captain Newport left for England late that year, he carried with him "tryals of Pitch, Tarre, Glasse, Frankincense, Sope Ashes; with that Clapboard and Waynscot that could be provided." Of what this first "tryal of glasse" consisted, the record gives no hint. It may have included only a few simple objects, but it must certainly have been sufficient to show the officials of the London Company that glass-making was a reality in the new province.
After the fall of 1609 and Captain Smith's return to England all
but 60 of the 500 colonists died glassmaking was halted.
1622 Indian Massacres halted Captain Willaim
Norton's attempt to renew glassmaking.
Glassworks were established in at least six states.
Glass production started at Pittsburgh.

Glassmaking at J amestoivnAmerica's First Industry
24 Glassmaking at J amestoivn-America's First Industry

II. Description of Methods, Past & Present
A. Preliminary steps
B. Crown Glass
1. The factory
2. The method
C. Broad Glass
D. Plate Glass
E. Mechanized Processes
1. Sheet Glass
a. Machine Cylinder Process (Lubbers)
b. Fourcault (vertical)
c. Colburn (Horizontal)
2. Cast Plate
a. Bicheroux Process
b. Ford Process (Continuous plate)
c. The Float Glass Process

A. Preliminary Steps
Three preliminary steps, typical to the production of crown, broad and plate glass were employed.
The first step was to heat or "calcine" the raw materials into a single mass. This was known as "frit." The drawing below shows the worker raking the frit from the furnace.
Vol. X, Verrerie en Hois, PI. XV,

In the plate below the workmen are implementing the next step, that of mixing the frit with "cullet," broken or left over scraps of glass.

Here the workers are employing the last step typical to all the processes, that of charging the main furnace with the calcined material, where it will be fused to a liquid glass.
Vol. X, Verrerie en Bois, PI. XVII

A. Crown Glass
"The following plates illustrate the production of crown glass. It meant glass which had been blown into a bubble, pricked at one end, whirled flat into a disc, and used as we use plate glass. Each 'table' of crown glass bore the distinguishing characteristic of a 'bulls-eye', which, like the rough imperfection in the base of hand-blown bottles, was the mark left in the glass when the blower detached his .punty rod.
The great advantage of crown glass was that it presented a brilliant surface unmarred by the marking and dulling inevitable in rolled or poured glass. But despite its beauty, crown glass has become obsolete. Casting or pouring plate glass lessened the manufacturer's dependence on highly specialized skills and produced much larger sheets for use in mirrors or fine windows. For panes of ordinary quality, crown glass was supplanted by broad glass. The crown glass process was expensive and wasteful. Cutting the round table into rectangular sheets required heavy trimming, and besides this the bull's-eye had to be eliminated.
Normandy was the center of the manufacture of crown glass. In that province there persisted into the 18th century the peculiar privilege which gave the master-blowers an aristocratic status. The profession of blowing crown glass was restricted to four families, who had enjoyed this monopoly since the 14th century. Thet were the Bongars, the Brossards, the Caquerays, and the Le Vaillants.
The following plate 'gives a view of a crown glass factory, set in the forests which provided it with fuel and designed in the style characteristic of Norman architecture. The furnace is in the great hall (a). There are no chimneys. Smoke pours out of the windows near the ridgepole. When a run is on, a passerby might think the whole factory about to go up in flames. The entrance is to the right. An arbor (d) has been planted under which the blowers may rest and refresh themselves. The wing at the left (e) contains warehouse and workshops for making pots and incindental equipment. The pool (f) serves for cleaning tools, some of which (g) are lying beside it.
At the extreme right is a shipping container packed with circular tables of glass, each four feet in diameter. Fig. 3 gives a more detailed picture."

Plate 235 Crown Glass I

The next drawing depicts the inside of a crown glass factory
showing the procedure in operation. (b) is the main furnace which was usually located in the center of the factory. (d) shows the oven in which the frit is calcined, with the entrance to the main furnace at (e) .
The blowing pipes are being preheated by the assistant for the gentleman-blower (p), who is in the process of lengthening a bubble by rapidly rotating it as it begins to cool.
(q) Another gentleman-blower is rolling a bubble or "bosse1 on the marver, usually of marble..
(r) Has just spun the bubble open. These steps will be shown in greater detail on the following pages.
After the bubble reaches its desired size, a solid rod, the pontil/or punty rod is attached to the opposite tip of the bubble with a small blob of molten glass. The blowing tube is then cut loose and the open bubble placed back into the furnace.
i '/

In the plate below, the blower is spinning the opened bosse, while reheating it. This is an extremely delicate time, as the experienced blower must spin the work just so, as it is reheating, softening the glass. '.Tie bosse then flares out into a bell and then into a flat "table" of glass.

When the table has sufficiently hardened it is then placed in a prepared depression (d) in the sand.
Vol. X, Verrerie en Bois, grande verrerie, PL XV

Now the purity is removed, that blob of glass,-which is the "bull's-eye" is still soft enough-to-pull the rod away.

Too rapid cooling of the glass can cause the release of stresses in the table, resulting in a shattering of the glass. ' For this reason it is placed in the annealing oven, shown below, allowing' these stresses to slowly dissipate leaving a single solid piece of glass.
Vol. X, Verrerie en liois, grande verrerie, PI. XVI

Two examples of more recent crown glass follow, the first showing its application, the bull's-eye being used as an ornamental element and the second photo the method of production.

Vol. IV, Glaces sou/lees, l'l. XXi
Broad Glass
In the glass industry of Lorraine and the northeast, a different technique was used for opening bubbles of blown glass into sheets. The first steps were similar to the crown glass process, but then instead of opening the glass into a bell, the blower (f) rotated the glass vertically into a hollow cylinder. This was then slit down the side and pressed into sheet glass. This was the method of the glass industry of Bohemia and Germany, of wliicli Alsace and Lorraine were, in certain ways, technological outposts. For this reason broad glass was also called "German sheet."

The next few plates are self explanatory and are taken directly from Diderot's book. One should take note that this is the "hand cylinder method," the process used in making the glass for the Crystal Palace in 1851.

Plate 250 Broad Glass II
Vol. IV, Glaces souflees, PI. XXXVI.
Plate 250 Broad Glass II
An artisan (a) takes the glass on a punty at the opened end, while the blower (b) knocks it off his pipe. A third artisan (d) spreads the neck so that the cylinder is of roughly constant diameter, after which it is carried up on a sort of platform so that the slitter (g) can cut it with his shears. After slitting, the glass is unrolled as a sheet on the table at the rear (h).

Plate 251 Broad Glass III
Vol. IV, Glaces soujlees, PI. XXXVU.
Plate 251 Broad Glass III
Annealing broad glass.
Plate 252 Broad Glass IV
Here are the tools used in the manufacture of broad glass.
The toise of the scale at the bottom is approximately six feet. It is evident that the sheets of glass must be fairly smallabout three feet by four.

Vol. IV, Glaces souflees, PL XXXVIII.

Plate 253 Broad Glass V
Vol. IV, Glaces, PI. XXXIX.
Plate 253 Broad Glass V
Since broad glass was unrolled on a flat table while still soft, its surface was dulled and scarred. For this reason, and also because the sheets were never uniformly thick, it had to be evened and polished by grinding. Even so, it never presented the brilliant surface which was the great characteristic of crown glass. Despite the expense of the extra step of polishing, however, the broad glass process required less dexterity than did crown glass, and since the product was rectangular and contained no bull's-eye, it could be used in larger panes.
This is a grinding shop. Glass is made to grind glass. Large sheets are plastered to the tables, while smaller sheets are fixed either to slabs of limestone fitted with handles (b, c, and d), or to the spokes of a wheel (a). Emery and wet sand are spread on the surfaces, which then grind each other smooth. Unground sheets are stored in the crude rack (e) at the back. No doubt the windows of the shop itself are glazed with factory rejects.

Plate 254 Broad Glass VI
Vol. IV, Glaces, PL XXX XII.
Plate 254 Broad Glass VI
Finally, broad glass had to be polished. The surface was furbished with felt buffers affixed to jointed ribs which maintained a constant pressure. Bits of grit or sand were brushed off with a silken brush. The wall mirror on the workbench in the background is the sort that was made of glass of this quality. Larger and finer mirrors, however, like those used in elegant salons or boudoirs, could be made only of the best grade of plate glass.

D. Plate Glass
None of the processes previously described have been of French origin. The process of casting plate glass, however, is exclusively French. Diderot claims this to be "the most important innovation since the prehistoric discovery of glass itself." No real argument, will be made here to contradict this statement, but it seems a great deal of credit is due the Phoenicians for their discovery of glass-blowing, a method sometimes still used today.
Although this method of casting glass was invented in 1668, resource material shows that it wasn't until 1688 that it came into production. This would have been 5 years after the death of Colbert who was Louis XIV's strongest supporter of French glass production. The date of 1688 therefore appears to be an error and should probably be attributed more to its date of invention, 1668. Three years earlier the one manufactory in which almost all of the 18th century plate glass was produced, was founded. It was a governmental concern, the Royal Glass Company. There was a plant at Saint-Gobain in Picardy, one at Tourlaville in Normandy and a polishing plant and warehouse'.; .'near Paris.
The casting process not only allowed for much larger and more uniform sheets, but also replaced a great deal of expensive skilled labor for that of unskilled.
The following six plates describe the casting method of producing plate glass.

Plate 266 Plate Glass X
Vol. IV, Glaces, PL XVIII.
Plate 266 Plate Glass X
A mixture of frit and cutlet is preheated in a kiln from which a workman (1) withdraws it. Transferring it to the main glass furnace is a continuous process. It is very hot and is carried around by scoopfuls (2), and thrust (3) through an opening into a pot where it is to be fused. The men return for another load (4) and await their turn (5 and 6). A furnace master supervises the progress of the melt (7). Apparently the heat is less intense than near a crown glass furnace, for the workmen are shielded by only their hat-brims. Fuel dries across the rafters.

Plate 269 Plate Glass XIII
Vol. IV, Glaces, PI. XX.
Plate 269 Plate Glass XIII
Impurities are skimmed from the surface of the melting pot (1). The molten glass adhering to the rod is scraped off on a slab (2), to be reclaimed for cullet. A stoker (3) adds wood to the fire.

Plate 271 Plate Glass XV
Vol. IV, Glaces, PI. XXII.
Plate 271 Plate Glass XV
A ladle of molten glass, ready to pour, is pulled out of the furnace onto a carriage. This is a tense moment. Everyone must move quickly to prevent the glass from overcooling. The furnace-master (1) himself takes a hand. The carriage will be wheeled rapidly over to a casting table in front of one of the annealing ovens in the background.

Plate 273 Plate Glass XVII
Vol. IV, Glacet, PL XXIV.
Plate 273 Plate Glass XVII
Pouring a glass plate is the climax of the operation. It requires speed and minute attention from every workman. The glass must be poured (1 and 2), and rolled out (3 and 4) evenly. Under the copper roller it handles like dough under a rolling pin and presents a workman with the same inert resistance to his will. Assistants (5 and 6) guide the roller. Others (7 and 8) are alert to catch and pluck out any bits of dust or scale which miglit fall onto the still soft surface.
Behind the two rollers, another pair (9 and 10) loosen the iron strips rimming the table before the glass has hardened to them, but- only ivhen it is stiff enough not to run. The hoistman (11) gives the pourers the height they need. The master (12) keeps his eye on everything, and the porters (13) take the carriage back to the furnace to get another ladle, for which the table will be moved along to the next oven.

Plate 274 Plate Glass XVIII
Vol. IV, Glaces, PI. XXV,
Plate 274 Plate Glass XVIII
The plate is slid into the annealing oven. Annealing plate glass required ten days. This shop has sixteen annealing ovens in operation to handle the output of a single furnace.

Plate 275 Plate Glass XIX
Vol. IV, Glaces, PI. XXVI.
Plate 275 Plate Glass XIX
Drawing the plate from the annealing oven (1-7) and carrying it away (8) to be ground and polished. The artist has probably exaggerated ils transparency. As a rule the surface of plate glass was so dulled by casting and rolling that it was almost opaque before being polished.

The two illustrations on the following pages show an early 19th
century use of plate glass in London, England. The first is the
shopfront of Saunder's and Woodley's and the second is Sangster's
Umbrella shop both on Regent Street and both built in the late 1830's. 8p-197

E. Mechanized Processes

1. Sheet Glass
In 1903 American J. H. Lubbers was the first to successfully produce window glass by the machine cylinder method. In this method, a drawing mechanism which represents the old blow pipe, is lowered into a crucible filled with liquid glass on the glass house floor. The glass adheres to the tip, or "bait" and is drawn vertically upward. As it is being raised, compressed air is forced through the pipe pushing the glass outward xf,rom the center and stretching out into a cylinder. The speed of the ascent and air pressure are necessarily controlled to give the proper surface smoothness and required diameter. I-154 See the photos on the following page.^ 152,153
a. Machine Cylinder Process

Fig. 48. The Lubbers cylinder machine. The cylinder is being drawn from the pot of molten glass by a flanged metal disc or 'bait'; the diameter of the cylinder is kept constant by compressed air blown down the blow pipe attached to the centre of the bait. When the pot is nearly empty the cylinder breaks away from the bottom of the pot and is then lowered onto supports.
Fig. 49. The Lubbers cylinder machine. The supports into which the cylinder of glass is lowered for splitting, flattening and annealing.

1. Sheet Glass
a b c
Fig. 50. The Fourcault process. The sheet of glass is drawn vertically from the furnace and narrowing of the sheet is prevented by forcing the molten glass to rise under hydrostatic pressure through a slit in a fireclay float (a) depressed below the surface of the glass (b). As the glass emerges from the slit it is drawn upwards by a bait and solidified by two water-cooled tubes placed against the sides of the slit. The sheet moves upwards continuously (c) guided by asbestos rollers.
b. The Fourcault Method1-157'4-155'3-130
Emile Fourcault was granted a patent for his method of making sheetglass in 1904. However, it was not produced commercially until 1913.
This method was accomplished by placing a fireclay float, its density less than that of the liquid glass, into the metal and forcing it below its natural floating level. A slot in the middle allows the glass to rise up through it (hydrostatic pressure) where it is attached to a bait and drawn up vertically.
The thick ribbon is drawn away and formed into a continuous sheet by asbestos covered rollers all of which are in a heated tower, acting as the annealing oven. As the solid glass reaches the top, it is cut into sheets.
See illustration below.

1. Sheet Glass
c. The Colburn Method1"159' 4"155' 5'13
While Fourcault was working in Belgium on his method of producing sheet glass, Irving W. Colburn was working in the U. S. A. on a similar process. His process was not commercially successful until he shared his ideas with Messrs Libbey and Owens in Toledo, Ohio. The primary difference was that shortly after drawing the glass vertically as in the Fourcault Method it was reheated and turned 90 degrees so as to move horizontally. Because Fourcault1s glass was continuously moved upward against gravity, the weight of this much material produced a much slower production than Colburn's.
Where statistics are concerned, Colburn'-s-. method produced standard window glass at a rate of 30,000 sq. ft. in 24 hours (1250 sq. ft./hr. more than twice the rate of the Fourcault process and 8 times more than that of the best hand blowers.

2. Cast Plate
This method was introduced in the early 1920's and was basically the same principal as the 17th century French method: that of pouring the molten glass onto a casting table and rolled out.
This process differed in that the metal was poured into a pair of rollers which pressed the glass between and through the two and then on to the casting table. In this way flatter plates which re-
quired much less grinding were produced. See illustrations below.4-158
Fig. VIII. 9. The Bicheroux process. [Above) Removal of pot from the furnace. (Belou-) Rolling of the sheet.
Fig. 51. The Bicheroux process. A. the casting table before rolling; B. the casting table raised to permit rolling; a. casting poi; b. mass of glass; c. casting table; d. inclined plane; e. rollers, rotating in directions shown by arrows; f. transportation table; g. rolled glass ihect.
a. Bicheroux Process

Cast Plate
The Ford Process
Henry Ford, out of necessity to produce glass at a more rapid pace to keep up with his production of automobiles, joined with the
successful method of making the glass flow continuously. In this method the glass flowed directly from the furnace to the rollers. Here the glass moved from the tank to the annealing lehr continuously, whereas in the Bicheroux proces^ the glass is poured intermittently from pots and then cut and carried to the lehr.
Included in the chain of production of glass was the continuous grinding and polishing, necessary because the glass made was, at first, translucent.
Pilkington Brothers, experienced glass manufacturqrs and devised a
Fig. VIII. 10. Diagrammatic representation of the continuous rolling process.

2. Cast Plate
c. The Float Glass Process. (Note: The information quoted below and on the next page came 4-160, 161, 162, A History of Glassmaking.)

160 A history of glassmaking
twin grinder and polisher (Figure 52) was never completely successful and usually the ribbon was cut and the large sheets polished separately, but on a continuous machine as in the first continuous grinding and polishing process.
The flow process used in conjunction with the twin grinder was adopted by the world's main manufacturers of plate glass for most of their production; other methods, being slow and very uneconomical, fell into disuse. Plate glass production has always required a greater capital outlay than is necessary for other types of glass manufacture, and tire introduction of the continuous process intensified the trend towards fewer units supplying a larger share of the total market demands.
Fig. 52. The twin grinder and polisher. The continuous ribbon of rough plate glass from the annealing lehr passes between twin grinding heads which grind both sides of the glass at once, after which it is polished.
The float-glass process
Glass produced by the continuous plate process is very flat, but requires grinding and polishing. Sheet-drawn glass on the other hand, has a brilliant fire-polished finish but the sheets show distortion caused by small differences in viscosity which affect the sheet thickness during the upward pulling
Flat jlass 161
motion of the forming machine. The ideal form of flat glass would therefore combine the flatness and freeness from distortion of plate glass with the natural finish and cheapness of sheet glass, and this ideal was achieved in the recent development of the float-glass process by Pilkington Brothers.
The aim of the float process was to reheat the newly-formed ribbon of glass and allow it to cool without touching a solid surface. In October 1952. Alistair Pilkington began experiments using liquid metal to support the glass ribbon as it emerged from the rollers. He says that:
The basic idea is a continuous ribbon of glass moving out of the melting furnace and floating along on the surface of molten metal at a strictly controlled temperature. Because the glass has never touched anything while it is soft except a liquid the surface is unspoiledit is the natural surface which melted glass forms for itself when it cools from liquid to solid. Because the surface of the liquid metal is dead flat, the glass is dead flat too. Natural forces of weight and surface tension bring it to an absolutely uniform thickness.
The float process is shown diagrammatically in Figure 53. The batch is melted in an oil-fired regenerative furnace and the glass emerges as a ribbon to float on the surface of molten tin at a carefully controlled temperature. The furnace atmosphere is controlled to prevent oxidation of the tin and heat applied from above melts the glass sufficiently so that it can conform to the flat surface of the molten tin. After sufficient cooling the plate can be fed onto rollers in the annealing lehr without affecting the surface finish and the annealed glass is cut into the required lengths.
This process took seven years of intensive work before it became a commercial proposition in 1959. A pilot plant was running in 1954 which provided a continuous flow of glass but many problems had to be solved, including die control of the float bath atmosphere, glass flow and the formation of the ribbon.
When these problems had been solved the float process was found to have many advantages. The brilliant ribbon of glass emerging from the float bath has few surface flaws and suffers little distortion. The speed of ribbon formation appears only to be limited by the melting capacity of the furnace and speeds in excess of fourteen metres per minute have been attained, the glass moving continuously through a horizontal annealing lehr to final inspection and packaging. Float-glass can be produced in a wide variety of widths and thicknesses at a cost very much less than the equivalent cost for the twin grinding process.
The first glass made by the float process had a natural thickness of about six millimetres determined by the forces of gravitation and surface tension acting upon the ribbon. Within five years of its commercial introduction methods had been developed to make thinner and thicker ribbons, the former by stretching the glass in a gentle and controlled way and the latter by

Flat glass 163
allowing the glass to build up to a certain extent within the float bath. By these methods thicknesses of between three and fifteen millimetres were produced.
The electro-float process
The electro-float process, announced in 1967, is a method whereby the surface of the clear ribbon glass can be modified during manufacture without shutting down the production line or requiring a separate unit for manufacture. As the glass passes through the float bath chamber an electric current is made to flow from an electrode above the glass through to the tin. By a rapid process of ion replacement between metal and glass, metallic particles such as copper can be implanted in the glass to controlled levels and densities.
Varying combinations of metals and temperatures yield glasses with different heat and light transmission characteristics which can be used for such purposes as reducing glare and cutting down heat transmission. Such glasses are most useful for the glazing of buildings containing large window areas, and can also be used to advantage in cars and aircraft.
1. Medieval stained glass, H.Hutter, (trans.) A. Shenfield, Methuen and Co. Ltd., 1964.
2. Some comments on the medieval glass industry in France and England, G. H. Kenyon, J. Soc. Class Techno!., 1959, 43, 17N.
3. The glass industry of the Weald, G. H. Kenyon, Leicester University Press, 1967.
4. A historybfindustrial chemistry, F.Sherwood Taylor, Heinemann, 1957.-
5. The Bicheroux process of making plate glass, Glass Ind., 1927, 8(9), 207.
6. Review Lecture, The float glass process, L. A. B. Pilkington, Proc. R. Soc. Lond. A, 1969, 314, 1.
7. Glass in the modern world, F. J. Terence Maloney, Aldus, 1967.
8. Pilkington Brothers and the glass industry, T. C. Barker, George Allen and Unwin, 1960.
9. Revolution in glassmaking, W. C. Scoville. Harvard University Press, 1948.
10. A textbook of Glass Technology, F. W. Hodkin and A. Cousen, Constable and Co. Ltd., 1925.
11. A history of Chance Brothers and Company, J. F. Chance, privately printed by Spottiswoode, Ballantyne and Co. Ltd., 1919.
12. flie development of coloured glass in England, H. J. Powell, / Soc. Glass Techno!., 1922,6,249.

III. Ornamental Architectural Glass (to 1940)
A. Stained Glass
B. Leaded Glass
C. Cast and Moulded Glass
P Engraving and Cutting
E. Sandblasting (Sand etching)
F. Acid Etching
G. Rolled Figure Intaglio
H. Glassblocks
I. Decorated, Enamelled, Sanded

Stained Glass
& Its
The following information is mostly in quotes and greatly reduced in content, since reports previously written have extensively covered the subject.

Empire in (lie fourth century ad, and may have been the forerunners of the Norman 'crown glass' which was made in the Middle Ages.
After the decline of the Roman Empire, window glass was still made in the West although on a reduced scale. Its chief use appears to have been for the glazing of churches, as is noted in several manuscripts of the period. An account of the miracle of St Ludger, who died in AD 809, mentions glass windows in many colours; they may have been ornamented with painting and there were probably also various ground colours. Bede states that in AD 675 French craftsmen came to glaze the church at Monkwearmouth: 'they not only did the work required but taught the English how to do it for themselves'. Before the end of the seventh century AD glass had replaced the linen and perforated boards in the windows of York Minster, but any training that the foreign glassmakers may have provided appears to have been unsuccessful, for in AD 758 the Abbot of Jarrow had to send to the Rhineland for glaziers to work on his monastery.
Stained glass and the medieval church
By the year AD 1000 conditions in Europe were becoming less warlike and church building began to flourish. A writer of that period spoke of the 'white robe of churches' which covered the land. However, the windows of these Romanesque churches were very small because the walls and pillars had to be massive and strong in order to bear the weight of structure above them. In 1137 Abbot Suger of St Denis, near Paris, started rebuilding his abbey church, and erected the first church to be designed in the Gothic style. This style, typified by the introduction of the pointed arch and later of flying buttresses which took much of the stress off the walls, enabled the builders to open up and lighten the structure and to insert jewel-like panes of glass which formed a 'wall of light', filling the church interior with glowing, insubstantial colour, and reflecting the theological idea of God as the source of perfect light. They also showed the Bible stones in vivid simplicity for those who could not read. Visitors to a strange church were instructed, in an ancient catechism, to pray to God and then to wander around the building looking at the stained glass. The popularity of stained glass spread with die new architectural style and the production of window glass began to grow at a time when vessel glass was suffering a period of decline in Europe.
Heinrich von Veldeke in his Aeneid of AD 1200 describes the effect made by the stained glass in a sepulchral chapel:
'Of garnets and of sapphires, Of emeralds and rubies, Of chrysotites and sardius, Topazes and beryls. The beauty of the early stained glass is enhanced by its very imperfection and by the weathering which has taken place over the centuries. Bubbles and
striatums present in all old window glass cause variations in refractive index throughout the material and the finished panes, when viewed from a distance in their window setting, have a richness of colour which might be lacking in more perfect glasses. Figure 41 shows the beautiful window in Chartres Cathedral, known as 'Notre Dame de la Belle Verriere', part of which dates from the mid-twelfth century AD. The blue of the Madonna's robes and halo is paler than that of the side pieces which date from the beginning of the thirteenth century. This comparatively light colouring is characteristic of the stained glass of the twelfth century, and as the window areas grew larger in proportion to the wall, so the colours became stronger and brighter.
This great expansion in the use of coloured glass must have resulted in the medieval glassmakers performing many experiments on the production of suitable colours for the varied scenes which the windows depicted. Their colouring agents were in general those used by the ancient glassmakers as far back as Egyptian and Babylonian times, but whereas we generally have to rely upon modern chemical analysis for our understanding of these ancient glasses there is additional information about medieval glasses to be gleaned from the contemporary texts which were written about all aspects of the manufacturing industries of the time.
Although, in modern terms, only a few elements were used as colouring agents, they were obtained from many sources and their preparation was complex. Blue was one of the most important colours and was obtained from zaffre, an Arabic word for cobalt oxide; the material containing diis oxide had to be brought from the Levant at great cost and was known as Damascus pigment. Later, cobalt Ores were exported on a large scale from Saxony. The cobalt was extracted by roasting the cobalt mineral (cobalt arsenide or sulpharsenide, with various other metallic sulphides) so as to remove sulphur, arsenic and other volatile matter. Another source of cobalt used by glassmakers to give a blue colour, according to Agricola, was the residue left behind when bismuth was separated from its ore. This residue must have contained cobalt with traces of nickel, and probably of iron and copper, and the blue produced would differ from that due to pure cobalt. Combinations of zaffre and copper compounds (e.g. calcined brass) gave a sea-green.
Copper and iron compounds were used to give greens and reds. Copper was generally added as ferretto. or burnt copper, made by a recipe dating from classical times. The copper was heated with sulphur to give a black mass of copper sulphide which was then roasted until it was converted into the red ferretto. Alternatively the copper was heated with blue vitriol, or copperas, copper sulphate, the resulting material probably containing a large proportion of basic copper sulphate. Copper could also be added by the inclusion of calcined brass, or brassmakers' scales, the mixture of oxides of copper and zinc formed when brass was heated; crocus martis, ferric oxide, made by the same process as ferretto, but using iron instead of copper, was widely employed for yellows and browns. As with many medieval materials, the

136 A history of glassmaking
Fig. 41. 'Notre Dame de la Belle Verriere,' mid twelfth and thirteenth centuries AD. The beautiful colours of this window, predominantly blue, were produced by the medieval craftsmen using a very restricted range of colouring agents, most of them known since ancient times.
Flat glass 137
words crocus marlis covered many reddish compounds of iron. e.g. ferric acetate, nitrate or chloride made by treating iron filings with vinegar, aqua fortis (nitric acid) or aqua regia (a mixture of nitric acid with hydrochloric acid or a chloride). Opaque white glass was produced by tin oxide, and purple by manganese.
Deep red, the colour most frequently used after blue, could be produced by iron mixed with a little calcined brass but the finest reds or 'rubies' were made with copper. Copper ruby glasses could be produced by fusing glass containing copper with a small amount of tartar (potassium hydrogen tartrate) which reduced the copper to the cuprous state; the reducing atmosphere of the old furnaces also had this effect. On reheating the glass a colloidal dispersion of copper and probably of cuprous oxide was produced, giving a fine red colour. Copper ruby glass has a very deep red colour, and in order to produce a glass of the required transparency the panes of red were flashed, i.e. a bulb of clear glass was dipped into a pot of copper ruby glass to give a thin transparent layer of ruby glass on top of the clear-glass. All medieval ruby glass was flashed, but the art of making copper ruby seems to have died out during the early seventeenth century. The French glassmaker Georges Bontemps revived its production at his factory in Choisy-lc-Roi in 1826, and later brought his knowledge to Chance Brothers in England where copper ruby glass was produced on a large scale during the nineteenth century. '
Gold ruby glass is generally not thought to have been made before the late-sixteenth century but a manuscript of the fifteenth century preserved in the convent of S. Salvatore, Bologna, gives recipes for dissolving gold in aqua regia and precipitating it by means of a solution of tin. and for obtaining a rose-coloured glass with the precipitate. A manuscript also exists in the British Museum, dated 1572, which was transcribed from an 'old copye'. and gives several recipes in which gold was used as the colouring agent. Thus it seems that the seventeenth-century makers of gold ruby glass, notably Kunckel. may have had some knowledge of these earlier methods.
Details of a scene on the stained glass window, such as die folds of drapery, were applied by means of painting, and then firing, a black enamel pigment derived from iron. From the thirteenth century a second pigment in the form of a 'stain' of silver chloride or sulphide was used. When the stain was applied to die clear glass and fired, colours varying from yellow to orange were produced. The stain was later applied to blue glass to give a green colour, making possible die depiction of blue sky and green fields on the same piece of glass.
Crown glass
The Normans made crown glass, although its origins dated back to Roman times, and it was in general use until the end of the eighteenth century.

chapter vi
The history of coloured glasses is longer than that of the transparent colourless variety. For many centuries before clear glass appeared a large number of different coloured forms had been in common use.
Some of the glazed tiles covering the walls of early Egyptian chambers were magnificent examples of colouring, and polychromes, as well as single colours, were achieved. Pieces of a vase have be,en found, bearing the name of a King living in 5000 b.c., which was glazed in green inlaid with purple. Some of the most important colouring oxides employed in glass-making to-day, including copper, manganese, and cobalt, were known to the Egyptians.
Egyptian glasses of the following colours have been found: black, blue, green, opal, red, and yellow. According to Sir Herbert Jackson cuprous oxide was the colouring agent of the brilliant scarlet ^Egyptian glass, specimens of which date from as far back as the" eighteenth dynasty up to Roman-Egyptian times. One example examined by Sir Herbert appeared to be an ordinary lead glass (30 per cent PbO), containing 8 to 10 per cent ofCuaO. He reproduced this glass after a few trials. Transparent glass did not appear in Egypt before about 660 b.c., when bottles and a few other objects were made of it.
From these early beginnings in Egypt the art of coloured glass-making gradually spread. One of the first uses of coloured glass in England was in cathedral windows. It will be understood that the glass was self-coloured, and used only in very small panes. The staining of glass and the introduction of large figure subjects came much later.
Coloured glasses, whether for use in cathedral windows or for any other purpose, are of two main kinds, (1) Pot metal, of the same colour throughout; and (2) Flashed or coated metal, where a thin coating of coloured glass is superimposed upon a* foundation of clear glass. In the second type, used for very dense colours, the saving in the cost of the quantity of coloured glass is offset by the difficulty of welding two glasses of different natures

the manufacture of glass
together. In recent years, however, ornamental glass of three layers has been successfully made. Upon a clear body a coating of white enamel glass has been placed, which in turn has served as a background for a final layer of glass in blue, red or other colours. By cutting through the upper layers to the clear foundation attractive results arc obtainable. It is necessary that glasses used together in this way should, as far as possible, be of similar composition.
A common belief is that wc do not know how to reproduce the brilliant colours of the early cathedral windows. This was partly true up to about a hundred years ago, but to-day the glass technologist certainly knows far more about colours in glass than ever before, and any failure comes about through the stained-glass artist not having sufficient inspiration to make good use of the material provided for him.
A further point in connexion with old cathedral glass is that when originally placed in position it was often very crude in colour and dark in shade: it has taken hundreds of years to become mellowed. Atmospheric attack on the outside of church windows has been the chief agent in creating the beautiful effects Ave know to-day. Throughout the centuries there has been steady abrasive action through the rain being driven against the glass by the Wind, and chemical dissolving action through the slight acid content of much of our rain-water, especially in towns. The combination of these two forces has gradually changed thick and comparatively dark glass to glass which in some cases has only the thickness of tissue paper and which has now light and vivid colours.
Even the impurities in the metal, which the early glass-makers could not control, gave variety to their products. It was of little consequence if the glass they supplied for church windows never had quite the same colour twice. Exciting surprises were possible, unlike the dull modern world in which glass-making materials and processes are so carefully and completely controlled that results are (or should be) well ascertained in advance.
Most of the advances made in recent years in connexion with the technology of coloured glasses have been the outcome of new research. At the same time some information has been gained by the investigation of the chemical composition of old glasses. Not long ago, for example, some of the thirteenth and fourteenth century glass in the Cathedral and the church of St. Remi at Reims was analysed, with the following result
colours in glass 51
Cathedral St. Remi Cathedral St. Rem
SiO, 53'9 50-10 53-50 54-10
Cad 19-30 18-60 1780 i6-6o
KaO 12-20 16-70 15-00 15-10
A1203 3-90 4-30 3-00 3-30
MgO. 4'10 4-70 6-io 4-7o
Na20 1-90 2-60 1 *8o 1-go
CuO 0-13 0-08 0-13 0-20
Mn304 3-03 0-63 o-86 1-22
Fe203 0-79 o-95 0-84 1-90
H,0 0-40 0-30 0-40 0-50
PbO trace o-io 0-03 o-io
TiO, 0-20 trace trace o-io
Bud,. 0-02 0-08
S03 0-20
CoO 0-25 1-03
100-32 100-17 99-46 99-72
It is convenient to consider in turn each of the common colours in glass and the agents for producing them. In certain cases this will mean that the same chemical will appear under different headings.
Cobalt and ,-nickel oxides are used together to give opaque black glasses. Nickel glasses are noted by Professor Woldemar Weyl as absorbing nearly the entire visible spectrum with the exception of the extreme red.
A small antique glass bowl shown to the author on one occasion by the late Sir William Boyd Dawkins, D.Sc, F.R.S., and of an almost jet black colour, had certainly a high concentration of manganese as the principal colouring agent, possibly with the addition of a little cobalt.
Antimony oxide can be used for the production of black glass. When the antimony is introduced in amounts of 10 to 30 per cent no colour is produced immediately, but upon reheating the glasses become greyish-black to black in colour.
Cobalt and copper serve as the usual agents for colouring glasses blue. Of the two cobalt is the more commonly used. It transmits a deep red band in addition to blue. Accordingly, if a cobalt-blue is combined with a copper bluish-green the latter
Blue Glass Red Glass

the manufacture of glass
absorbs the red and an excellent blue glass is obtained. If, however, the colour required is to be midway between blue and green, as in signal lenses, copper is used, in association with a small amount of iron.
Cobalt is so powerful that only a very small quantity is needed to produce a rich coloration. The following well-proved batches will indicate this and may be taken as normal melts
Light Blue Opaque Glass. Sand 100 parts, potash 19, soda ash 15, red lead 8, borax 2, bone ash 17, saltpetre 2, copper sulphate 1, cobalt oxide o-oi.
Dark Blue Opaque Glass. Sand 100 parts, potash 20, soda ash 15, red lead 8, borax 3, bone ash 18, saltpetre 2, cupric oxide 1-5, cobalt oxide 0-04.
It will be observed that so far as the copper is concerned, only as little as approximately 0006 per cent is required.
Weyl observes that from the viewpoint of glass technology it is only necessary to consider cobalt compounds which are derived from the divalent cobalt ion, as trivalent cobalt is not stable in the temperature range required for glass melting. It has been found that potash glasses give a purer blue than the corresponding sodium glasses, and that borosilicates yield a reddish hue.
An interesting modern use of cobalt is in connexion with lighting glassware. If small amounts of cobalt, manganese, and copper oxides are used together in the batch what is now known as the daylight effect is obtained. What happens is that these oxides eliminate part of the light from the artificial light sources, and the filtered remainder more closely resembles natural sunlight.
Sir Herbert Jackson suggested that the production and properties of glasses containing copper are representative of those in which the colouring is diffused in very minute particles throughout the glass, and which can be compared with colloidal solutions. In contrast to this cobalt glasses are examples of those in which the colouring agents are in a state resembling solution, which might be compared to aqueous solutions of coloured salts.
Less than 1 per cent of copper oxide is required to produce colours ranging from light blue to dark indigo blue. It is believed that the colour is intensified by the presence in the batch of
colours in glass
boron, barium, or lead, and soda glasses are generally found to be darker than potash glasses. Copper nitrate (Cu(N03)2.6H20) also gives blue to indigo colours, sometimes stronger than those produced by the oxides.
In the opinion of Granger, copper colours lime-soda and lime-potash glasses blue, but the colour changes to green as the concentration is increased. Glasses rich in silica tend to give a green colour with copper, while a large lime content is more favourable for the production of red copper glasses.
Gmelin noted in 1779 that glasses containing iron became blue when founded under strongly reducing conditions, but no explanation was then possible. The colour was later attributed to a modification of ferric oxide, which is only stable in the presence of ferrous oxide. Weyl points out that this involves the existence of the two states of valency in the glass, or the simultaneous presence of FeO and Fe203. The addition of limestone to the melting glass may cause the colour to change to blue, as it may contain a certain amount of organic material which acts as a reducing agent.
Gold is capable of producing a blue colour in glass. A gold-containing glass cooled quickly tends to be colourless, a red colour being developed by re-heating, and if kept too long at a high temperature a blue colour can be observed.
For the production of green glasses iron, copper, and chromium are employed. Other possible agents, such as stannic oxide, are very rarely used.
Iron, which can be added as ferric oxide or as hydrated ferrous sulphate, exercises a greater colouring action in the ferrous state when present in small quantities, but the reverse when present in large quantities. The presence of 0-23 per cent Fe203 gives an almost colourless glass which is greenish-blue in thick layers, 048 per cent a light sea-green, 0-73 per cent a bright sea-green, 1-23 per cent a bright yellowish-green, 5-56 per cent a deep yellowish green, but still transparent, 8-23 per cent a dark olive

the manufacture of glass
green, still transparent even in thick layers; 11 12 per cent again a dark olive green, but opaque in thick layers. The percentages stated were obtained by the analysis of test glasses, and therefore represent the amount of iron present in the finished glass.
As might be expected, the colours given by copper depend upon the conditions under which the glass is melted. A blue or green glass is obtained from copper compounds under oxidizing conditions, and a ruby colour under reducing conditions. For green glasses the black or cupric oxide is used.
To obtain the highest transmission of blue and green, and the lowest of red, a copper glass should contain magnesia. On the other hand neither boric acid nor alumina should be used in the batch, as they decrease the green-blue transmission. It has been found that glasses containing soda give better results in this respect than potash ones.
An opaque Seladon green results from a batch composition of sand 100 parts, potash 18, soda ash 16, red lead 8, bone ash 18, borax 3, saltpetre 2, cupric oxide 1, sodium uranate 9-9. An Iris green, also opaque, is obtainable from the same batch with the sodium uranate omitted and the addition of 0-3 parts of potassium chromate and 0-4 parts of ferric oxide.
Chromium was discovered in 1795 in the Russian mineral crocoite, which is a lead chromate, and has been used for colouring glass since the early part of last century. The name is derived from the Greek word ckromos, colour, and indicates that the chromium compounds have vivid hues.
Chromium is used in the forms of Cr303, K2Cr2Ov, and K2Cr04 for the manufacture of green and yellow-green glasses. The introduction of 0-05 per cent of Cr203 into the batch gives a green colour, which takes a yellow tint in the presence of an oxidizing agent such as a nitrate. To produce a bright emerald green, and to avoid the yellow cast, additions of reducing agents such as As2Os or Sb203 are necessary. The use of K2Cr04 gives a colour richer in yellow than that due to Cr2Os, and the yellow coloration is still more pronounced when K2Cr207 is the agent. The green coloration has been ascribed to Cr204 and the yellow to Cr03, but it is possible that the colours may result
colours in glass
from action upon the iron oxides present. In general, the colours from chromium in glass are yellowish-green, so that chromium glasses have a high transmission in the red part of the spectrum. For this reason chromium should not be used as the main colouring agent in a signal-green glass.
A common defect in chrome green glasses is the presence of black specks. This troublesome characteristic is due to the limited solubility of chromium oxide in glass, which causes part of it to remain undissolved. The occurrence of the specks usually shows that the chromium compound has not been mixed sufficiently intimately with the remainder of the batch. Sometimes, however, the cause is lack of purity in the chromium used, or a low furnace temperature. If a soluble chromium salt is being used, such as potassium bichromate, the difficulty of good mixing can be overcome by dissolving in water and spraying the solution over the sand, which can then be dried and mixed with the other chemicals.
Nearly all actinic glasses are made with chromium as the colouring oxide. In tank furnaces the chromium is usually added as a chromate salt, when satisfactory mixing and distribution are readily obtainable. In pot furnaces, on the other hand, and it is in such that the bulk of actinic green glass is made, chromium is used either, in the form of a bichromate or of chromium oxide. Of the two the oxide is the more liable to give black specks because the bichromate is much the more fusible.
At certain concentrations of some colouring oxides a second phase appears. It is possible to make a chrome-aventurine glass in which the second phase can be seen, it being chromium oxide that gives this effect. Like most chromium glasses, the aventurine variety is decidedly more yellow-green than blue-green.
To produce grey or "smoked" glass the colouring agents used are combinations of (a) cobalt, nickel, and uranium oxides, or (b) manganese, iron and copper oxides. The glass may be of either the soda-lime or lead-potash type. The amount of the colouring material naturally depends largely upon the ultimate purpose of the glass. Thus thin sheets for use in spectacles require more colouring matter than the thicker sections used in windows. Typical batch mixtures are
(1) Sand 100 parts, soda ash 30, limespar 24, manganese 3, iron oxide 3, copper oxide 2.

the manufacture of glass
(2) Sand 100 parts, red lead 82, potash 25, nitre 10, uranium oxide 2, nickel oxide 2, cobalt oxide 0-5.
(3) Sand 100 parts, soda ash 30, potash 6, talc 5, bone ash 4, iron oxide 1, manganese 0-05, cobalt oxide 0-05.
It has been shown that Bismuth basic carbonate (2(BiO)2. C03.H20) imparts a colour varying from grey to brown to the majority of glasses when present to the extent of 2-5 to 5 per cent. Glasses of the borosilicate type showed the least colour, while potash glasses not containing boron had the deepest shades when experiments were made. The brown colour developed on reheating, in some cases causing opacity. Bismuth subnitrate (Bi(OH)2.NOs) produces colours less pronounced than those obtainable from the basic carbonate, but on re-heating the difference disappears.
Opal glasses fall into two main classes. First, that in which the opacifying agent dissolves, leaving the glass transparent while molten, but causing it to become opaque upon cooling. Second, that in which the opacifier never wholly dissolves, but remains very evenly and finely distributed throughout the molten metal. In the one type the glass is quite transparent when gathered from the crucible, and gradually becomes opal as it cools. In the other the metal is already opal upon gathering, but becomes more so as the temperature falls.
In most opal glasses the opacity is dependent upon the separa- -tion of either finely divided silica or some metallic oxide or both from the mass of glass. The separation is assisted by the presence of fluorides and phosphates. A certain degree of opalescence may result from the separating out of the fluoride (say as aluminium fluoride) or the phosphate themselves from the glass, but as a rule the batch is such that they remain combined as fluosilicates and phosphosilicate, throwing out. of solution the silica. The source of the opal colour in glass, therefore, may be either excess of silica; excess of some metallic oxide such as alumina, tin or antimony; or the presence of fluorides or phosphates.
Felspar and Fluorspar
The most commonly used materials for producing opalescence are felspar and fluorspar. The batch composition varies greatly,
colours in glass
partly in accordance with the degree of opacity required, but the following may be given as examples of typical melts
Milk-white Glass. Sand 100 parts, soda ash 15, felspar 18, fluorspar 9, cryolite 8, tin oxide 4.
Alternative Milk-coloured Glass. Sand 100, soda ash 12, potash 5, felspar 15, fluorspar 10, saltpetre 2, cryolite 10, nickel oxide 4.
Enamel-white Glass. Sand 100, potash 20, saltpetre 5, tin oxide 12, red lead 80, borax 4, bone ash 12.
Opalescent Glass. Sand 100, soda ash 30, felspar 34, fluorspar 18, red lead 6, nitre 4, borax 0-5.
Imitation Mother-of-Pearl. Sand 100, soda ash 25, red lead 15, saltpetre 25, cryolite 1-5, bone ash 5, borax 25, nitrate of bismuth 3, nitrate of copper 0-25, fluorspar 1-5..
Opal Glass for Illuminating Ware. Sand 100, soda ash 30, fluorspar 20, felspar 35, cryolite 8, manganese 0-25.
Silicon, Boron, etc.
Duval d'Adrian of Washington, Pennsylvania, has recently sought to show that, by using the complex fluorides of silicon, boron, tin, zirconium, and titanium with the fluorides of the alkaline earths and heavy metals, and adding various mixtures of these to an ordinary glass batch, more satisfactory results are obtained than with simple fluorides. An example of a batch used by him for this purpose is: Sand 100, soda ash 35, felspar 20 to 30, magnesium silicofluoride 0-5 to 5, barium stannofluoride
-5 t0 5-
Tungslen and Molybdenum
Weyl suggests that the technical applicability of tungsten and molybdenum compounds in glasses, apart from serving as constituents of special glasses with high refractive indices, seems to be limited to their use as opacifiers.
Opal glass is being increasingly used for a wide variety of purposes. It is now commonly found in the form of illuminating globes and bowls, sheet glass, towel rails, imitation candle tubes for electric lighting, containers for pomades and other druggists' sundries, and linings for electro-plated articles.
It has been shown by Ebell and Seleznew that alkali sulphides, unlike the sulphates, are soluble in silicate glasses. A later development is the making in Bohemia of a molybdenum sulphide

the manufacture of glass
glass from the following formula: Sand 700 lb, soda ash 1 o, potash (85 per cent) 30, limestone 18, sodium sulphide 4, molybdenite 2. A complex sodium sulphomolybdate is probably responsible for the orange colour.
According to Sir Herbert Jackson the famous Chinese sang-de-bmuf or ox's-blood glazes were due to dispersed finely-divided copper in amounts of the order of 0-5 per cent, the reducing conditions under which they were made avoiding any tinge of green through the presence of cupric oxide.
Purple Manganese
The compounds of manganese are among the oldest colouring agents used in glass. Dralle and others have stated that the purple colour is produced by the trivalent manganese oxide, Mn203, in equilibrium with MnO.
To secure a deep colour with manganese the founding has to be under oxidizing conditions, as reducing agents destroy the purple tint. Mn203 is readily reduced by the oxides of arsenic and antimony. It has been found that glasses melting at low temperatures, rich in alkali, are more suitable for use with manganese than hard glasses rich in silica.
Two pieces of transparent glass of an intense purple colour were sent to the late Sir William Crookes from South America. They had been found on rubbish heaps at a high altitude and had been exposed to the action of the sun over a long period. The probable cause of the colour was manganese, which would be affected by the solar rays of short wave-length present at an altitude of 4000 metres, but absent at sea-level owing to atmospheric absorption. The colour had penetrated the whole mass, but disappeared when the glass was re-heated to softening-point. By exposure to radium rays the colour could quickly be reproduced.
Red and Ruby Antimony
A deep red colour in glass can be obtained by the use of antimony. The colour does not strike below the softening range, and Weyl therefore suggests that the development of the colour should be combined with the moulding operation.
colours in glass
Copper oxide in the cuprous form, melted under reducing conditions, gives an excellent ruby. No nitrates or arsenic should be present, and if the glass contains lead this should be introduced as litharge and not as red lead. The average quantity of copper used is from 51b to 10 lb, with an equal weight of tin oxide, to every ton of batch. The colour is caused by the deposition of exceedingly minute particles of copper throughout the body of the glass. In order to impart a scarlet tinge to the glass it is only necessary to add to the batch one oz of ferrous oxide to every four oz. of cuprous oxide used. If a copper ruby is re-heated slowly for some time the particles of copper aggregate and become visible to the naked eye, giving what is known as an aventurine glass. This type of glass was first made at Murano, the name aventurine being from the Italian avventurino, "chance," indicating its accidental discovery. It has been described as a semi-opaque glass filled with golden yellow spangles, the effect being produced by the separation of thin reddish-yellow plates of copper in a greenish glass.
Copper is remarkably useful to the glass manufacturer, for besides reds it is possible with it to colour glass brown, yellow, green, blue, purple, and black. And with careful control of conditions and the amount of copper compounds present these colours are available in an almost endless variety of shades.
Ruby glasses are used probably more frequently than any for t casing purposes. It has been found with copper ruby glass that a simple way of overcoming the welding difficulty is to make 25 per cent of the batch for the coloured glass consist of powdered cullet from the clear glass which is to be used as a base. The use of the cullet from the clear glass in this way obviously provides a useful if not very scientific method of seeing that the two glasses are allied in nature, and therefore have similar co-efficients of expansion.
Three examples of batch compositions for copper rubies are
English Cherry Red Glass. Sand 100 parts, potash 30, borax 7, saltpetre 5, red lead 20, cream of tartar 2, cuprous oxide 6, stannous oxide 6, iron oxide 0-5.
American Ruby Glass. Sand 100, potash 26, white lead 30, borax 10, soda ash 13-2, cuprous oxide 3-5, tin oxide 2.
German Ruby Glass. .Sand 100, potash 30, soda ash 2-5, limespar 12, copper sulphide 002, sodium sulphide 0-026, borax 0-024.

the manufacture of glass
Gold was for long a traditional source of pure ruby colour in glass. The brown gold (gold chloride) used is put into solution with nitric acid and sprayed over the batch of the remaining raw materials. The colour obtained is probably richer than any other red. On account of the high cost of production gold rubies are now, however, made much more rarely than in the past. Colours almost equally good are obtained from copper and selenium. In the case of gold one part of gold to 50,000 parts of glass will give a bright red colour. On the rare occasions when a gold ruby glass is made to-day, it is generally prepared from a soft lead batch containing an excess of antimony and arsenic. The gold is added in the form of a solution of gold chloride, obtained and used in the manner described a little earlier, in the proportion of 1 oz of metal to 200 lb of batch.
Selenium was first isolated by Berzelius in 1817, who used the mud of sulphuric acid plants as his raw material.
Although selenium is now being used more and more as a source of red colour in glass the colour is difficult to retain, because if great care is not taken the selenium has a strong tendency to volatilize. Considerable experience is needed, adapted to the conditions ruling in each factory, to cover batch composition, furnace and melting procedure, and the actual working out of the molten metal. What has proved a good formula for blown selenium ruby ware may prove an entirely unsatisfactory one for pressed ware.
The part played by selenium in the batch is indicated if increasing amounts of it are added to a cadmium sulphide glass. The original pure yellow colour of the latter changes first to orange and then, as the proportion of selenium is made greater, it becomes a brilliant red. Even when the amount of selenium present is ample the proofs of the melts may be colourless, and the red coloration may appear only after the glasses have passed through the lehr or annealing oven. Such colourless proofs exhibit a blue fluorescence, which increases with the selenium content. Examination of the glasses by the ultra-microscope show this fluorescence to be due to colloidal particles in the case of the glasses rich in selenium.
Selenium was used on the Continent for the production of ruby glasses for a considerable time before it was introduced in
colours in glass 61
Britain, where the Manchester firm of Butterworth Bros., Ltd., was the first glass works to make selenium ruby glass successfully on a commercial scale. The following Continental and English batch formulae may be compared
Mo. i Continental Selenium Red. Sand 100 parts, potash 30-3, soda ash 30-3, zinc oxide 21-2, cadmium sulphide 1 -51, selenium 1-51, borax 1-51.
No. 2 ditto. Sand 100, soda ash 29-8, hydrated lime 27-7, cadmium sulphide 1-35, selenium 1-04.
Mo. 1 English Selenium Red. Sand 100 lb, soda ash 62 lb, zinc oxide 151b, cryolite 41b, cadmium sulphide 1 lb 4 oz, selenium 6 oz.
Mo. 2 ditto. Sand 100 lb, soda ash 50 lb, zinc oxide 151b, cryolite 25 lb, cadmium sulphide 10 oz, selenium 4 oz.
An unusual but quite satisfactory means of producing in glass a colour varying from a light henna to a deep ruby is by adding to the batch uranium oxide, together with an oxidizing agent such as sodium nitrate. The amount of uranium oxide required is from 2 to 12 per cent, in accordance with the depth of colour desired. A batch giving a beautiful red colour with uranium is: Sand 100 partSy litharge 93, soda ash 107, borax 27, nitre 40, uranium oxide'32.
Violet Titanium
Colours ranging from violet to purple can be obtained by adding less than 1 per cent of titanium dioxide (Ti02) to the batch mixtures of certain borosilicate or phosphate glasses. A curious feature is that titanium dioxide itself is white, and the salts of tetravalent titanium are colourless.
Yellow and Amber
It is believed that at present no single element can be used to isolate spectral yellow in glass. Glasses of varying shades of yellow are, however, produced by means of cadmium, silver, sulphur, and uranium.
Carbon is very commonly used in the manufacture of yellow glasses, being entered in the form of coke, graphite or anthracite. It is the sulphate-containing impurities in the carbon which are the true source of the colour. This was demonstrated among

the manufacture of glass
others by Splitgerber, who in 1839 melted two carbon-containing batches, the first of which had 1-75 per cent of sodium sulphate, ^but the other none, and only the former developed a yellow colour. Again, in 1865, Pelouze obtained similar results, finding at the same time that other substances, such as silicon, boron and phosphorus, which had the power of reducing sulphates, gave rise to a yellow colour. Even hydrogen, at incipient red heat, produced a similar colour with a sulphate-containing glass.
Glasses can be coloured yellow by colloidal cadmium sulphide. Thus in Germany, while China-yeilow glasses are coloured with sodium uranate, the rich Kaiser-yellow glasses are coloured with cadmium sulphide. The latter brilliant yellow colour cannot be obtained by any other means.
On melting a cadmium sulphide batch a colourless glass results, which has to be heat-treated to cause the striking of the colour. When photographic filters are being made the glass is first blown in the form of cylinders, which are afterwards slit lengthwise and placed in a muffle furnace to open out into the final form of flat sheets. It is while the glasses are in the muffle that the colour strikes, two manufacturing operations being thus accomplished at the same time.
The yellow stain so favoured in church windows is obtained by firing a glass with silver, and is due to finely divided colloidal silver. The colour is secured most easily with potash-lime glasses, for with hard glasses there is a tendency for the silver to be deposited metallically.
In the making of amber glasses it is usual to employ a mixture of sulphur and charcoal as the colouring material. The batch must of necessity be melted in a reducing atmosphere. Experiments by Fenaroli and others led to the conclusion that the elements of the group giving yellow colours are only effective in colouring glasses when they are present as polysulphides, poly-selenides or polytellurides. Quite small amounts of iron, in the form of sulphoferrite, have considerable effect, much stronger than if the sulphur is present as alkali polysulphide.
colours in glass
It was in 1789 that Klaproth isolated a new element, uranium, from the mineral pitchblende. He obtained sodium uranate as a bright yellow precipitate, which soon came to be used as a colouring agent for glass and glazes.
Uranium can be used with antimony to give a stable yellow in lead-containing glasses. Under reducing conditions uranium compounds give dark green or black glazes, but under oxidizing conditions a good yellow is given in glazes having little or no lead.
Modern Uses for Coloured Glass
In addition to the new uses for coloured glass already mentioned there are almost countless others. Black, alabaster, or coloured glass has recently been used for wash-basins, fire-places and book-cases. Polished glass in the form of tiles has very great durability and cleanliness, and is therefore excellent for use in laboratories, operating-theatres, bathrooms, etc. For sky-signs alone a great demand has been created for domes and tubes in glass of vivid and unusual colours, although the latest advances have tended to take advantage of the colours of electrical discharges in certain gases, and to use these gases in transparent glass tubes.
In artistic glass? both in Britain and France, colour has been gaining in popularity. Success in this sphere has naturally depended upon discretion and good taste, which have fortunately been available.

B. Leaded Glass
Although leaded glass windows do not define a particular type of glass to be used^some examples will be shown on the following pages to show how lead has been used, both functionally and ornamentally.

Both glassmaking and the use of lead have a long history behind them, however the combination of the two apparently did not occur until as late as the 7th century A. D. By the 12th century, "the art of leading had advanced to ease and proficiency."'''0^-''^
The width of the lead strips were rarely over 3/8" and was very pliable and easy to work with, but staybars were frequently used as protection against both the wind and man.10p~7^

20. 1 929
Page 79

TTT ir4
casement c,t_ms rn.oM
OLD LEADED GLASS AND FITTINGS original sketches by s. e. castle

A more recent use of lead in windows has come about with lead playing the major role and the glass used to fill the voids, as can be seen in the photo below, thus the lead is seen in silhoette and forms the design.
This photo is from a front entrance to the S. 0. Merriman residence in Jamestown, N. Y.^P ^

Several examples of leaded glass are found on the pages following.
A window panel, Grand Central Art Galleries, designed by J. Scott Williams, the lead of which
is painted in polychrome

A door panel in a New York apartment. Craftsmanship by G. Owen Bonawit
Panel for a door in a country house. Craftsmanship by G. Owen Bonawit

^TT*^ r-TT-^
,0.-. -.tV, .VJ.-. ,U; i,c-t
* \ 'V^- \- v 1'.
window in the Northern States Insurance Building, Hammond, Ind. Childs & Smith, architects; craftsmanship by G. Owen Bonawit
Half of a window in the Northern States Insurance Building, Hammond, Ind. Childs & Smith, architects; craftsmanship by G. Owen Bonawit
f^m iii i

January, 1932

C. Cast & Moulded Glass
(Including relief and sculptured glass)

In obtaining moulded glass the architect is required to furnish the manufacturer with a drawing or a model from which a mould is built. The glass is then blown or pressed into the mould forming the configurations of the pattern.
The design can either be executed on the front, in "relief," or on the back "intaglio." Intaglio appears to be in relief, however the front is actually smooth for ease of cleaning.
The refracted light effect is obtained by lightly sandblasting the pattern only, and when illuminated from the edge the pattern is lit up without affecting the surrounding clear glass.^lp~53
As some of the following photographs illustrate, moulded glass can be obtained in sheets or small cast units, either in standard patterns or individually designed pieces.
r..-,&C.V.^ wii i i ...- .-------.....-.....i-------

oot^ntialities of some rial pressed glass sug-_se might yield inter-
roiatively slight cosr. 'pressed on one side
other being polished
Left: Designs moulded in relief g translucent glass. Note that only t.. patterns and the reverse are uu. Below: A pair of panels moul:; in relief illustrate .how degree modeling is emphasized by light-*.

SMALL UNITS OF CAST GLASS are combined not only to produce an effective ceiling but also to conceal the source of light in the lobby of 530 Seventh Avenue, New York. Ely Jacques Kahn, architect
i MAY 1932

(Reproduced from an original design by Keppler Relief Class)


i w! i i
m i i ^ r
mmm$m. mm
i .j

"For purely decorative uses, an architectural glass has been developed by the Corning Glass Works. Frederick Carder, for many years art director of the firm, who is not only an eminent artist but one of the world's greatest authorities on glass, has given his attention to the development of what is in reality sculptured glass. Cast in panels and columns of a variety of shapes and sizes, both in solid reliefs and in pierced grilles, the beautiful products of Mr. Carder's art have a wide range of adaptability. They may be built into lightings fixtures, screens, grilles, or wall panels, and can be illuminated either from behind, or, with striking effect from above or below.* p~
See the examples on the two pages that follow.

Relief design on this side of oloss-
Linolite Lamps
Aluminum Coaied "Inside Metal Case
Panel with design moulded in relief, illuminated by lamps at top and sides. Note fhat lamps are placed behind the plane of the glass rather than in the same plane
60 Watt Lumiline
White Mat.________
Reflecting Surface
40 Watt Lumiline Lamps
60 Wall Lumiline \a Highly Pallidal V'
Even distribution of light in ^> pilaster secured by placing a reflecting surface behind Lumiline lamps. Compare with pilasters on facing page. The grille is illuminated by light sources of unequal intensity controlled to accentuate the modeling. Plate glass protects lamps from accumulation of dust

Polished Face
-^-Polished Edges
40 Watt
.^Lumiline Lamp
Design Side
^- Black Background
Top lighting of intaglio design by Lumiline lamp placed on same plane as glass
Polished Face i, Back-
.^40 Watt Lumiline Lamps
Polished_ Edges
Design in this side
Black Back Ground
Edge lighting of intaglio design by Lumiline lamps of equal intensity. Compare with upper panel. Note effect of apparent relief and the manner in which black background causes flat surfaces to be invisible
White Mot Surface Reflecfoi
40 V/att Lumiline Lamp White Mat Surface Reflector
Concealed lighting of pilasters. I. Effective treatment where bold decorative effect is desirable. 2. Even distribution of light through use of curved reflector. 3. Spiral effect in illumination may be secured by placing a reflector behind a series of Lumiline lamps set at an angle > >

Table showing a practical and decorative use of the material. Note the thickness of the glass and the interesting effect of light on the design. Courtesy of Eny-Art, Inc.
I ABORS OF HERCULES. Six pressed glass panels from a set of twelve designed by L Walter Gilbert of England for use in a series of bronze elevator doors. From an international exhibition of contemporary glass and rugs which will be shown in Philadelphia, Chicago, St. Louis, Pittsburgh, Dayton, Cincinnati and Baltimore by the American Federation of Arts.
i 42
the american architect

Pressed glass has been very little used in glazing, its most frequent applications being for jewels, plaques and in leaded combinations, the one exception being railway car deck-lights. For this purpose, being made and annealed in one piece to fit the size of the opening, its generous thickness successfully withstands shock under which ordinary glasses will break. In this connection, quite the most interesting recent development in glass-making is the production by an American firm of "Design Glass," similar to the illustration marked
fit*.* Ai.\ \ I.
Y/ixt:tttV i /* t try r,\ ;fit x tvt< [ '\v r >". i "vr x'-' l-
. a >, If
:txt\ l.;At-rttym t;
unr >:\y /jx > x if r v
>j crvY ;.:vi.' r >r \*
U.i ,,\ \* /V, ijl
Ar, ')()/ -fx>.v 'vn.' >
:y.v ; '/r v v k i/ ;,x x x *;>. \r /*xxrs .;
S v r. Si tfm;
'<<',- k X < V r <>' *// /A\ x x i
rrf 'vn -\ '' ">r--r xv ,"'* *firrxij *> -A > r.'fvv i.ww/ < \r.o*:vv/i if'' St J J/ (.
"Imperial Design."
"Imperial Design." Made first about two years or so ago, for decorative windows in Pullman cars, it opens an interesting field. Being pressed and annealed in one piece, it resists shock and can be readily cleaned, as there are no lead seams to catch dirt. It would seem especially applicable wherever a large number of lights of similar size and design can be used in a building, as in liffht walls and elevator shafts.

D. Engraving & Cutting
"Wheel cut glass in which the pattern is formed by cutting and polishing on emery wheels, is a method of decroating plate glass panels, such as doors or mirrors, where the pattern is made up principally of straight-lines. It is possible to make deep curved incisions with the emery wheel but difficult to manipulate a large piece of glass during the process. The cut is usually V-shaped or U-shaped in section following the shape of the emery wheel'.1-'--'-?""5

A design by Richard Siissmuth combining leaded and engraved amber glass
Moulded glass unit designed by Frederick Carder, executed by Coming Glass Works
RICHARD SUSSMUTH 0/ Pensig, Silesia, a leader in the revival of glass cutting and engraving, uses the glass-cutter's wheel as the painter does his brush. A panel by Siissmuth is shown above. At left is an exquisite decantor designed by Simon Gate of Sweden. Courtesy of E. Schoen, Inc.

"Max Ingrand and his wife Paule are young French artists whose work in glass has received wide recognition. Examples of their skill are to be found all over Europe and a few are shown here. Ingrand has developed a series of new techniques, variations of the wax and hydrofluoric acid method of etching, sand engraving, and direct painting which mark a tremendous advance in the craft. He frequently works on both sides of the glass, thereby obtaining surface as well as color variations. The illustrations show a glass ceiling and an over-door treatment."7p-85

The photo below shows the method of engraving, or brilliant cutting a large plate of glass. The weight of the glass is offset by balance weights suspended from an overhead beam. The glass cutter can then manipulate the plate up to the cutting surface of the wheel.

the manufacture of glass
are not unbreakable, but they are so strong that they survive many of the knocks and blows usually fatal to a drinking glass. When dropped accidentally on the floor they will usually bounce where an ordinary tumbler would break.
Edge-lighted Mirrors
Mirrors are now being used in a brilliant new way, namely with illumination from the edges. This became possible with the development of interior fluorescent neon lighting, by means of which the light is diffused through a whole mirror panel.
The uses of these illuminated mirrors are various. Their attractiveness for decoration is obvious, but they give much service apart from their artistic side. In the way of plain utility one function they fulfil to perfection is to act as shaving mirrors. An edge-lighted mirror can be placed anywhere in the bathroom, for with it dark corners no longer exist. And a mirror of this type has no shadows, the reflection being always uniform over the whole surface and at a maximum as regards quality. One of the earliest applications of this kind of mirror was in theatrical dressing-rooms.
The largest edge-lighted mirror murals ever made were recently installed by the Pittsburg Plate Glass Company in the cocktail lounge of the Penn-McKee Hotel, McKeesport, Pennsylvania (Fig. 64). The central theme of a Greek dancing girl, life size, is executed by sand-blasting on a single piece of polished plate glass over 7 ft high, slightly less in width, and \ in. thick. Edge-lighted by concealed neon fluorescent tubes on all four edges, the panel is supported by hidden brackets that do not permit it to come in contact with the neon tubes.
Flanking this central design are smaller panels, three feet wide, sand-blasted in a leaf design, and also edge-lighted. The entire installation consists of two murals of the same design reversed, placed between three fluted pilasters painted eggshell white. It extends for more than 30 ft along the wall facing the dance floor.'-'"'""
In the lounge all lights are soft and indirect, each mirror receiving its illumination only through its edges, and hence the entire mural appears to be suspended in air with its glowing figures in graceful motion.
Reflection in the murals of the entire cocktail room, with its diners and dancing couples, adds to the effect of spaciousness. These mirrors are admirable for use in the public rooms of

E. Sandblasting (Sand etching)
This process is used quite often in decorating doors, windows, panels, lighting fixtures and mirrors. To produce the etching effect the area to remain smooth is masked off and fine particles of sharp edged sand under compression are forced against the unprotected surface. This procedure results in effects similar to intaglio but is less expensive.
Depth of the cut is advised to be not more than 1/3 the original thickness.

Many types of glass can be cut and decorated by the sandblasting process. Here in a "man from Mars" outfit, a sandblaster is finishing a set of glass letters for Pittsburgh Plate Glass. A sandblast surface is obtained by blowing, with steam or compressed air, a granular abrasive such as sand or carborundum against the glass. Coarse sand pro-
duces a rough ground surface and fine sand a smoother, shallower tone. For super-smoothness a fine carborundum is used. To obtain sandblast designs a blast-resistant cover is placed on the glass and the pattern of the design is cut away, leaving sections of the bare glass open to the attack of the abrasive.

the manufacture of glass
West Virginia, ,161, 166 Westlake machine, 201-2 Westminster, 211
Weyl, Prof. Woldemar, 51-3, 57-8 Wilmore, Dr. A., 238 Window glass, 13, 24, 45, 70, 100-
214-15 Wine-glasses, 22, 110 Wired glass, 141, 218-19 Wisconsin University, 161 Witherite, 37 Wood, Frank, 225
I Worm reduction gear, 85
X-ray examination, 47-8 j glass, 221
Yellow glass, 4, 61-3 York, 207-8 Yorkshire, 175, 249
- Glass Manufacturers Association,
Zinc, 26,39
By the courtesy of The Architectural Review

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