• TABLE OF CONTENTS
HIDE
 Front Cover
 Front Matter
 Title Page
 Frontispiece
 Foreword
 Table of Contents
 Abstract
 Introduction
 Laboratory equipment
 Mold making problems
 Basic ingredients of porcelain
 Preliminary experiments with body...
 Soft feldspathic porcelain...
 Glaze compositions
 Underglaze colors
 Summary
 Bibliography
 Appendix
 Advertising














Title: Development of ceramic compositions suitable for the production of porcelain type artware
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Title: Development of ceramic compositions suitable for the production of porcelain type artware
Series Title: Development of ceramic compositions suitable for the production of porcelain type artware
Physical Description: Book
Creator: Thorngate, Bruce W.
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Table of Contents
    Front Cover
        Front Cover
    Front Matter
        Front Matter
    Title Page
        Page 1
    Frontispiece
        Page 2
    Foreword
        Page 3
    Table of Contents
        Page 4
    Abstract
        Page 5
    Introduction
        Page 6
        Page 7
    Laboratory equipment
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
    Mold making problems
        Page 13
        Page 14
        Page 15
        Page 16
    Basic ingredients of porcelain
        Page 17
        Page 18
        Page 19
        Page 20
        Page 21
    Preliminary experiments with body materials and mixtures
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
        Page 32
        Page 33
        Page 34
        Page 35
    Soft feldspathic porcelain bodies
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
        Page 46
    Glaze compositions
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
    Underglaze colors
        Page 56
        Page 57
        Page 58
        Page 59
        Page 60
        Page 61
        Page 62
        Page 63
    Summary
        Page 64
        Page 65
        Page 66
        Page 67
    Bibliography
        Page 68
    Appendix
        Page 69
        Page 70
        Page 71
        Page 72
    Advertising
        Page 73
        Page 74
Full Text





COLLEGE OF ENGINEERING BULLETIN
UNIVERSITY OF FLORIDA



SEPTEMBER, 1946




DEVELOPMENT OF CERAMIC

COMPOSITIONS SUITABLE

FOR THE PRODUCTION

OF PORCELAIN TYPE

ARTWARE



by
B. W. THOBNGATE
Associate Research Engineer




C-S

9





BULLETIN No. 11.
FLORIDA ENGINEERING AND INDUSTRIAL EXPERIMENT STATION
GAINESVILLE, FLORIDA






The Florida Enginering and Industrial
Experiment Station
The Engineering Experiment Station was first approved by
the Board of Control at its meeting on May 13, 1929. Funds
for the Florida Engineering and Industrial Experiment Station
were appropriated by the Legislature of the State of Florida in
1941. The Station is a Division of the College of Engineering
of the University of Florida under the supervision of the State
Board of Control of Florida. The functions of the Engineering
and Industrial Experiment Station are:
a) To develop the industries of Florida by organizing and
promoting research in those fields of engineering, and the re-
lated sciences, bearing on the industrial welfare of the State.
b) To survey and evaluate the natural resources of the
State that may be susceptible to sound development.
c) To contact with governmental bodies, technical societies,
associations, or industrial organizations in aiding them to solve
their technical problems. Provision is made for these organ-
izations to avail themselves of the facilities of the Engineering
and Industrial Experiment Station on a co-operative financial
basis. It is the basic philosophy of the Station that the indus-
trial progress of Florida can best be furthered by carrying on
research in those fields in which Florida, by virtune of its loca-
tion, climate, and raw materials, has natural advantages.
d) To publish and disseminate information on the results
of experimental and research projects. Two series of pamphlets
are issued: Bulletins covering the results of research and in-
vestigations by staff members; and Technical Papers, reprinting
papers or reports by staff members which have been published
elsewhere.
For copies of Bulletins, Technical Papers or information on
how the Station can be of service, address:
The Florida Engineering and Industrial Experiment Station
College of Engineering
University of Florida
Gainesville, Florida
JOSEPH WEIM, Director









Florida Engineering and Industrial Experiment Station
of the

University of Florida
and

Southern Forest Experiment Station
(Lake City. Fla.. Branch)

Forest Service

United States Department of Agriculture


Bulletin No. 11 Septemlber, 1946




DEVELOPMENT OF CERAMIC

COMPOSITIONS SUITABLE

FOR THE PRODUCTION

OF PORCELAIN TYPE

ARTWARE

by

B. W. THORNGATE
Associate Research Engineer













COLLEGE OF ENGINEERING
UNIVERSITY OF FLORIDA








69P


"I ,









Decorated and Glazed Figurine 8" High, Produced with
FP-23 Body and V-7 Glaze.
'I' I




41

Deccorated and Glazed Figurine 8" High, Produced with
FP-23 Body and V-7i Glaze.

















FOREWORD


Small handicraft industries have a very important patented
value to the State. The opportunity for disabled veterans and
others to establish themselves gainfully is uppermost in the
minds of the staff of the Engineering and Industrial Experiment
Station. The work described in this bulletin had these ob-
jectives.
It is hoped that the information will encourage artistically
inclined individuals to translate their ideas into porcelain objects
of distinction. The opportunity to produce a new Florida de-
sign has not yet been touched.
If these data should encourage one individual to produce a
worth while porcelain object, they will have justified the time,
effort and money expended. The possibilities are there, the
methods of procedure are described but the individual with the
will to achieve the desired end must supply the initiative to
translate the possibilities into actualities.


JOSEPH WEIL, Director






LIST OF ILLUSTRATIONS
Figure Page
Glazed and Decorated Figurine .... .... ............. Frontispiece
1. P otter's Jolly .................................. ........ ............. ........... 9
2. B lunger .. ............................. ........................... 11
3. Ball M ill ................................... ... .. .... 12
4. Large Drying Oven .................................. .. .............. 14
5. Spray Booth ................................ ... ........ ......................... .. 15
6. Modulus of Rupture Apparatus .. ............................ 17
7a. Electrically Fired K iln .............................................................. 21
7b. P laster M old ............................... ................. ...... ...... ........... 25
8. Interior of Gas Fired Kiln ................................. ............. 26
9. Photom eter .............................. .... ............. ........ .... 29
10. Photomicrograph of Section of FP-10 Body ........................... 33
11. Triaxial Diagram -.-.... ........................... .----- .. ........ 35
12a. Unfired Figurine of Original Model ......... ....... ......... 38
12b. Incised Vase ................................. ........ ............ 44
13. Applying Underglaze Colors to Biscuit Ware 53
TABLE OF CONTENTS Page
Frontispiece .. ...................................................... .......... ........... 2
F orew ord .................................................................... 3
Abstract ..................... ...................................... ..................... 5
Introduction ................................ ...... ..... ...................... ...................... 6
Laboratory Equipm ent ............................... ............................ ... .... ... 8
M old M making Problem s .................... .................................................... .. 13
Parting Compounds ...................... ..................... ....... 14
Pinhole Defects .............................................. .. ..... ......... 16
Basic Ingredients of Porcelain ................. .......................... .. 17
Principle of Body Composition ..........................2................ 21
Preliminary Experiments with Body Materials and Mixtures ....... 22
Method of Body Preparation ..............2....................... .... .................. 24
Bodies Containing Mineralizing Preparations ......... ... ..... 28
Measurement of Translucency ......................... .. ................ 32
Petrographic Examination of Samples .... ................ 34
Soft Feldspathic Porcelain Bodies ......................... ....................... 36
Method of Compounding Bodies .............................. .......... 36
Cracking Defects ................................ ..... ....... .... ..... 39
Effect of W ater Im purities ................................. ..... ....... .................. 39
Investigation of Body Cracks .......................... ................. ..... 40
Group I Body Trials ..................... ... .......................... .......... 41
Factors Affecting Castability .............. .................... ............... 42
Group II Body Trials ........................... .... .. ......... ... ... 45
Limits of Body Composition ................................ ................. ...... ..... 48
Group III Body Trials ................. .......................... 46
Glaze Com positions ..................................................................... ........................ 47
General Discussion .................... ........................................ ..... 47
Principle of the Glaze Formula ...................... ............... ....... 48
Complexities of Composition ............................. ................ .... 50
Method of Glaze Preparation ................. .................. .............. .... 51
Glaze Compositions and Their Fired Characteristics ...... ........... 52
Underglaze Colors ..................... ............................... ................ 56
Nature of Underglaze Colors .......... ............................... ...... .. 57
Experimental Work with Fluxes, Colors and Vehicles ........................ 59
Useful Fluxes for Underglaze Colors ........................................... .... 60
Final Evaluation of Color-Flux Mixtures ............... .............. 60
Vehicles for Underglaze Colors .................... ............ ........... 61
Summary ........................................................ ................. ....... ....... .... 64
Acknowledgments ........................ ................ ....... ... ... .... ...... 67
Bibliography and References ........ .............................. .. ...................... ... 68
Appendix ....................... ................ 69





DEVELOPMENT OF CERAMIC COMPOSITIONS SUITABLE
FOR THE PRODUCTION OF PORCELAIN
TYPE ARTWARE

ABSTRACT
The process of "casting" is a suitable method for the pro-
duction of ceramic artware of the porcelain type. Ware may
be produced successfully with a minimum of equipment; the
principal prerequisite is a kiln which is designed for tempera-
tures of 24000F or above. Other necessary equipment is de-
scribed and illustrated.
The quality of the ware is influenced by the character of the
plaster molds. The defects generally encountered and the
methods of overcoming them are discussed.
The use of various mineralizers to induce the formation of
mullite crystals in an experimental porcelain body was attempted.
Lepidolite appears to exert a favorable effect.
The cause of cracking in cast ware was investigated. The
results of the trials indicate that the impurities in ordinary tap
water may be responsible for the defect.
A series of body mixtures was made and a satisfactory
porcelain body evolved. The body is of the "soft" type and
matures at 2380 F (cone 10). Glaze compositions were also
tested and a suitable composition was developed. The glaze
did not craze or peel and it is properly balanced for use with
underglaze colors. The glaze matures at 22000F (cone 5).
Figurines were produced by casting in molds made from a
commercial model and from an original model. Each figurine
was formed in a seven piece plaster mold. Other items such as
vases and animal shapes were produced with the same body
and glaze compositions.
The ware was hand painted with underglaze colors which
were properly adjusted for use on the vitreous body. A variety
of commercial underglaze colors was utilized for'the hand painted
decorations. These colors were suitably fluxed. Spirits of tur-
pentine and polymerized turpentine ("fat oil") were used as
vehicles with the underglaze colors.






INTRODUCTION


Any attempt to provide the word porcelain with an inclusive
definition is difficult and subject to compromise. The common
conception of porcelain is, that it is a fine, white, translucent
type of glazed ware which has been fired to high temperatures.
In general, such a definition is entirely adequate. However,
there are several types of porcelain which in turn are subdivided
into a variety of compositions with special characteristics.
The porcelains are classified broadly as belonging to one of
two generic types of translucent wares. These two principal
types are the hard and soft porcelains; the terms indicating the
temperatures to which the products are fired. The hard porce-
lains are fired at relatively high temperatures, the soft porce-
lains at lower temperatures. Technically, the hard porcelains
are once fired, that is, the body and glaze mature simultaneously
at the same temperature. This type of porcelain was first
developed by the Chinese at an early date and the other porce-
lains are derivations of it.
The early soft porcelains were the result of the European
potters' attempts to imitate the Chinese ware without having
sufficient knowledge of its composition. These porcelains are
really complex glasses and have little similarity to more recently
developed soft porcelains. As time passed, further develop-
ments in body compositions resulted in the production of very
fine soft porcelains such as English bone china, Belleck ware
and the porcelaine nouvelle of Sevres.
Between the hard and soft porcelain types, there is a variety
which is characteristically American as regards composition. It
has become known as true china, American hotel china or vitri-
fied china. In some respects it approaches a modified form of
hard porcelain, although it is produced by the same methods used
in the manufacture of the soft porcelains.
In the production of true china, which may be considered a
special type of porcelain, the body is fired to a temperature suffi-
cient to develop impermeability and translucency. The fired
ware subsequently is glazed and receives a glaze firing at a
temperature considerably lower than that of the biscuit or body.
The early Chinese potters frequently adopted the same pro-
cedure in the course of making some of their finest porcelains.
It is interesting to note the comments of Burton (5) with refer-






ence to this practice. He states ". . many of these glazes, espe-
cially the turquoise, purple, antimony-yellow and clear iron-
yellow afford indubitable proof that the Chinese first fired their
porcelain to the biscuit condition, and then glazed it at a lower
temperature, whenever it suited their purposes so to do."
Although the technique of making the various kinds of
porcelain is different, all types have two essential characteris-
tics in common; namely, translucent and vitreous bodies. Con-
sequently, the experimental work described in this bulletin is
based upon the premise that a ceramic body containing certain
raw materials which exhibits the properties of translucency
and vitreousness after firing, regardless of the temperature
employed for firing the glaze, may be classified as porcelain.
In the following pages the term porcelain has been used with
this interpretation.
The primary objective of this bulletin is to describe the de-
velopment of a porcelain type body and glaze which will serve
as a basis for mixtures intended for use in commercial produc-
tion. The nature of the experimental data has been influenced
by several major factors. Most of these factors have an im-
portant relationship to the problems often associated with pro-
duction in a small factory or workshop engaged in porcelain
manufacture. Some of the factors recognized as a basis for
the experimental data are:
1. The choice of production methods which require a minimum
of equipment.
2. The establishment of moderate fuel costs by employing tem-
peratures under 2400 F.
3. The employment of fabricating techniques most likely to
incur minimum losses.
4. The development of a reliable body mixture which can be
manipulated by unskilled labor.
5. The introduction of a palette of underglaze colors.
6. The application of glaze by the spray gun method so that
the preparation of a minimum amount of the material would
be feasible.
7. The use of a portable type of muffle kiln, suitable for firing
small quantities of ware at frequent intervals.
Since the experimental data furnish technical information
relating to the manufacture of porcelain artware, it is hoped
7






that the presentation of these data will create a wide interest
in the unusual opportunities which the production of fine cera-
mic articles offers in the State of Florida.
LABORATORY EQUIPMENT
The principal pieces of equipment utilized to carry on the
experimental work in the laboratory consisted of the following
items:
1. A variable speedy jolly, Figure 1, sometimes called the
potter's wheel. This equipment has a circular head attached to
a vertical shaft which rotates by means of a device driven by
an electric motor. A rheostat is used to regulate the speed of
the head. The jolly is used to make plaster models designed
to be shaped in the round, as in the case of a bowl, and to
form articles of plastic clay bodies.
2. A blunger, Figure 2, also called a bucket mixer. The
blunger is used to prepare relatively small quantities of ceramic
materials with water. The mixer arms are attached to a vertical
shaft and may be raised to facilitate placing the charged bucket
in position. The shaft is motor driven and the arms rotate
approximately 85 revolutions per minute.
3. Ball mills, Figure 3, used to grind body mixtures, glazes
and colors. The cylindrical jar, of 11/4 gallon capacity, is sup-
plied with a cover which is locked in place by means of a metal
cross-bar and a neckband with loops. The jar carries a charge
of flint pebbles or porcelain balls ranging in size from 1/2 inch
to 3/4 inch in diameter. The cradle of the mill is provided with
brass straps which hold the jar in place as it rotates horizontally
at the rate of approximately 70 revolutions per minute. The
pebbles or balls in the revolving jar provide a grinding action
not obtainable with the blunger. The particles of the ingredients
in the mill jar are reduced in size and at the same time more
uniformly distributed.
4. Two sizes of drying ovens, Figure 4. These are used for
drying the unfired samples and plaster molds. The smaller
dryer, about 12"x12"x15", is a constant temperature appliance,
electrically heated. The larger dryer also is heated electrically
and thermostatically controlled and has an air circulating sys-
tem. A pressure type of pyrometer is used for recording the
temperatures. The dimensions of the dryer chamber are ap-
proximately 4'x5'x71/2'.







































































Fig. 1.--Variable Speed Jolley Showing Operator with a Plaster
Model of a 5" Bowl.


'Y;L"r
'C~r:








r.

; 6i-
:






5. An exhausting spray booth, Figure 5. The exhausting
chamber of the booth measures 36"x38"x48". The front is
open to permit the operator to apply glaze to ware with a spray
gun. The biscuit item, supported by a banding wheel, is placed
at about the center of chamber. The dust from the atomized
glaze is drawn out of the rear of the chamber by an exhaust fan.
6. A motor driven compressor. The air used in the glaze
spraying operations is supplied by this equipment. A 1/ H.P.
electric motor operates the compressor. The air is pumped by
the compressor to a tank equipped with a safety valve, drain
cock and pressure gauge. The two cylinder head type of com-
pressor displaces approximately 7.5 cubic feet per minute.
7. An adjustable spray gun. The gun is utilized to apply the
fluid glaze to biscuit ware and is a light weight, suction feed
type of gun with a valve to adjust the width of the spray. In
order to obtain uniform air pressure from the tank a light duty
transformer was installed.
8. A metal banding wheeL This item is made of a spindle
6 inches long attached vertically to a suitable base. At the top
of the spindle there is a flat circular head 6 inches in diameter,
held in position by a sleeve which fits over the spindle. The
wheel head is rotated by hand and generally is useful when
glazing or decorating ware.
9. A modulus of rupture apparatus, Figure 6. The apparatus
is used to determine the transverse strength of clays and body
mixtures. The sample is made into a bar about 6 inches long
with a 3/4 inch square cross section and placed horizontally on
two triangular knife-edge supports which are parallel and spaced
4 to 5 inches apart. The dry bar is broken by means of pressure
applied to a third triangular rod placed on top of the test piece.
The load is gradually increased until the sample breaks. The
number of pounds required to rupture the bar and the width
and depth of the bar at the break are recorded. The modulus
of rupture of the sample is then calculated. The formula used
to determine the traverse strength of the sample, in pounds
per square inch, is as follows:
3 PI
M-=
2 bd'
where P equals the pounds required to break the sample, 1, the
distance in inches between the knife edge supports, and b and d,
the width and depth of the test bar respectively in inches.
10






10. Plaster of Paris bats. Many of the body mixtures made
during the experimental period were converted from the slip
state to a plastic mass by the use of plaster bats. The bats
are made of potter's plaster and are circular forms 16 inches
in diameter and 5 inches in height. A concave depression in
the bat, 12 inches in diameter and 21/ inches deep, provides the
cavity which holds the clay mixture.
11. An electric kiln, Figure 7a. The furnace is utilized pri-
marily for firing small test pieces. It is equipped with 4 rod
shaped heating elements and is intended for temperatures up
to 25500F. The inside measurements of the furnace muffle are
71/2"x71/"x10 %3/". The transformer of the furnace has primary
connections for a 115 or 230 volt, 60 cycle power supply. The
furnace is also equipped with a pyrometer. The voltage is
altered by using various combination settings on the control
switch board.


Fig. 2.-Blunger Used for Preparation of Body Mixtures.
Capacity of Bucket, 5 gal.






12. A portable gas-fired kiln, Figure 7b. This equipment is
used to fire samples which are too large to be placed in the
electric furnace. A greater number of items can be accommo-
dated in the gas-fired kiln. The kiln is equipped with two burn-
ers and has a muffle constructed with multiple tubes. The inside
dimensions of the muffle are 141/2"x18"x22". The combustion
chamber, floor and floor supports are made of sillimanite rl-
fractories. Temperatures up to 25000F are obtainable with this
kiln. The fuel utilized in firing the kiln is butane. The kiln
as modified in the laboratory is equipped with a manometer to
indicate the pressure of the gas in the burner valves, in inches
of water. The burners are the Venturi inspirating type, and
are fitted with needle valves so that a uniform adjustment of the
gas pressure may be obtained. A special unit of two metal
strips, % of an inch wide and 1/16 of an inch thick which bisect
each other perpendicularly, is used in each burner nozzle to
stabilize the flame. The gas orifice of the burner is 9/64 of an
inch in diameter. An emergency shut-off valve, a gauge and a
pressure reducing and regulating valve are attached to the main


Fig. 3.-Ball Mill Equipment. Large Jar (1% gal.) for Grinding Bodies
and Glazes, Small Jar (1 qt.) for Underglaze Color Preparatiun
12






supply line, near the rear of the kiln. Other items of kiln
equipment are a pyrometer and a rare-metal thermocouple. The
thermocouple projects into the muffle about two inches through
an aperture in the rear of the kiln. During the firing operation
it is used primarily to ascertain the rate at which the tempera-
ture inside the muffle increases. This instrument is supple-
mented by pyrometeric cones, placed advantageously on the
shelves of the muffle, and are observed through small openings
in the kiln door. When the desired temperature is reached these
cones change their upright position by bending. Fire-clay
shelves and posts of various heights also are part of the kiln
equipment.
In addition to the items described above, other accessories
such as sieves, brushes, scales, bowls, and an electric hot plate
are required.
MOLD MAKING PROBLEMS
With a few exceptions, all laboratory test pieces made with
porcelain body mixtures were formed by the method known
as "casting". In this process, molds prepared from potter's
plaster are filled with a viscous mixture called "slip" which is
composed of the body ingredients and water. The porous walls
of the mold absorb water from the slip and the body mixture
forms a coating on the interior of the mold. The coating inside
the mold is then allowed to dry until it has hardened sufficiently
to allow removal of the mold sections. An illustration of cast
ware with several of the mold sections removed is shown in
Figure 8.
Plaster of Paris. Potter's plaster is a grade of plaster of
Paris which has been prepared specially to meet the require-
ments of the pottery industry for forming and shaping ceramic
products. The properties of the plaster allow the porosity and
strength of the material to be varied to suit the conditions of
the user and these characteristics are often utilized to advantage.
This is accomplished by varying the water-plaster ratio of the
plaster mixture.
The usual proportions of plaster and water used in making a
mold intended for casting are 5 parts of dry plaster by weight
and 4 parts of water by weight. The plaster model from which
the working molds are formed must be treated with a parting
compound before the fluid plaster is poured over it. This com-
pound prevents the newly formed mold from adhering to the







model. Soft soap is a substance commonly used as a parting
compound and is termed, "size". Frequently a different type of
separating medium, such as those described below, is applied to
the plaster model before the soft soap is utilized.
Parting Compounds. At the start of the experimental work
some difficulty was encountered with a one-piece mold which
failed to release properly from the model. A series of experi-
ments were made whereby various parting compounds were ap-
plied to rectangular slabs of set plaster and allowed to stand for
24 hours. The slabs were then given an application of soft soap
and liquid plaster poured on the treated surface of the plaster
slabs. The relative ease with which the two sections of the
individual plaster samples separated was noted.
As a result of these experiments three mixtures, Nos. 5, 9
and 10 were chosen as the most satisfactory. The No. 9 mix-
ture is composed of 100 grams of No. 3 cup grease and 500 cc.
of kerosene. The No. 10 stearic acid solution is similar to a


Fig. 4.-Large Drying Oven Utilized to Dry Plaster Molds and Unfired
Ware. Oven Chamber Dimensions, 4'x5'x7%'.





mixture frequently used in various fields of plaster work and
has the following composition:
Stearic acid ................................ 28 grams
Kerosene .................................... 118 cc.
Aerosol O. T. 100 per cent ........ 7 grams
To prepare the No. 10 parting compound, the three ingredients
are mixed and heated to the boiling point with constant stirring.
Proper precautions must be taken during the preparation as this
mixture is inflammable. The solution is applied to the plaster
with a soft brush while warm.
The No. 5 mixture, a saturated solution of toluol and paraffin,
has been utilized with excellent results as a basic separating
compound on plaster models used to make working molds. The
models were given one application of the No. 5 solution after
which the regular size was applied by the usual technique.


p..
--


iI


Fig. 5.-Spray Booth Showing Transformer and Spray Gun at the Right.
Model of Deer 6" high, Rests on Banding Wheel.






Pinhole Defects. Excessive pinholing is a common difficulty
with working molds. One of the most important requirements
of a satisfactory pottery mold is an interior surface free from
pinholes. Wiss, Camp, and Ladoo (16), in discussing some of
the troubles encountered in the use of plaster, make the follow-
ing comments regarding the problem of pinholes:
"Plaster, being an extremely fine powder, normally carries
air around the surface of each particle. In addition, it is evi-
dent that the water removed from the gypsum particles in
calcination has been displaced by air; when this finely-powdered
plaster is added to excesses of water, the air around each particle,
as well as the air within the particle will have a tendency to
flow together into small bubbles of visible dimensions in the mix.
These bubbles, if they are not removed during the blending
or the mixing process, remain more or less uniformly distributed
throughout the mass of the mold . ."
Experiments were conducted to determine the cause of the
excessive number of pinholes in the working molds made in the
laboratory. Molds showed the least amount of pinholes when
the tap water was allowed to stand several hours after being
drawn, when distilled water was used and when the powdered
plaster was permitted to blend or soak in the water from 4 to
5 minutes and the mixture then carefully stirred for 4 to 6
minutes. The bubbles which came to the top of the plaster as
scum were removed by skimming.
The conclusion reached after the plaster trials were com-
pleted, indicate that extreme care is required in both the blend-
ing and mixing operations with hand-mixed plaster in order
to obtain maximum freedom from pinholes.
Numerous other defects may appear in set plaster mixtures.
Consequently, in an effort to avoid making unsatisfactory cast-
ings, the following factors are recognized as essential to the
production of satisfactory molds:
1. Ratio of dry plaster and water
2. Blending and mixing time of the material
3. Correct mixing procedure
4. Fluidity of the mixture and the time of pouring
5. Strength of the set casting
6. Absence of pinholes
7. Proper temperatures for drying the molds






At one period of the mold making experiments the time re-
quired to bring the plaster-water mixture to a creamy con-
sistency was far in excess of that normally experienced. The
reason for this condition was not determined although the
causes usually attributed to the retarded setting of plaster were
investigated. The failure of the fluid plaster to become viscous
within the normal stirring time was corrected by increasing the
solubility of the plaster. This was effected by treating the
water with 0.25 to 0.50% of dilute hydrochloric acid (1 HCl-
1 H20) before the dry plaster was added.
BASIC INGREDIENTS OF PORCELAIN
Porcelain bodies and numerous other whiteware mixtures
usually contain three basic ingredients; namely, clay, flint and
feldspar. Reference to these materials is made later to indicate
clearly the function of each in the body mixture.


-4

minL*


Fig. 6.-Modulus of Rupture Apparatus. A Simple Device Construct-
ed in the Laboratory for Determining the Relative Transverse Strength
of Clays and Body Mixtures.


C.
r:T






Composltion of Clay. Clay is the weathered product of sili-
cate rocks and consists of a mixture of indefinite composition,
composed of various mineral fragments of different sizes The
physical, chemical and mineralogical properties of individual
clays vary greatly and those chiefly used in ceramic products
belong to that class of minerals known as alumino-silicic acids.
The kaolins, china clays and ball clays appear to consist essen-
tially of one or more of these alumino-silicic acids and therefore
are expressed chemically by the formula, A12Os. 2 Si02. 2 HtO.
Properties of Clay. When clays are used for ceramic pur-
poses, the plasticity, shrinkage, transverse strength, degree of
hardness after firing and the fired color properties of the material
are of special importance.
Plasticity. Plasticity is that property typical of clays, which
permits them to be molded into definite shapes by pressure after
they have been mixed with a suitable quantity of water. Clays
which require relatively large amounts of water to make them
workable generally are the most plastic. Thus the water of
plasticity of ball clays usually ranges between 35 to 60%
whereas some other types of clays whose potential plasticity
is low, require a smaller percentage of water for workability.
Shrinkage. Plastic clays and clays rich in colloidal matter
exhibit a greater degree of shrinkage than do those with low
plasticity. This contraction is due to the evaporation of water
during drying and fusion during firing and the resultant move-
ment of the clay particles towards one another. If the shrink-
age does not proceed uniformly throughout the mass, internal
strains are set up which cause the clay to rupture or become
distorted. The size of the clay particles and the rate of drying
are also factors which influence shrinkage.
The hazard of removing the shrinkage water from clay may
be minimized by several methods, the most common being the
addition of non-plastic materials to the clay substance. Typical
non-plastic materials, utilized for this purpose are flint, feldspar
and calcined clay.
Transverse Strength. The strength of a clay when in the
dried condition often is important. Since the amount of clay in
a porcelain body seldom exceeds 50%of the total ingredients,
the dry strength of the body is dependent upon the proper choice
of the clays used. Obviously the dried ware must be sufficiently
strong to withstand handing during the various stages of pro-






duction. When ready for the kiln an article also must be strong
enough to support the weight of other pieces which may be
placed upon it. The necessary equipment and method of de-
termining the modulous of rupture of clays and bodies previously
have been described.
Fired Hardness. When clay is fired to temperatures above
10000F. physical changes occur in the material as a result of the
expulsion of the chemically combined water and the gradual
vitrification of the fusible constituents. The temperature re-
quired to produce vitrification is dependent upon the type of
clay and the fluxing impurities present. This reaction may begin
as low as 17400F. The degree of hardness of a sample of clay
after it has been fired is determined, largely, by the extent to
which it is vitrified. An example of this is shown by a sample
of Florida clay which was submitted for testing. After a test
bar of the clay had been fired to approximately 1650F the
specimen could be scratched with a knife. The test bar had
low transverse strength and when the apparent porosity was
determined it was found to have 27.3' absorption. Another
sample was fired to 2380cF and a dense vitrified specimen re-
sulted. The strength of this bar was much higher than that of
the previous one and a diamond pointed tool was required to
scratch the surface. Due to the high vitrification of the speci-
men the absorption was reduced to 3'%.
Color of Clays. The color of unfired clay seldom indicates
the nature of the color after firing. Most china clays and kaolins
are light colored in their natural state and usually fire to a white
or cream color dependent on the proportion of iron oxide and
titanium oxide present. The ball clays exhibit a variety of
colors in their natural condition as a result of the presence of
carbonaceous matter.
Flint. Quartz (SiO2) in the form of potters' flint is a non-
plastic material which is an important ingredient in porcelain
bodies. When added to the unfired mixture it reduces the
plasticity and workability of a body and it is an aid in reducing
the drying shrinkage and increasing the rate of drying.
In the fired ware flint lowers the fired shrinkage, reduces the
tendency to deform or warp and improves whiteness. Also it
is a factor in regulating the volume changes in the body effected
by heat during the firing operation.





Feldspar. The feldspar group of minerals include several
types but those most extensively used in white wares are the
potash and soda varieties. Potash feldspar has been found to
be more satisfactory than soda feldspar in body mixtures largely
due to a longer firing range. Although several investigations
in the use of soda feldspar in whiteware mixtures have been
made, a recent study of this problem (10) indicates that further
experimental work under factory conditions is desirable.
Potash feldspar also forms a lower melting mixture when a
small percentage of lime is added as an auxiliary flux. This
action may be used to advantage in lowering the softening
range of the body. (See page 23.)
The addition of lime to soda feldspar does not give similar
results. Whereas the fusibility of potash feldspar is noticeably
increased by such additions, soda feldspar melts more readily
when free lime is absent. The higher viscosity of the glassy
matrix formed by fused potash feldspar reduces the tendency
of the body to deform during the firing and therefore the use
of potash feldspar in a body is especially desirable. Frequently
however, a mixture of potash and soda feldspars may be utilized
with satisfactory results.
The minerals of the feldspar group are not found pure in
nature. They usually are obtained from pegmatites, a variety
of granite composed essentially of feldspar, quartz and mica.
An analysis of a typical potash feldspar of commercial grade
is given below:
Chemical Compound Per Cent
Silica .................................................... 65.60
A lum ina .............................................. 18.60
Iron oxide ............................................ 0.05
Calcium oxide .................................... 0.10
Magnesium oxide ............................ Trace
Sodium oxide ...................................... 2.30
Potassium oxide ................................ 13.00
Loss ................................. ............... .. .30

The potassium oxide content of a feldspar should be at least
9% in order to classify the material as belonging to the potash
group. The formula for a theoretically pure potash feldspar is
KO Al20s. 6 SiO...
Principle of Body Composition: If a body mixture is com-
pounded of kaolins only, the fired product usually possesses
good white color and is quite porous even at high temperatures.




















































Fig. 7a.-Electrically Fired Kiln Used to Test Small Porcelain Samples.
Inside Dimensions of Muffle, 712"x71~"x0I4".


JA



"A






The vitrification of the mixture may be accelerated by the addi-
tion of ball clay, but if the percentage of ball clay added does
not fall within certain limits the fired color will be off-white.
Another objectionable feature of such a mixture is that of ex-
cessive shrinkage in drying. Therefore several steps are neces-
sary to counteract these results. First, the ball clay and kaolin
ratio is adjusted so that the fired color is not objectionable.
Second, a non-plastic is added to the mixture in the form of
flint which modifies the high shrinkage of the clays and pro-
motes an improvement in the color. Third, a fusible material
is incorporated into the mixture in order to provide a glassy
matrix for the other materials and to develop the property of
translucency. Thus, feldspar, sometimes augmented by small
amounts of other fluxes, becomes an important ingredient of
the body composition.
When the proportions of these various ingredients are
properly balanced and the body fired to a temperature consistent
with the fusibility range of the mixture, a hard, white and
translucent sample of porcelain is obtained. However, in some
special types of porcelain mixtures there are numerous varia-
tions of the above principle.

EXPERIMENTS WITH BODY MATERIALS AND MIXTURES
Tests of Body Materials. An important principle of ceramic
body compositions requires a knowledge of the physical prop-
erties of the raw materials used. Therefore, tests were made to
furnish information pertaining to some of the fired characteris-
tics of the body ingredients. The results obtained from the kao-
lin type clays fired to 23800F (cone 10) are indicated in the
following data:
Per Cent
Fired Total Linear
Name and Type Clay Color Shrinkage
H & G China clay ....................... chalk white .............................. 9.0
M & M China clay ..................... light cream .......................... 13.0
No. 27 Georgia kaolin ................. cream white ............................ 8.5
Dawson kaolin .......................... ...cream white ......................... 12.0
Aetna washed kaolin ....................light cream .............................. 16.0
Pioneer kaolin ................................ light cream .............................. 14.0
Florida kaolin ........... ........... light cream ........................ 22.5

Ball clays were mixed with 50'; of flint to facilitate removal
of the pressed bars from the molds and to counteract the tend-
ency of the clay to warp during drying and firing. The test






bars were fired to cone 10 and the total linear shrinkages and
fired colors evaluated as follows:
Per Cent Total
Name and Type Clay Linear Shrinkage Fired Color
O. M. No. 4 ball clay .................. 11 ................ .. light cream
Bell's dark ball clay .................. 9 ............ cream white
Victoria ball clay ........................ 10 .. ........ light cream
H & G black ball clay ....... 10 .. ... ......... light cream
Enfield ball clay .................... ....... 12 ........... .. ..... ... gray cream
H & G light ball clay .................... 4 .................. .... deep ivory
Feldspar was tested for fusibility at cone 6. A test piece
was made in the form of a cone which had a height of 25 mm.
and a base diameter of 40 mm. when dry. After firing and
cooling the test cone was sufficiently vitreous to be impervious
to water. It exhibited a faint pinkish-gray color and after mea-
surement revealed that there had been a decrease in linear
height. Several cones of the same dimensions composed of feld-
spar and other ingredients, were fired simultaneously with the
feldspar cone at the same temperature. The purpose of these
experiments was to determine the deformation characteristics
of the mixtures at a given temperature. The data for the fired
cone mixtures are shown below:
Per Cent Per Cent
Trial Per Cent Whiting Per Cent Linear Height
No. Feldspar (CaCO,) Flint Decrease
1 100 .... .... 6.4
2 88 12 .... 18.0
3 02 8 .... 29.4
4 94 6 .... 34.7
5 75 15 10 11.5
6 78 12 10 21.6
7 82 8 10 17.3
From the above tabulation it will be noted that sample
No. 4 with 94'/ of feldspar and 67 of whiting has the highest
slump value of the group. However, if the whiting is decreased
to approximately 3'. of the mixture and the potash feldspar
raised to 97 7,, a eutectic is produced at the relatively low tem-
perature of approximately 2000F. (3). This fact is frequently
taken advantage of to mature a body at a lower temperature
without appreciably increasing the percentage of fluxes.
When a comparison of the No. 1 feldspar cone and the No. 4
mixture was made, the difference in appearance immediately
was evident. The latter mixture appeared as a smooth, milk-
white, opaque cone and had been converted to a glass-like mass
due to the fluxing action of the whiting.






The fineness of the flint was tested by screening samples
mixed with water through a 200 mesh sieve. Acceptable flint
stock leaves no residue on this screen. The color and refractory
properties were determined by firing a smkll quantity of the
material to cone 10. The fired sample should show a chalk
white color and should be crushed quite readily to the powdered
form with the fingers.
Method of Body Preparation: In general the porcelain body
mixtures formulated in the laboratory were prepared as follows:
The proper amount of ball clay required for the desired compo-
sition was added to an excess of water. The clay-water mixture
was then blunged mechanically for 1 to 2 hours. The mixture
was subsequently lawned (screened) through a 200 mesh sieve
to remove impurities such as lignite and sand. After the lawn-
ing operation was completed the other ingredients of the body
mix were added to the ball clay-water slip and the mixture
stirred in the blunger for another hour. The fluidity of the
slip was adjusted by the addition of water until a mixture of
creamy consistency was obtained.
After the completion of the last blunging operation the
mixture again was lawned through a 200 mesh sieve, then
placed in a laboratory ball mill and ground for an hour or more.
The exact grinding time depended upon the special conditions
attending the individual mixture which was to be tested. The
time factor for grinding is specified in later discussions of the
various body compositions.
After the grinding was completed the slip in the mill jar
was emptied into a plaster bat. When a magnetic separator
is available the slip should be passed through it to remove any
metallic iron. The mixture was allowed to remain in the bat
until a portion of the slip water had been absorbed and a plastic
mass formed. The plastic mixture usually contained 20 to 25
per cent water and corresponded to the "filter cake" prepared
in pottery plants by the use of the filter press. The "filter cake"
from the bat then was stored, until required.
The casting slip was prepared in the following manner: The
percentage of moisture in the "filter cake" was ascertained and
small pieces of the cake were then added to a known quantity
of water in a container. When this mixture was ready to be
placed in the blunger to blend the clay and water into a slip,
deflocculating agents were added. The mixture was agitated






from 1 to 5 hours and acquired a uniform, creamy consistency.
A deflocculant is a substance used to make the slip fluid even
though there is relatively little water present. (For an example,
see page 42.) A small percentage of water in the slip is de-
sirable as a means of prolonging the life of molds and in reduc-
ing the time otherwise required for casting.
Body Mixtures and Trials. Several bodies were formulated
and tested at the beginning of the experiments dealing with
body compositions. Two mixtures were chosen from the group
as having the greatest promise as the basis for further experi-
mentation. The compositions of the mixtures were as follows:
Ingredient Body FP-1 Body FP-3
Per Cent Per Cent
English China clay ......... .. .... ........ 17.0 25.0
Florida kaolin ............ ................ ... 5.0 ......
Calcined Florida kaolin .................... 3.0
BDK ball clay ...................................... 10.0 10.0
Potash feldspar ................................ 35.0 22.0
Flint ................................. .................... 30.0 35.0
Calcined alum ina ................................ ...... 5.0
Flux N o. 12 ..................................... .. .... 3.0

Flux No. 12 is a mixture of 1 part of zinc oxide and 2 parts
of strontium carbonate, by weight. The materials were ground
wet in the ball mill and then thoroughly dried.
In the preparation of the FP-1 mixture, 1000 grams of the
dry ingredients were added to 750 cc. of water and the body
blunged and lawned. Subsequently, the mixture was ball milled
for 2 hours. Casting slip was prepared from the plastic mass
















Fig. 7b.-Plaster Mold with Several Sections Removed from a
Figurine 9" High.
25






or "filter cake" of this mixture, water and "N" brand sodium
silicate. The amount of the deflocculating agent, sodium sili-
cate, usually required for the various experimental bodies, ranges
between 0.1 7 and 0.5 of the dry mixture. For the FP-1 body
0.25% of deflocculant was used.
FP-3 mixture was prepared in the same manner as the FP-1
body, except that 1000 grams of the dry mixture were added to
700 cc. of water.


V
L*^


Fig. 8.-View Inside Muffle of Gas Fired Kiln Showing Biscuit Ware.
Inside Dimensions of Muffle, 14%"x18"x22".
26





In these experiments test pieces of all the experimental
bodies were made by pouring casting slip into small bowl molds.
Larger items also were cast with some of the body mixtures
and are discussed later.
Data pertaining to the above bodies fired to 2380F (cone 10)
are shown in the table below:
TABLE I
Data on Fired Properties of Initial Body Mixtures
Per Cent
Body Wall Trans- Color Total Linear
Thickness lucency Shrinkage
FP-1 3 mm. fair cream white 11.5
FP-3 3 mm. fair chalk white 12.0
The biscuit samples of FP-1 have a dull, smooth surface and
exhibit a fair degree of translucency. The ware is slightly
cream in color. The fired body of the FP-3 mixture has a texture
which is semi-glassy and consequently has a greasy appearance.
Use of Mineralizers. Certain compounds termed mineralizers
sometimes are incorporated into ceramic mixtures as a means
of assisting specific ingredients in the mixtures to convert from
one form to another while under the influence of high tempera-
tures. Some of the compounds known to be active as mineral-
izers are beryl, lime, sodium tungstate, lepidolite and some of
the borates and phosphates. Luks (11) states that beryl aids
the mechanical strength of porcelain and believes that this
result no doubt is due to beryllium oxide's strong mineralizing
characteristics. Other published experimental work (6) reports
on the effect of beryllium oxide upon the formation of mullite
and presents data to show that it is the best of several oxides
in that respect. Betz (1) found that when lepidolite was added
in small amounts to ceramic bodies, it was an effective mineral-
izer which helps promote rapid solution and subsequent re-
crystallization.
Further analysis of this type of action in ceramic mixtures
suggests that the presence of a mineralizer increased the crys-
tallization capacity of some substances, which are capable of
crystallizing when conditions, influenced by such factors as
temperature and viscosity, are favorable. Therefore, it was
decided to investigate the use of mineralizing substances which
might aid in the formation of mullite crystals in porcelain
bodies fired to cone 10. According to one authority (12) in
a mixture such as is used in the manufacture of porcelain, the
27






solvent action of the feldspar on the clay and flint is noticeable
at about 23350F and mullite begins to crystallize out at ap-
proximately 23750F. Since both of these temperatures are
within the range proposed for the experimental bodies of the
project, trials were made to ascertain the effect of mineralizers
on the growth of mullite crystals at cone 10 in a body of the
FP-3 type.
Mixtures to be tested for their effectiveness as mineralizers,
with one exception, were prepared as calcines and fired to cone 1,
(21200F). The hardened mass formed by the calcined ingredi-
ents was crushed, wet ground, dried and then added to the
other body materials.
The compositions of the mineralizing mixtures are shown in
the following table.


TABLE II
Percentage Composition of Mineralizing


Ingredient M-2
Beryl ........ ................ 83
Bone ash ........ ........
Borax ........................
English China clay ....
Lepidolite ...... .....
Nitre ...................... 17
Strontium carbonate ....
Zinc oxide ...................
Not calcined, ground
Bodies Containing
similar in composition


M-3
78
15


7
wet and


M-4* M-6


67




35
dried.


Mixtures
M-7


M-8 M-9


15
50 35 35 30
35 50 50 50
10 12
3 10 5
5 5 .


Mineralizing Preparations: A mixture
to No. FP-3 was formulated and used as


the. basis for the bodies in which the mineralizing preparations
shown in Table II were used.
The body compositions of the trial batches are given below
in Table III.


Percentage Compositions of Ca


Ingredient Body
FP-6
English China clay 20
Florida kaolin .... 5
BDK ball clay .... 10
Calcined alumina 5
Potash feldspar .. 22
Flint .................... 35
M-2 Calcine .......- 3
M-3 Calcine ...
M-4 Mixture ......
M-6 Calcine ........
M-7 Calcine ........
M-8 Calcine ........
M-9 Calcine ....


Bod
FP-
20
5
10
5
22
35


TABLE III
ist Bodies Containing Table
y Body Body Body
7 FP-8 FP-9 FP-10
20 18 20
5 5 5
10 10 10
5 6 3
22 22 22
35 35 35


II Preparations
Body Body
FP-11 FP-12
20 20
5 5
10 10
8 3
22 22
35 35

























































Fig. 9.-Cross Section of Photometer Apparatus. Upper Shelf Supports
Porcelain Sample. Cabinet Dimensions, 6"x6"x12".






A special body also was compounded which was intended
as a standard for petrographic comparison with the bodies con-
taining mineralizing preparations. It was composed of the fol-
lowing ingredients:
Special Body FP-5
Ingredient Per Cnt
English China clay 20.0
Florida kaolin .. 5.0
BDK ball clay..... 10.0
Calcined alumina 5.0
Potash feldspar 22.0
Flint ........ ...... .... 36.5
Flux N o. 14 ........... ..... .. 1.5
The No. 14 flux contained 1 part of zinc oxide and 4 parts
of strontium carbonate, by weight. The mixture was wet
ground, lawned through a 200 mesh sieve, thoroughly dried and
then added to other body ingredients.
The special body FP-5 and the trial bodies (FP-6 to FP-12
inclusive) were prepared by wet grinding in a porcelain ball mill.
TABLE IV
Data Relating to Body Grinding Operation
FP-5 FP-6 FP-7 FP-8 FP-9 FP-10 FP-11 FP-12
Iry batch weight in
grams ... 1000 1000 1000 1000 1000 1000 1000 1000
Per cent water ....... 40 40 40 40 42 42 42 42
Grinding time in hours 3 3 3. 3% 4 4 4 4
Mill capacity in gals. I1' 1 % 14 1 % 14 14 1 i 1 i
Weight of flint pebbles
in lbs ... 9 9 9 7 7 7 7
Speed of mill. R.P.M. 70 70 70 70 70 70 70 70
After the grinding operation was completed the slurry of
each body was placed in a plaster bat and a portion of the water
eliminated. This procedure provided a plastic mass or "filter
cake" which contained 20 to 25'; moisture. A typical batch
of casting slip, prepared from the plastic body mixture was
Plastic Body (20',; moisture) .... 1000 grams
W ater ... ..... .... .... ... ..... 180 cc.
"N" Sodium silicate ..... .. ...... 0.3 per cent
The 180 cc. of water were placed in a copper container and
small pieces of the plastic body gradually added. At the same
time the mixture was agitated by a high speed laboratory stirrer.
The deflocculent was added in increments of 0.1 cc. to the slip
during the stirring operation. The prepared slip of each mixture
was cast into plaster of Paris molds and the pieces subsequently
dried and fired. The results obtained from the test pieces fired
to 23800F (cone 10) are given in the following tabulation:





TABLE V
Data Relating to Fired Properties of Bodies. (FP-5 to FP-12 inclusive)
Per Cent
Sample No. Thickness Translucency Color Total
Factor Rating Shrinkage
FP-5 3.5 mm. 4.28 A 13.6
FP-6 3.5 mm. 0.85 A 13.6
FP-7 3.0 mm. 9.33 A 13.6
FP-8 3.5 mm. 8.00 A 14.5
FP-9 3.5 mm. 6.57 A-i 14.5
FP-10 3.5 mm. 7.14 A-i 13.6
FP-11 3.5 mm. 12.85 A 13.4
FP-12 3.5 mm. 10.00 A 13.4
An evaluation of the various properties of the fired samples
as shown in Table V, indicate that the mineralizing mixtures
had little effect upon the color and shrinkage. The color of FP-9
and FP-10 samples appear somewhat superior to the others.
However, the composition of the individual calcines which were
introduced into the mixtures influenced the property of trans-
lucency in each body mixture, as indicated in column 3 of the
table. This translucency factor is determined by dividing the
reading of a photometer in foot candles by the thickness of the
sample in millimeters. The highest number in column 3 of the
table indicates which sample has the greatest translucency. (See
page 32.)
Factors Influencing Translucency. Translucency is that prop-
erty of certain materials whereby sufficient light passes through
a thin section to permit objects, placed behind it, to be seen
indistinctly. It is one of the most essential properties of
porcelain and serves as a means of distinguishing that type of
body from other types such as earthenware and stoneware.
The property is intermediate between transparency and
opacity and is due largely to the high proportion of clear, glassy
matrix within the body structure. Translucency is dependent
upon several factors, the chief of which is chemical composi-
tion. The importance of composition upon the translucency of
porcelain is such that the raw materials must meet rigid speci-
fications. The percentage of fluxes present influences trans-
lucency but tests frequently will show that increases beyond
certain limits are not advantageous.
Temperature and the duration of the firing also are factors
which must be considered to obtain maximum translucency in
porcelain. A porcelain mixture showing slight translucency
when fired at a certain temperature may only develop increased
31





translucency when fired at temperatures 80-1000F higher. The
duration of the fire is important because vitrification or the for-
mation of the glassy matrix necessary for translucency, may be
increased in many instances by prolonged heating.
The amount of mullite which has been formed during the
firing process also is a factor in increasing the translucency of
ware. These needle-like crystals serve as a form of reinforce-
ment which in turn permit a greater quantity of the glassy
matrix to be present without deformation of the ware. Aside
from the foregoing factors which determine the extent of
translucency in the ware, in the final analysis, the thickness of
the object controls the degree of translucency present. In gen-
eral the amount of light transmitted through a section of
porcelain is inversely proportional to its thickness.
Measurement of Translucency: There are several methods
by which translucency may be measured. When specially de-
signed laboratory equipment is not available, a practical and
inexpensive method such as described by Hursh (9) may be
used to obtain the relative translucency of various bodies.
The principle applied in this method involves the use of
wire screens and an incandescent electric light of known candle
power. The procedure is to place the sample in contact with
a piece of screen of known mesh and to illuminate the latter
with light at a distance of approximately three inches. By
determining the finest mesh of screen, the wire of which is vis-
ible through the sample, relative translucency values may be
established for the individual samples.
The results obtained for bodies FP-5 to FP-12 inclusive by
this method are given in Table VI.

TABLE VI
Data on Determination of Translucency by Screen Method
Body Number FP-5 FP-6 FP-7 FP-8 FP-9FP-10FP-11 FP-12
Sample thickness, mm. .. 3,5 3.5 3,0 3.5 3.5 3.5 3.5 3.5
Mesh number .................... 6 4 6 6 6 6 8 6
Distance, light to screen 3" 3" 3" 3" 3" 3" 3" 3"
Light source, candle power 83 83 83 83 83 83 83 83

A further evaluation of the translucency of the above bodies
was made with a photometer, Figure 9. The results of these
determinations are shown in column 3 of Table V.
32





The apparatus consists essentially of a plywood box ap-
proximately 12"x6"x6" in dimensions. One side is comprised
of a hinged door to facilitate putting the sample and meter in
place. Inside at the top of the box a porcelain socket is secured
to hold a 75 watt incandescent electric light bulb in a downward
position. Below, within 5 millimeters of the bulb end, there is
a shelf with dimensions equal to the inside cross sectional area
of the box. This shelf has an oblong opening of the same
shape and size as the cell eye of the meter and is located directly
beneath the electric bulb. Another shelf a short distance below
the first, holds the photometer. A small sized, General Electric
light meter, calibrated in foot-candles is used.
The measurement of the degree of translucency of a speci-
men is made by placing a thin porcelain tile over the opening
in the top shelf. The photometer is then put directly under
the specimen so that the outside edges of the meter around
the cell eye make contact with the bottom side of the shelf
along the rim of the opening. The meter edges must fit well
against the shelf bottom so that no other light except that
which passes through the specimen strikes the cell eye.


Fig. 10.-Phuoomicrograph of a Section of FP-10 Body. Magnification
approximately 100X.
33





The door of the box is closed and the specimen illuminated
by switching on the electric light. An aperture in the door
which is in alignment with the photometer face permits the
operator to record the foot-candles registered by the cell eye.
In making the measurements for translucency of the FP-5
to FP-12 series of bodies by the photometer method, three sep-
arate readings for each sample were taken. The results were
uniformly consistent and provided a greater degree of accuracy
than that of the first method described which involves the use
of wire screens. Nevertheless correlation of the two methods
is interesting in-as-much as the results agree to the extent of
showing which bodies exhibit the lowest and highest degree
of translucency.
Table V shows that body FP-6 has the lowest translucency
factor, while FP-11 body has the highest. Similarily in Table
VI, body FP-6 required the largest mesh whereas a much smaller
mesh, number 8 was visible through the FP-11 fired body.
Petrographic Examination of Samples: In order to determine
the extent of the influence which the mineralizers exert on the
formation of mullite in the samples at the cone 10 (23800F)
temperature, a petrographic examination was made of three
bodies. The bodies chosen for the purpose were FP-5, contain-
ing no calcine; FP-7, having 3% of M-3 calcine which has beryl
as the mineralizer and FP-10 with 5% of the M-7 calcine con-
taining lepidolite. The FP-5 sample was selected in order to
furnish a basis for comparison in determining the degree of
crystallization in the other two samples.
The report of the petrographic examination (15) reveals
that the three samples are very much alike. The quartz grains
are angular and show no signs of solution in any of the samples.
The feldspar glass is present in two distinct forms; one is clouded
with inclusions of clay, the other is clear but contains minute
fibers of mullite. Pale brownish particles of an unknown ma-
terial also are present in the field.
The proportion of the two forms of fused feldspar varies
in the three samples and the FP-10 sample is much richer in
the glass containing the mullite than the other samples. The
proportion of the constituents within the microscopic field was
made by areal measurement. The results are listed in Table VII.
The compositions are approximate but the evidence would seem
to indicate that lepidolite has a favorable influence on the forma-
tion of mullite crystals.





TABLE VII
Data on Petrographic Determination of Body Constituents in Per Cent
Constituent Body FP-5 Body FP-7 Body FP-10
Quartz (angular fragments) ........ 30 32 30
Feldspar glass with clay .............. 57 64 44
Feldspar glass with mullite .......... 7 6 18
Unknown particles ......................... 6 8 8
Figure 10 shows a photomicrograph of a section of the FP-10
body, with an approximate magnification of 100 diameters. The
white quartz grains are readily distinguished by their angular
shapes. The glassy matrix of feldspar with inclusions of clay
particles and mullite appears as a greyish background. The
numerous dark spots seen are of different origin. These, in
part, are due to grinding dust caught in the pores of the sample
during the sectioning. The brownish, unknown material, pos-
sibly some iron-titanium compound, accounts for other smaller


100%.CLAY (M I TURE)





















100 FELDSPAR 100% FLINT

Fig. 11.-Triaxial Diagram. The Diagram is Useful when Blending Three
Components to Develop Ceramic' Mixtures.
35





spots. The mullite crystals are not apparent in the photomicro-
graph due to their fine, needle-like character. However, at a
magnification of 1000 X the mullite is visible in the microscopic
field as a felted mass of crystals.
The results reveal that the problem of increasing the crystal-
lization capacity of the porcelain ingredients by means of min-
eralizers, requires further consideration of several important
factors. In addition to the presence of mineralizers, crystal-
lization is influenced by physical conditions such as the vis-
cosity of the molten fluid, the temperature employed, the dura-
tion of the firing and the rate of cooling. A careful investi-
gation and study of these factors involve an extensive program
of research. It was decided, therefore, to attack the problem
of the development of a porcelain body suitable for commercial
production by another procedure.
The results of the Lests carried on in the second phase of
the experimental work involving body compositions is described
in the following pages.

SOFT FELDSPATHIC PORCELAIN BODIES
In addition to translucency and whiteness which are essen-
tial characteristics of porcelain type wares, several other factors
must be kept in mind in order to make a satisfactory body. The
body composition must permit the various forming operations
of casting, jiggering and pressing to be carried on successfully
and allow the fired glaze to appear without defects. Further,
the occurrence of warping or cracking in the fired ware is fre-
quently due to incorrect body composition and must be controlled
by a proper adjustment of the mixture.
Method of Compounding Bodies: Satisfactory bodies for a
given temperature are not obtained by the mere application of
a scientific equation or specific formula. Such mixtures are
largely the result of tedious experimentation by the trial and
error method. However, there is a large reservoir of technical
information available which is helpful and necessary to con-
sider if the desired results are to be obtained with a minimum
of time and effort.
Practical experience has shown that a high fired (2450F
and above) porcelain body may contain approximately 50%
of china clay while the balance of the mixture is composed of
non-plastic materials. By using this formula as a basis for





experimentation, the forming, drying, shrinking and firing char-
acteristics of a series of mixtures may be determined as follows:
A batch of ball clay and china clay, with a ratio of 2 to 5
by weight, was prepared by wet ball milling after which it
was carefully dried to constant weight.
A number of mixtures were then weighed out according to
the proportions given below:
Constituent Per Cent
Clay mixture 25 30 35 40 45 50 55 60 65 70 75
Flint .......................... 75 70 65 60 55 50 45 40 35 30 25
Triaxial Diagram:
Assuming that the mixture containing 40% of the clay
mixture and 60% of the non-plastic flint possesses several of
the desired properties, the work was continued by the use of
the triaxial diagram shown in Figure 11.
These diagrams are specially helpful when blending three
components to make a mixture and the next step in compounding
the body trials involved the use of an additional material. A
portion of the flint was now replaced with feldspar until trans-
lucency was obtained by the formation of an adequate amount
of glassy material in the fired body. In this case the small circle
on the diagram indicates the point at which the proportions of
each material was such that a promising mixture was produced.
To obtain the composition of any point on the diagram, a
perpendicular line is drawn from each vertex to the opposite side.
By reading the percentage lines from each side line of the diagram
toward the corresponding vertex until the specified point is
reached, compositions may be determined. Thus the compo-
sition of the trial mixture as indicated by the circle in the
diagram of Figure 11 was, clay mixture 40%, feldspar 30%
and flint 30%. Since the clay mixture was composed of 71.4%
of china clay and 28.6% of ball clay, the body mixture therefore
corresponds to the following:
Ingredient Per Cent
China clay ........................ 28.6
Ball clay .............................. 11.4
Feldspar .................... .... 30.0
Flint ............. 30.0
Other modifications in the body composition are now pos-
sible. The proportion of the ingredients may be altered; various
brands of clay may be substituted in order to obtain a suitable
37









/- '

ji
/' ^


I~g 12a. Unfirevd Figurine .-f Original 'Mmlto". High.






drying shrinkage, to improve the body color or to develop a
better casting mixture. An example of this procedure may
be noted by comparing the above mixture with FP-1 body given
on page 25. This latter mixture was used as the basis for
the compositions of the experimental trials described below.
Cracking Defects: In the development of a porcelain body,
for use primarily in making statuary and various decorative
figures, the problem of forming the articles by the casting
method must be given special consideration. Such objects are
frequently intricate in design and consequently the interior
of the molds may have narrow openings and deep recesses.
These conditions require that the slip be adjusted correctly so
that all parts of the mold are completely filled and a uniform
rate of casting is obtained in addition to allowing the surplus
slip to drain freely from the mold during the emptying period.
At one period of the experimental work, when slip casting
trials were made in small open molds, the body developed cracks
soon after the casting operation was completed. At first it was
thought the body ingredients had been modified unfavorably
as a result of excessive grinding. Other factors which were con-
sidered as possibilities were the non-uniform absorption of the
slip water due to imperfect molds and the nature of the water
used in making the slip.
Effect of Water Impurities: The erratic effect which hard
water produces on many ceramic mixtures is well known in the
pottery industry and a review of the experimental data of the
laboratory revealed an interesting fact in this connection. In
earlier casting trials no difficulties with cracking had been ex-
perienced and it was noted that the water used in the body
mixture at that time had been processed through an ion ex-
changer. When tap water from the city supply line was substi-
tuted in the same body composition and the slip cast in the same
molds, cracks developed in the ware regardless of the age of
the slip.
The solids present in hard water often exert a detrimental
influence upon clay mixtures because of the electrolytic action
of the former. These solids are usually compounds of calcium,
magnesium or sodium in the form of carbonate, bicarbonate,
sulfates and chlorides. When water is treated by the addition
of small amounts of alkalies these impurities generally can be
removed as insoluble precipitates. Numerous materials may be




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