/ ( FOREST PRODUCTS LABORATORY t FOREST SERVICE
U. S. DEPARTMENT OF AGRICULTURE
SPECIAL METHODS OF SEASONING WOOD -- \
CHEMICAL SEASONING \ .- \
Chemical seasoning consists of treating green wood with--a"iygroscopic chemi-
cal and air drying or kiln drying the treated material. The present process
involves comparatively short treating periods. The chemical is used to reduce
or prevent surface checking during seasoning, rather than to accomplish actual
drying or speed the drying.
The process was not fully understood until the U. S. Forest Products Labora-
tory began an investigation in 1930. In 1938, the Laboratory and the West
Coast Lumbermen's Association developed a process using urea to reduce
checking in Douglas-fir timbers. This treatment was widely used in the sea-
soning of high-grade, Douglas-fir timbers, particularly flat-grained clears
and ponton stock during World War II.
Since 1947, very little use has been made of the chemical seasoning process
for Douglas-fir. Increasing use has been made recently, however, of a pro-
prietary sodium chloride preseasoning treatment for hardwoods in the South
and East. Chemical treatment adds to expense, and the presence of the chemi-
cal in the wood may be a disadvantage. The process, therefore, is economic-
ally practical only when the increased value of the chemically seasoned lumber
offsets the added expense.
The objective of the preseasoning treatment is to impregnate the outer zone of
lumber with chemicals to a depth of about one-tenth of the thickness, with the
highest concentration at or near the surface. The chemicals maintain the outer
zone at a higher moisture content during drying than occurs with untreated wood.
The hygroscopic nature of the chemical tends to keep the wood moist. This
reduces the shrinkage of the outer zone and lessens its tendency to surface
check. Some chemicals impart a permanent antishrink effect which is separate
from the actual moisture effect.
The presence of the chemicals in the outer zone does not interfere with drying.
Moisture from the untreated interior passes through the impregnated outer
zone and evaporates. For some species, such as Douglas-fir, chemical sea-
soning cuts drying time by permitting the use of faster kiln-drying schedules.
In most cases, however, treatment reduces surface checking without decreasing
drying time. Kiln schedules must be modified somewhat to bring the initial
relative humidity below the relative humidity in equilibrium with the saturated
Rept. No. 1665-6 (Revised)
t Maintained at Madison 5, Wisconsin in cooperation with the University of Wisconsin
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Numerous chemicals have been tried. Common salt is cheap and effective
in reducing surface checking, but it corrodes metals and may damage dry-
kiln equipment, woodworking machinery, and hardware fastened to the
treated wood. Urea may not always be as effective in reducing surface
checking in other species as it is in Douglas-fir. Under some conditions,
urea-treated oak may check worse than untreated oak. Urea corrodes
copper-bearing metals and discolors wood. Other chemicals investigated
include invert sugar, molasses and other low-grade sugars, diethylene
glycol, and a urea-formaldehyde mixture.
The proprietary sodium chloride mixture,which consists principally of
common salt, has corrosion inhibitors in it. This material, which can be
called a buffered sodium chloride, appears as effective as pure salt in
controlling seasoning checks. Recommendations of the manufacturer call
for applying the chemical to green wood only, then air drying the wood
enough so that final kiln drying can be carried out with a relative humidity
sufficiently low that moisture does not condense on the wood. When the
wood is kiln dried immediately after treatment, the inhibitors are not ef-
fective enough to prevent corrosion of the kiln equipment.
The most efficient treating method, so far as reducing surface checking is
concerned, is the soaking method. However, this requires a large invest-
ment in chemicals, tanks, and other equipment, and the solution becomes
discolored with use and discolors the wood. Dry spreading, the cheapest
method of application, has been used quite extensively in the recent appli-
cation of buffered sodium chloride on hardwoods. It also was used in the
wartime treatment of Douglas-fir. The lumber and chemical are piled in
alternate layers, using from 35 to 140 pounds of chemical per 1,000 board-
feet. When the chemical has been absorbed, the lumber is piled for sea-
Chemical seasoning agents can reduce the strength of wood and sometimes
cause problems in gluing and finishing or corrosion during use. Although
large quantities of wood treated with such chemicals have been used suc-
cessfully for a variety of purposes, some consideration should be given
to these disadvantages.
Tests have shown that red oak treated with either sodium chloride or
buffered sodium chloride, partially air dried, and then kiln dried with
a final temperature of 165 F. can suffer a reduction in toughness of
about 30 percent. The corresponding reduction in bending strength
would be 10 to 15 percent. One test on maple showed a smaller reduc-
tion--only 10 percent in toughness, equivalent to 3 to 5 percent in bend-
ing strength. When oak treated with sodium chloride was air dried to
Rept. No. 1665-6
20 percent moisture content, then kiln dried with temperatures not ex-
ceeding 130 F., no significant reduction in toughness occurred. Com-
mercial driers of sodium chloride-treated oak have used final kiln
temperatures up to 150 F.
In limited tests on the gluing of salt-treated red oak with liquid ani-
mal glue, hot animal glue, and polyvinyl resin-emulsion glue, the
hot animal glue and polyvinyl glue were not affected by the treatment.
For the liquid animal glue, joints made with salt-treated wood were
very significantly lower in dry strength than those made with normally
seasoned wood. No reports have come to the attention of the Labora-
tory on the salt-treated wood being difficult to glue other than with
liquid animal glue. Salt treatment, however, is reported to prohibit
at least some applications of electronic gluing because of arcing.
The finishing characteristics of salt-seasoned wood have not been
studied. Presumably a considerable amount of wood so seasoned has
been used for furniture and other items without difficulty in finishing.
Although the surface layers that contain the greatest concentration of
salt are removed in the machining operation, some salt remains in
the surfaced wood and couldinterfere with finishes if used under condi-
tions involving extremely high relative humidity. Under such conditions,
the salt could attract enough water to the wood to disrupt the finish.
Poor results during manufacture were obtained with one finishing sys-
tem, however, by a firm that makes a practice of carefully controlling
the moisture content of the wood going into its furniture production.
When the proprietary sodium chloride mixture was introduced several
years ago, it was hoped that the corrosion-inhibiting chemicals would
be effective during use. This has not proven to be the case. Under
relative humidity conditions above 75 percent, metal in contact with
the treated wood has corroded. Where it is known that the wood will
be subjected to high humidities, salt treatment should be avoided.
When it is used, care should be taken to avoid use of excessive amounts
of chemical or prolonged treating time.
Because of modification of the electrical properties, the moisture
content of salt-treated wood indicated by an electrical moisture meter
will be considerably higher than the actual moisture content.
Obviously, it is pointless to use chemicals as an aid to seasoning items
that, without treatment, can be satisfactorily air dried and rapidly
kiln dried without surface checking. Generally, 4/4 and thinner oak
and other hardwoods can be satisfactorily seasoned without predrying
Rept. No. 1665-6
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treatment, provided proper procedures are used. There are advan-
tages in using chemical pretreatment for thicker material, however.
Whether or not to use chemicals in seasoning items of lumber that check
in drying will depend upon the economics of each situation.
Colgrove, William H. Chemical Seasoning of Lumber. Forest Prod.
Jour. 6(10):417-419, 1956.
Jordan, Jack. Chemical Treatment of Douglas-Fir in Seasoning for I
Hot, Dry Climates. Western Building, Sept. 1955.
Loughborough, W. K. A Forward Look at Chemical Seasoning of Lum-
ber. National Hardwood Magazine, Dec. 1955.
Lund, A. E., and Taras, M. A. Kiln Drying Chemically Treated
Scarlet Oak and White Oak Lumber. Forest Prod. Jour. 9(11):
Peck, E. C., Baker, Gregory, and Carter, R. M. Chemical Treat-
ment and Seasoning of Thick Beech Stock. Forest Products Labo-
ratory Report No. 1708, reviewed and reaffirmed 1956.
Torgeson, 0. W. Effect of Yard-Piling Methods and Salt Treatment on
Checking of 5/4 Red Oak Lumber. Forest Products Laboratory
Report No. 1759, reviewed and reaffirmed 1956.
Revised by J. M. McMillen
Forest Products Laboratory
Madison 5, Wisconsin
Rept. No. 1665-6