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.. H..HNICAL NOTE NUMBER 254
FOREST PRODUCTS sABO7TDYj UNITED STATES FOREST SERVICE
MAU~~iN[RSRIYCOQ N WI~LAotY F ~pi i 14
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CED4ICAL BROWN STl N PINE
Stains that develop in logs and it-n ^ fi ffi ed eier
by fungi or by chemical changes that occur in the wa e -cill rtr
tires. Although the stains produced by fungi are more common and cause
greater financial losses, the chemical stains may become troublesome
during the seasoning of certain species. Among the commercial species
that are subject to objectionable chemical stains are ponderosa pine, the
true white pines, western hemlock, Noble fir, redwood, and several hard-
woods, including maple, birch, hickory, persimmon, basswood, and magnolia.
Chemical brown stain in pine is not expected to appreciably alter the
physical properties of the wood other than that of appearance.
Occurrence of Stain
Chemical brown stain in the pines darkens the wood to colors
ranging from buff to dark brown. It is usually most conspicuous at or
near the surface, but may penetrate throughout the wood. The stain is'
more pronounced in areas where most evaporation occurs, as at end sur-
faces and on tangential surfaces.
The occurrence of brown stain is closely associated with the
drying of wood. It appears in the zone where the water vaporizes, thus
depositing the solutes or extractives. The stain appears beneath the
surface if the porosity of the wood and the severity of the drying condi-
tions promote rapid drying of the surface fibers. Under such conditions,
a steep moisture gradient is established with a moisture content of the
surface fibers well below the fiber-saturation point. When the water
moves so freely through the wood that the surface fibers remain moist f:r
some time after exposure to a drying atmosphere, the stain tends to develop
at or near the surface of the lumber.
In pine, chemical brown stain develops in either sapwood or heart-
wood. Very commonly it occurs in the area of the junction of heartwood
and sapwood. Blue and brown stains due to fungi are generally confined to
the sapwood only. Chemical brown stain is more prevalent in lumber saved
from old logs than in lumber sawed from newly cut logs. As recently shown
by the Western Pine Association it is also more prevalent in boards that
have been solid piled for two or more days immediately following saying
than in boards piled for drying immediately after sawing.
Cause of Stain
The exact nature of the chemical changes responsible for chemical
brown stain is not fully understood. However, experience has shorn that
the stain occurs only as the wood dries, appearing during both air
drying and kiln drying. The degree of staining is influenced by the
temperatures and relative humidity employed in drying, that is, a high
temperature-high humidity drying schedule produces greater staining than
does a low temperature-low humidity schedule. Warm and excessively
humid conditions seem to increase brown stain in air drying.
Chemical brown stain is believed to be caused by chemical reactions
that occur in the water-soluble extractives as they are concentrated and
deposited during drying. Evidence for this belief was obtained when brown
stain was produced in a nonstaining wood by impregnating it with an ex-
tract isolated from green sugar pine selected for its known tendency to
brown stain in drying. Furthermore, sugar pine, freed from its extractives,
could be dried under severe kiln conditions without stain. Additional
work on the separation of the various chemical components present in ex-
tracts from staining wood showed that the principal stain producing sub-
stances are the sugars. A lesser degree of staining was produced by
tannins. Further research is needed, however, to develop a complete chemi-
cal understanding of the brown stain phenomenon.
Control of Brown Stain
No completely successful way of preventing chemical brown stain is
known. Certain means can, however, be recommended for limiting its occur-
The substance or substances responsible for the brown stain are
insoluble or only moderately soluble in cold water, but are readily soluble
in hot water. Therefore, the use of mild temperatures during seasoning
will decrease the quantity of soluble material carried by the water toward
the surface of the lumber, where it is deposited during the process of
The temperature of the free water in wood at the start of a kiln
run approximates the wet-bulb temperature. This is especially true of
woods in which the movement of the water is rapid and the wood dries al-
most as fast as the moisture can be evaporated from the surface. Conse-
quently, the wet-bulb temperature is an important factor to be considered
in the control of brown stain, and it should be kept low, especially during
the early stages of drying of stock that is likely to stain.
Since the zone of extractive concentration should be kept well
below the surface, as low a relative humidity as is possible without devel-
oping serious surface checking should be maintained by proper ventilation.
High air velocities are required when' kiln drying wide truck loads of
edge-to-edge piled stock to prevent excessive increases in the relative
humidity of the air moving across the load.
As previously indicated, an important factor in the development of
chemical brown stain is the length of storage of the logs and lumber. The
most effective method of keeping brown stain to a minimum is to cut the
logs into lumber soon after felling the trees and to avoid solid piling
the green lumber for more than a day.
Brown stain develops less during air drying than during kiln
drying, and thorough air drying prior to kiln drying, therefore, will
reduce training. This is particularly true of lumber that has been air
drying during the cold winter months, when the least amount of stain-
producing extractive is in solution.
Ponderosa pine lumber cut from freshly felled logs and promptly
kiln dried with initial conditions of 120 F. and 55 percent relative
humidity and final conditions of 160 F. and 24 percent relative humidity
has a minimum of chemical brown stain. Eastern white pine is being kiln
dried to produce commercially acceptable stock with initial conditions of
1500 F. and 80 percent relative humidity and final conditions of 180 F.
and 30 percent relative humidity.
Z M. 6770o F
UNIVERSITY OF FLORIDA
III 12 fl 1ii im3 at tn il tIIl lI Iii
3 1262 09216 7369 I