at with great rapidity and finally becomes saturated when it has
lost all of its ability to cause evaporation. In this respect it is
more moist than air when it comes in contact with bodies which are at
a lower temperature. When saturated steam is used to heat the lumber
it can raise the temperature of the latter to its own temperature, but
cannot produce evaporation unless, indeed, the pressure is varied.
Only by the heat supplied above the temperature of saturation can
evaporation be produced.

Impregnation Methods

Methods of partially overcoming the shrinkage by impregnation of the
cell walls with organic materials closely allied to the wood substance
itself are in use. In one of these which has been patented, sugar is
used as the impregnating material, which is subsequently hardened or
"caramelized" by heating. Experiments which the United States Forest
Service has made substantiate the claims that the sugar does greatly
reduce the shrinkage of the wood; but the use of impregnation
processes is determined rather from a financial economic standpoint
than by the physical result obtained.

Another process consists in passing a current of electricity through
the wet boards or through the green logs before sawing. It is said
that the ligno cellulose and the sap are thus transformed by
electrolysis, and that the wood subsequently dries more rapidly.

Preliminary Treatments

In many dry kiln operations, especially where the kilns are not
designed for treatments with very moist air, the wood is allowed to
air-season from several months to a year or more before running it
into the dry kiln. In this way the surface dries below its
fibre-saturation point and becomes hardened or "set" and the
subsequent shrinkage is not so great. Moreover, there is less danger
of surface checking in the kiln, since the surface has already passed
the danger point. Many woods, however, check severely in air-drying or
case-harden in the air. It is thought that such woods can be
satisfactorily handled in a humidity-regulated kiln direct from the
saw.

Preliminary steaming is frequently used to moisten the surface if
case-hardened, and to heat the lumber through to the center before
drying begins. This is sometimes done in a separate chamber, but more
often in a compartment of the kiln itself, partitioned off by means of
a curtain which can be raised or lowered as circumstances require.
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