KILN DRYING HANDBOOK 65 
small portion, about one-twentieth, less than 5 per cent. The moisture 
range in such stock as it comes from the kiln is probably somewhat 
greater than this, and the range in common grades is still greater. 
The degree of uniformity of final moisture content can be increased 
by holding the stock in the kiln an additional 12 to 24 hours after 
the desired average dry moisture content has been reached. The final 
temperature and humidity conditions of the drying schedule should 
be maintained during this period. 
HARDWOODS 
In hardwoods the corresponding general situation is somewhat dif- 
ferent. The largest part of such stock is air-dried before it enters the 
kiln and thus much of the inequality in initial moisture content is re- 
moved. The amount of drying done in the kiln represents merely the 
taking away of a comparatively small percentage of moisture, the dry- 
ing is done quite slowly in comparison with that of softwoods, and 
the final moisture content is usually fairly low. AI] these factors con- 
tribute to uniformity of final moisture content. No adequate data are 
available; yet it is almost certain that the final moisture content in 
kiln-dried hardwoods is more uniform than that in kiln-dried soft- 
woods, 
TOLERANCES IN MOISTURE CONTENT 
Asa result of the entire situation just outlined, the term “average 
‘moisture content,” which appears in the paragraph on the factors 
influencing final moisture content, has much more significance for 
hardwoods, and also for softwoods redried at the consumer’s plant, 
than for softwoods dried at the producer’s mill. Although it is 
difficult, under present conditions, to have softwood stock kiln dried 
at the mills to a uniform specified moisture content, the average 
moisture content of individual hardwood and of redried softwood 
boards may undoubtedly be made to conform quite closely to the av- 
erage value specified for the lot. In any event, it is highly desirable 
that at the time of initial use or of remanufacture the moisture con- 
tent of every piece of wood be very close to that which it will nor- 
mally reach in service. The exact relation between these two values 
depends upon the final requirements. Softwood factory lumber that 
has comparatively shght shrinkage characteristics may show quite a 
range in moisture content, both in the average value of a lot and 
among individual pieces, and still be suitable for use in millwork. 
Hardwood furniture stock should at the time of manufacture be 
slightly drier than the equilibrium moisture content in service. Stock 
for gluing should be still drier, as should also stock intended to swell 
shghtly in service, and stock to be stored in an unheated space for 
some time between drying and final use must be so dry that the 
absorption of moisture during the time of storage will not unfit it for 
the service intended. 
In general, wood used indoors becomes drier than that used 
outdoors, and this fact is largely responsible for the differences in 
final moisture content given in Table 27. 
32006°—29——-5 
