extent, it becomes weaker as temperature increases. The
net result is that as wood dries it becomes stronger because
of the decreasing moisture content and can tolerate higher
drying temperatures and lower relative humidities without
cracking. This is a fortunate circumstance because as wood
dries, its drying rate decreases at any given temperature, and
the ability to increase drying temperature helps maintain a
reasonably fast drying rate. Thus, rapid drying is achieved in
kilns by the use of temperatures as high as possible and rela-
tive humidities as low as possible.
Drying schedules vary by species, thickness, grade, mois-
ture content, and end use of lumber. The two general types
of kiln schedules are moisture content schedules and time-
based schedules. Most hardwood lumber is dried by mois-
ture content schedules. This means that the temperature and
relative humidity conditions are changed according to the
percentage moisture content of the lumber during drying.
A typical hardwood schedule might begin at 49 °C (120 °F)
and 80% relative humidity when the lumber is green. By the
time the lumber has reached 15% moisture content, the tem-
perature is as high as 82 °C (180 °F). A typical hardwood
drying schedule is shown in Table 13–3. Some method of
monitoring moisture content during drying is required for
schedules based on moisture content. One common method
is the use of kiln samples that are periodically weighed,
usually manually but potentially remotely with load cells.
Alternatively, imbedded electrodes in sample boards sense
the change in electrical conductivity with moisture content.
This system is limited to moisture content values less than
30% (Simpson 1991, Denig and others 2000).
Softwood kiln schedules generally differ from hardwood
schedules in that changes in kiln temperature and relative
humidity are made at predetermined times rather than mois-
ture content levels. Examples of time-based schedules, both
conventional temperature (<100 °C (<212 °F)) and high
temperature (>110 °C (>230 °F )), are given in Table 13–3.
Some hardwoods used as structural lumber also use a time-
based schedule as shown in Table 13–3 (Simpson and Wang
2001, Ross and Erickson 2005).Drying Defects
Most drying defects or problems that develop in wood prod-
ucts during drying can be classified as fracture or distortion,
warp, or discoloration. Defects in any one of these catego-
ries are caused by an interaction of wood properties with
processing factors. Wood shrinkage is mainly responsible
for wood ruptures and distortion of shape. Cell structure and
chemical extractives in wood contribute to defects associ-
ated with uneven moisture content, undesirable color, and
undesirable surface texture. Drying temperature is the most
important processing factor because it can be responsible for
defects in each category.Chapter 13 Drying and Control of Moisture Content and Dimensional Changes
Figure 13–6. Package-loaded kiln with fans connected directly to motors.