Manual of Purpose-Made Woodworking Joinery

4 Softwoods and hardwoods

sap from the timber, until the required mc is achieved.
Strict control of heat and humidity has to be main-
tained to prevent defects occurring. Such defects
include warping, splitting – and case- hardening
(drying too quickly on the outside, causing moisture
to be trapped on the inside).
Although different types of kiln are used, the
principles are similar. In one type, the air in the kiln
is heated by steam pipes, humidified by sprays and
forced along a central floor duct by a fan near the
heater. The central duct has a number of openings
along its length. The conditioned air is forced out of
these and it passes through the stacks of piled timber.
Then it enters side ducts in the floor, to return to the
heater for recirculation.
Natural seasoning is more environmentally
friendly, but Kiln seasoning saves time and is there-
fore mostly used. By kiln seasoning methods, soft-
wood of 50mm thickness takes approximately 3 to 5
days to reduce to 12% mc. Hardwood of 50mm thick-
ness takes approximately double this time to reduce to
12% mc.

Combined air and kiln seasoning

By this method, timber is air seasoned to about 20%
mc, then kiln- dried for a relatively short period to
achieve the required moisture content. This usually
reduces the time in the kiln by approximately one
third.

TIMBER CONVERSION AND

DEFECTS

As mentioned above, logs are cut (converted) into
various- sized timbers before seasoning, using several
different cutting arrangements. These well- established
arrangements are governed either by economical con-
sideration and waste avoidance, or a need for quality
timbers with regard to the position of the annual rings
and (with certain hardwood species) the position of
the medullary rays. The different cutting arrangements
used for log conversion include:

Through- and- through sawn

Figures 1.2(a)(b): As illustrated, the log is sawn verti-
cally (or horizontally, depending on the machine used)
with parallel cuts referred to as through- and- through
sawn, flat sawn, plain sawn or slash sawn. Also referred
to in the USA as bastard sawn. This is the most eco-
nomical conversion, with very little waste. But by this
method, about two- thirds of the boards have annual

rings that are tangential to the face of the boards (as

seen in Figure 1.2(b)) and therefore are very likely

to be adversely affected by shrinkage and cupping (as

illustrated in Figure 1.2(g)).

Quarter sawn

Figures 1.2(c)(d): As the terminology suggests,

the log is first sawn into quarters (four quad-

rant shapes) and each of these can be converted

by many different sawing arrangements. This is

done either to display the medullary ray figuring

on the face of the boards, or to produce more stable

boards (or rectangular sections) with annual rings

that are at right angles to (or not less than 45° to)

the widest face of the board or section – as illustrated

Figure 1.2 (a) Through- and- through sawn log (shown

reassembled); and (b) Example of annual rings being

tangential to the face of the board.

Figure 1.2 (c) Quarter- sawn log (shown reassembled);

and (d) Example of annual rings being at right angles to

the widest face of the board.

(a) (b)

(c) (d)