Manual of Purpose-Made Woodworking Joinery

Seasoning 3

Spring and summer growth of annual rings

Figures 1.1(b)(c): Most annual rings show themselves
distinctly on the end- grain of converted (sawn) timber
and this is because the spring growth of the rings is
lighter in appearance (because the cells are larger)
and this contrasts (usually distinctly) with the summer
and autumn growth of the rings which are darker in
appearance (because the cells are smaller and denser).

SEASONING

Seasoning of timber after it is felled and converted
(sawn into a variety of sectional sizes), means drying
out and reducing the moisture and sap until a certain,
necessary percentage remains. This remaining percent-
age is known as moisture content (mc). If timber is not
properly seasoned, it will warp, twist, shrink exces-
sively and be more prone to rot. Commercially, timber
is seasoned before it is sold.
The required moisture content varies between 8
and 20% and should be equal to the average humid-
ity of the room or area in which it is to be fixed. 10%
mc is suitable for timber in centrally heated buildings;
and 10 to 14% in buildings without central heating.
External joinery should be 15% mc. Timber for first-
fixing carpentry jobs (i.e. roofing and floor joists, etc)

should be 16 to 18% mc; 20% mc being the maximum.

Timber with more than 20% mc is liable to be affected

by dry- rot and wet- rot decay. 20% mc is generally

considered to be the dry- rot safety line.

If the average humidity of a building is less than

the moisture content of the timber (as is usually the

case after installing central heating), the timber may

shrink and split. If the building’s humidity is more

than the timber’s mc (as often happens with conden-

sation problems), the timber may swell and rot. In

such cases, the first signs are usually the appearance

of a black mould growth on the timber (and other

surfaces).

Methods of seasoning

There are three methods used. These are (1) natural

seasoning, (2) artificial seasoning, and (3) a combina-

tion of natural and artificial.

Natural or air seasoning

In common with the other two methods, the branches

are removed just after the trees are felled, the trees are

then cut into logs and the bark is removed. Eventually,

the logs are converted into baulks (sized at least

150mm × 150mm) or planks (sized at least 300mm ×

50mm) and stacked to allow air to circulate around

them. The stacks may be out in the open, with rough-

boarded roofs over them or in open sheds, having

roofs and one or two walls.

Hardwood logs may be converted into planks and

stacked one above the other with, say 25mm × 25mm

softwood piling sticks between them, laid across the

planks at maximum 1metre spacing. This is done to

create all- round air circulation. Vertically, the piling

sticks between the planks must be placed carefully

above each other to achieve an equally distributed

load; in the drying- out stage, an unequally distributed

load may cause distortion of the timber. Also, the ends

of the planks are usually painted to prevent end-

splitting.

Air seasoning is a slow process and in countries

with moderate climates and average weather condi-

tions, it is usually not possible to reduce the moisture

content much below 20%. Softwood of 50mm thick-

ness takes approximately 3 to 4 months to reduce to

20% mc. Hardwood of 50mm thickness takes approxi-

mately 12 months to dry to the same amount.

Artificial or kiln seasoning

The converted timber is usually stacked on rail- trucks

(with piling sticks between each layer) and placed in

steam- heated kilns which force out the moisture and

Figure 1.1 (c) Artistic impression of the magnified cellular
structure of hardwood.

Annual

growth ring

Fine rays

Broad rays

Spring-wood pores or vessels

(c) Summer-wood pores or vessels