Manual of Purpose-Made Woodworking Joinery

2 Softwoods and hardwoods

three of these named hardwoods are very strong and
durable and are often used in the manufacture of
external joinery such as doors, door- and window- sills,
door- and window- frames, etc. Of course, for aesthetic
reasons, hardwoods from a wide variety of species are
also used for internal joinery.

BOTANIC TERMINOLOGY

Medullary rays

Figure 1.1(a)(b)(c): This term refers to thin bands
of cellular tissue, also called parenchyma or pith rays,
which serve as storage for food which is passed
through ray pits to the tracheids for distribution.
These rays radiate from the pith in the centre of the
tree, to the bark. The rays become very decorative
when exposed superficially in certain timbers such as
quarter- sawn oak, producing an effect known as silver
grain.

Sapwood

Figure 1.1(a): This is the youngest, active growth
of a tree, occupying a narrow or wide band, varying
from about 12mm width in some trees, up to half the
radial area of the tree’s trunk, adjacent to the heart-
wood which occupies the central area. It can often
be detected on the surface- edges of slab- sawn boards
as a light blue to greyish blue in softwoods – and

in hardwoods, the colour of the sapwood is usually

lighter than the heartwood. Because of the sapwood’s

open grain and its large amount of sap and mineral

content, it has a lower durability than heartwood and

is therefore less stable. However, it is still used, but

because it is more absorbent than heartwood, its posi-

tion on external joinery components – such as sills –

should (ideally) be on the internal part of the sill.

Heartwood

Figure 1.1(a): This is the mature central portion of a

tree, which – because of the effect of the stabilized

content of substances such as gum, resin and tannin –

is usually darker than the sapwood. Each year a band

of sapwood becomes a band of heartwood, as a new

band of sapwood is added from the cambium layer

around the tree trunk.

Cambium and annual rings

Figure 1.1(a)(b)(c): Each year a layer of new wood is

formed around the outer surface of the tree, varying in

thickness from 0.5mm to 9mm for different species.

This growth forms under the bark in the cambium

layer and – as mentioned above – this cambium layer

becomes an annual ring as it is superseded by a new

cambium layer.

Figure 1.1 (a) A cross- section through a tree trunk
showing a typical area of heartwood and sapwood;
annual growth rings; the pith or medulla; the radial medul-
lary rays; the outer bark; the inner bark, bast or phloem;
and the cambium layer.

Figure 1.1 (b) Artistic impression of the magnified cellular

structure of softwood.

Medullary rays

Pith or

Medulla

Outer bark

Inner bark

or bast

Cambium layer

Annual growth rings

HEARTWOOD

SAPWOOD

(a)

Tracheid cells

Spring

growth

Summer

and autumn

growth

Summer

and

autumn

growth

Medullary rays

Spring

growth

Tracheid

cells

Annual ring

(3 to 40

per 25 mm)

(b)