262 PAINTING MATERIALS
from needle- or scale-leaved trees, nearly all of which bear cones and have un-
covered seeds. Annual rings are clearly defined in soft-woods. Scattered among
the tracheids of some soft-woods and absent in others are minute spaces which
contain pitch and these are known as * resin passages/ The presence or absence of
resin passages in soft-woods is not such a distinguishing characteristic as the ring
and diffuse porosity of hard-woods.
A great deal of chemical research has been done on the composition of wood.
The bulk of the wood fibre is cellulose, a compound of carbon, hydrogen, and oxy-
gen, belonging to that great class of organic materials known as the carbohydrates.
The formula is expressed empirically as (C6HioO5)u where n is supposed to have a
value, roughly, of the magnitude of 5000. Besides the cellulose, there is a material
called lignin, a cementing substance which holds the cellulosic wood cells together.
With cellulose, it forms ligno-cellulose. There are, also, various minor components:
resins, tannins, essential oils, wood sugars, and waxes.
Wood substance is a typical elastic jelly. It is not soluble in ordinary organic
solvents. It is quite inert to the petroleum hydrocarbons, but it is swollen by
water and the lower alcohols, ketones and esters. It is also swollen by contact
with the vapors of these liquids. Not all the swelling manifests itself in exterior
dimensional changes of the wood; much of the swelling is absorbed in plastic
deformation within the wood structure.
Several properties of a physical nature make wood generally useful for pur-
poses of art and architecture, and it has certain physical properties which may be
regarded as shortcomings. Compared with many other materials that may be
used for paint supports, wood is a light material. It has limitations as to width of
single boards, but the ease with which it may be worked, joined, and glued allows
construction of fairly large, flat surfaces and panels. Woods vary in texture con-
siderably. Many of the conifers are fine-textured, because they have no pores,
and there is not much difference in density between the spring-wood and summer-
wood. Woods like oak which have large wood vessels are spoken of as coarse-
grained or textured woods.
The structure of wood is cellular, not labyrinthine. From one half to three
quarters of dry wood is air space. The structure is nearly like that of the honey-
comb in which there is no communication between cells. The air enclosed in wood
is mostly enclosed within the tracheids and wood cells and not between the cells.
There is not complete isolation between the tracheids of soft-wood because their
walls are penetrated by small pits (bordered pits) covered by membranes, which
are microscopic openings about o.ooooa mm. in diameter.
Wood readily absorbs or gives up water (moisture) and, consequently, it is
subject to shrinking and swelling. This change in dimension with changing mois-
ture content may be considered a shortcoming. There are three forms of water in
wood: (i) the water that is in chemical combination with carbon to form the
cellulosic material, (2) the water that is absorbed by the cell walls, and (3) the