SUPPORTS 265
summer-wood expands so much that the forces are greater than those of the
swelling of spring-wood, with a result that the spring-wood is often compressed.
Paint does not chip and flake from spring-wood so quickly as it does from summer-
wood, which indicates that there is less mechanical movement in the spring-wood.
Its natural expansion is balanced bv the compression effects exerted upon it by
summer-wood.
It is difficult to find published data which give the linear increase in the
swelling of wood with change in the moisture content of wood. S. T. C. Stillwell,
of the British Forest Products Laboratory, supplies the information (private
communication) that a quarter-sawed oak panel I foot in width, increases 0.023
inch for each increase in moisture content of I per cent. A tangentially-sawed
panel of the same width gains 0.038 inch. These movements were measured over a
range corresponding to relative humidity conditions of from 20 per cent to 80
per cent. This laboratory has also made some measurements on the expansion
and shrinkage of old panel paintings with changes in relative humidity. Their
graphs show that certain painted panels may change in width across the grain as
much as 1.5 per cent to 2 per cent in a range of relative humidity varying from
40 to 80 per cent.
Wood, deterioration and treatment. The decay of wood is always caused by an
external, living organism. All such organisms need for their growth food, water,
air, and warmth. In general, they belong to the mass of dependent plants known
as fungi and, so far as they infest wood, may be considered in three classes:
bacteria—unicellular organisms the action of which is still imperfectly under-
stood, though it appears that some of them may be destructive to wood tissues;
ascomycetes—chromogenetic fungi known as sap stains and having only a slight
influence on the strength of wood; and basidiomycetes, the highest class of fungi
and the most destructive to wood. These are made up structurally of many
branching, hollow tubes arranged loosely like mold or crowded into cushions.
The tubes grow principally in length. They may be formed into fruit-bodies,
plate-like, bracket-like, or flat, and assuming a variety of shapes. The character-
istics of some of the principal basidiomycetes that live on wood can be shown in a
tabular form (this is condensed from the table by Prof, Percy Groom in * Dry
Rot in Wood/ pp. 6 and 7).
An understanding of the conditions of growth carries with it sufficient indica-
tion of how to prevent that growth. Many of the fungi that cause dry rot will
attack sound wood. This is particularly true of the Coniophoraputeana (cerebella).
Inoculation occurs from a microscopic spore, and thin films of such materials as
wax would help to prevent this from getting lodged in the surface. None of the
fungi will live without an ample supply of water vapor, or without fresh air.
Extreme and prolonged dampness is therefore to be avoided and occasional
fumigation with thymol or some of the stronger gases would tend to kill any
active or dormant growth.