240 PAINTING MATERIALS
form of soft, irregular lines in tangential section and straight lines in quarter-
sawed section. The most distinctive feature is the appearance of the various
markings in reflected light; almost all of them appear relatively lighter or darker
than adjacent areas, depending upon the source of the light. As a paint support,
mahogany has been considerably used since the tropical regions of America were
opened to commerce and it is a favorite material for cradles and other accessory
supports. It is said to have been used as a furniture wood in the early part of the
XVIII century. The timbers of several other species of wood are used and known
as mahogany, and those of the West African Khaya senegalensis are known as
African mahogany (see A. Koehler, * The Identification of True Mahogany, Cer-
tain So-Called Mahoganies, and Some Common Substitutes/ Bulletin No. 7050,
U. S. Department of Agriculture, 1922).
Maple (Acer). The common maple of Europe (A. campestri) is not an impor-
tant timber tree and it appears that the wood is seldom used for painted panels.
In America the sugar or hard maple (A. saccharinum) furnishes timber of eco-
nomic importance. The sugar maple is hard, does not readily warp or shrink, and
has superior working qualities. The heart-wood of the hard maple is a light
reddish brown; it has no conspicuous figure, and is diffuse-porous, the pores being
very small. The annual rings are, however, fairly distinct. Rays are conspicuous
as fine dashes in quarter-sawed surfaces and are similar to those in beech.
Metals. Metals have never been used extensively as supports for paint. In
ancient times they were probably too highly valued for other purposes and were
not available in large sheets or panels. Sheets for armor and such uses had to be
beaten out laboriously by hand; methods for rolling metals were not developed
before the middle of the XVI century. Metal-rolling mills driven by water-power
were in use in Europe in the first part of the XVIII century but steam-power for
this purpose, with a subsequent large-scale production, did not come into general
use before the latter part of the same century.
The great difficulty with metal of any kind as a support for paint, at the
present day, is a flexibility greater than that of the paint, ground, and surface
films of the design itself. The value of a support is the strength and firmness it
supplies for those films. To make metal sufficiently rigid is to make it so heavy
that it becomes practically unmanageable. Expansion of metal with heat has been
argued against it, but since the highest coefficient of thermal expansion for any
of the commonly used metals is that of aluminum, given at 24 X io~6, and that
for oak is 54-4 X icr6 with other common woods all above 32 X io"~6, that ob-
jection does not hold comparatively. The real objection to it lies in its tendency
to be bent, dented, or flexed.
Below are listed some of the physical properties of the metals generally used.
(These are taken from Smithsonian Tables, Vol. LXIII, 1914, and from Handbook
of Chemistry and Physics, aoth ed. [Cleveland: Chemical Rubber Publishing Co,,
1934]. For other properties, see Aluminum, Copper, and Iron.)