_92	painting materials
it loses its combined water and is changed to aluminum oxide, A1_2O_3, which is of
no value for pigment purposes. Alum and similar substances were used as early
as classical times as a source of substrates for dye colors (see Bailey II,
Aluminum Leaf and Aluminum Bronze Powder are made from sheet alumi-
num by a beating and a stamping process, respectively. The name, 'bronze/ is
still retained, no doubt from its association with metal powders made from copper
alloys (see Bronze Powders), Although aluminum powder was probably available
as early as the middle XIX century, it was not until a decade or so after 1886,
when aluminum began to be produced in large commercial quantities by the Hall
process (see Aluminum, section on supports, pp. 221 and 222) that the powder be-
came readily available. It was first used for coating picture frames and radiators.
Aluminum powder did not become important as a pigment for commercial paints
until after 1920 (see J. D. Edwards, Aluminum Bronze Powder and Aluminum
Paint [New York: The Chemical Catalogue Co., 1927], pp. 26-29). Its develop-
ment for outside and for protective painting followed experiments and field tests
carried on in the Forest Products Laboratory, Madison, Wisconsin, and by the
H. A, Gardner Laboratory in Washington.
When aluminum bronze powder is stirred into a suitable vehicle like oil or
varnish, the flakes swirl and some come quickly to the surface layer where they
spread out to form an almost continuous film of flat particles. This phenomenon,
which is called 'leafing/ is caused by surface tension and is shown only to a
marked degree by the polished powder and not by the unpolished powder (see
J. D. Edwards, F. C. Frary, and Z. Jeffries, The Aluminum Industry -, II [New
York: McGraw-Hill Book Co., 1930], p. 803). No grinding of the powder and
vehicle is necessary. In pyroxylin medium (nitrocellulose) aluminum powder has
no leafing properties and does not form a durable film. Because of its leafing
properties, it is now finding wide use for moisture- and waterproof paints. For
exterior use, long oil spar varnishes are the best vehicle. Experiments at the
Forest Products Laboratory show that this coating has outstanding moisture
resistance and maintains its moisture-proofing efficiency over relatively long
periods of time.
Aluminum bronze leaf in a vehicle has a reflectivity of 60 to 75 per cent for
light, but it has low emissivity or radiating power for heat. At 40° C. the emis-
sivity of aluminum paint is only about 20 per cent of that of a * black body/
which is the theoretically perfect radiating surface (see Edwards, of. cit^ pp. 47
and 51).
Microscopically, the particles of aluminum bronze powder are irregular in
shape; in reflected light the individual flakes are lined with irregular, dark mark-
ings which are the result of having been stamped in contact with other flakes.
Although the flakes are very thin (in the order of I micron)_> they are opaque to
strong transmitted light.