222 PAINTING MATERIALS
early as 1893, the making of sheet metal did not assume large proportions until
about a decade later.
Aluminum is prepared by the electrolysis of the mineral bauxite (A12O3'2H2O)
in a fused bath of cryolite. For this reason, the commercial product is of high
purity. Sheet aluminum is obtained by either hot or cold rolling. Modern alloys
of aluminum have a wide range of hardness and working properties.
Because of its very recent commercial history, few paintings are done on this
metal. Its light weight, however, has suggested its use as a paint support and as a
new support for the transfer of old paintings. Previous use in the automobile body
industry shows that it gives a suitable surface for the application of paint. Highly
polished or * bright-finish * aluminum is too smooth for satisfactory coating.
* Gray plate,' which is made by putting the aluminum through rolls that are
covered with a coating of aluminum metal particles and aluminum oxide, has a
better' tooth.' It can be sand-blasted or roughened in other ways to give a rougher
surface. Gray oxide coatings that are adherent and protective may be applied
chemically (see Edwards, Frary, and Jeffries, II, 471) and electrochemically. L.
McCulloch has published a method for giving aluminum a dead-white finish.
The aluminum is boiled for some time in a mixture of lime and calcium sulphate;
the coating formed is fine-grained, is adherent, and does not separate from the
metal on bending.
Artificial Building Boards. In recent years a number of artificial building boards
have been developed and are coming into extensive use in house construction and
in remodeling. Although some of these may be considered as lumber or plaster
substitutes, others have specific purposes. They may all be divided into three
categories:
I. Fibre building boards
a. Homogeneous type (either porous or compact)
b. Laminated type
a. Mineral building boards
a. Asbestos with cement
b. Paper liner with gypsum core
3. Composite board (wood core with paper liner)
Low-grade vegetable fibres are largely used for the manufacture of the fibre
boards. They may be wood-pulp, bagasse (crushed sugar cane residue), straw,
corn-stalk, or sawmill waste. The porous, homogeneous fibre boards are made
primarily for heat-insulating purposes, but that made from bagasse is also used
for interior wall finishing. The laminated type, made by joining several sheets of
thin cardboard with an adhesive, is used for wall boards almost entirely. Water
resistance is produced in most boards by incorporating rosin size before forming
and, in addition to this internal sizing, many boards have applied to them paints,
gums, oils, or waxes to further increase water resistance. Both starch and water-
glass are used as adhesives in making laminated boards. Frequently, both sides are