SUPPORTS 223
finished with a ' liner/ a sheet of paper of better quality than the fibre in the
interior.
One type of fibre building board has aroused a considerable interest among
painters of the present. It is of the compact, homogeneous type and is made by
the so-called * Masonite process' from wood chips of the long-leaf yellow pine.
The wood fibres are torn apart by exploding the chips with high-pressure steam.
The natural wood lignins are used to cement the wood fibres together again on
large plattens with the aid of heat and pressure. The finished fibre board, which is
chestnut brown in color, has one smooth side and one rough side; this is caused by
imprints of the wire screen on which the board is formed. The rough side may be
coated with a gesso ground or it may be primed with a white paint. The wire
mesh imprint gives the surface a texture somewhat similar to that of coarse
canvas. The Masonite product is hard and dense; it does not bend or warp easily.
It is prepared in sheets four feet wide and up to twelve feet in length and from
one eighth to one half inch thick. Three general types of the Masonite product
are available: a thick, porous board for insulation purposes, a semi-hard board,
and a hard or * tempered * board.
Artificial building boards have one advantage: they are homogeneous in
physical properties in all directions. They have no grain and, hence, are not
subject to unidirectional shrinking and swelling. In large sheets, unless properly
supported, they are liable to twist and to warp from their own weight.
Besides the artificial fibre boards, there are two types of wall board that are
made almost entirely of inorganic or mineral materials. The first of these is made
chiefly from asbestos and Portland cement in varying proportions. A little sulphite
pulp for binding purposes and some pigment may be added. These asbestos boards
are hard and dense. Weight and brittleness are their chief disadvantages; they are
liable to fracture much like glass. When secured to a wall or properly re-enforced,
however, they offer an excellent surface for the direct application of paint. The
second type is made with a gypsum filler and a paper liner. This is a plaster
substitute; it is fragile, easily broken, and, unless carefully supported and held
at the edges, it can not be carried about and handled. It is of little use as a paint
support unless it has been previously secured to a wall.
A third general class of artificial building board is a composition board made
with a core of thin wood laths (frequently redwood) which are held together edge
to edge with sodium silicate cement and a paper liner on both sides. On the outside
they are similar in appearance to the laminated fibre boards. Boards of this type
have little to recommend them for painting purposes.
Asbestos is a magnesium calcium silicate mineral which occurs in various
combinations as white, grayish masses of long, compact, silky fibres, flax-like and
readily separated. (See also Artificial Building Boards.)
Ash (Fraxinus). The European ash (F. excelsior) is a wood which grows
widely from England to Asia* The American white ash (.F. americana) is a little