supports	1253
with the result that they bulge and crack. If plasters contain soluble salt impuri-
ties, such as calcium chloride or magnesium sulphate, they may effloresce with
changing humidity conditions. This may happen if sea sand or estuary sand is
used in the preparation.' Pitting ' or the appearance of small conical holes in the
surface of plaster is often the result of improper slaking in the preparation of the
mortar. After the plaster has set and hardened, unslaked particles of lime slowly
become air-slaked; they increase in volume and drop from the surface, leaving
tiny craters.
In the treatment of deteriorated or broken plaster, one of the first steps is to
provide a proper support if that is lacking. When plaster surfaces are not too
badly broken and disintegrated they can be repaired and filled up with materials
similar to those from which the plaster was originally made. Lime plaster can be
filled with fresh lime mortar or with plaster of Paris. When plaster is in bad
condition, A. Lucas (p. 185) recommends consolidation with melted paraffin wax.
Artificial resins and lacquers are finding a limited use for this purpose.
Plaster of Paris is the hemihydrated sulphate of calcium, aCaSOrHaO, and
is made by heating finely ground gypsum at a temperature around 145° C. When
mixed into a paste with water, it regains its water lost on heating and sets rapidly
to a moderately hard solid. If gypsum is heated to 200° C., all the crystal water
is expelled and the product will combine with water and set only very slowly.
The theoretical quantity of water necessary to set the plaster is about 18 per
cent of its weight, but, in practice, from 30 to 35 per cent is generally used. To
cut down the setting time glue, vegetable gums, or other colloidal matter are
added as retarders. Plaster of Paris expands slightly on setting and, hence, is
valuable for making casts and reproductions. The surface of set plaster of Paris
is not suitable for the application of paint because of its high absorbency. By
dipping plaster casts in melted wax, paraffin, or stearin, or by the application of
these in solvents, the pores are filled and the surface is made smooth so that dirt
will not adhere and the articles may be washed. Mixed with glue, plaster of Paris
sets to a much harder material which can be sanded and given an ivory-like
surface suitable as a ground for painting (see Gesso and Stucco).
Plywood and Veneer. Within recent years plywood has become an important
material in building construction. That now used is made from ' rotary * veneer
cut in thin sheets from a log of wood. Three and sometimes five or seven sheets
of the ply are glued together with the grain of each extending in a direction at
right angles to that of the adjacent sheet. Plywood can be recognized not only by
the laminations at the edge, but also by the wavy grain of the surface, particularly
in coniferous woods where alternate wide bands of spring- and summer-wood are
exposed by the rotary cutting of the log. Large sheets up to 8 X 15 feet in dimen-
sion are now available. Frequently, the 'core ^J or middle ply is thicker and of
cheaper wood than the surface plies. For furniture and interior paneling, the
surface ply may be of an expensive hard-wood like walnut or gum-wood, sliced or