250 PAINTING MATERIALS
difference in the density between spring- and summer-wood. There is practically
10 figure or stripe in longitudinal section. The hard-wood group, comprising the
fellow or pitch pines, is distinguished usually by deeper color, pronounced annual
•ings and by a stronger odor of the resin. Quarter-sawed, hard pine has a striped
tppearance. Pine wood, in general, is durable, does not warp badly, and is straight-
Drained. According to the records, pine was widely used in European panel
tainting, but in these records the term ' pine * probably includes woods from many
>ther conifers.
Plaster is the term given to the surface material, of a wall or building, which
las been applied as a plastic mass made by mixing certain dry materials with
vater and letting the mass set by drying, carbonation, or hydration. The term
plaster' is broadly applied to its three stages: the dry powder, the plastic mass,
>r the hardened surface. Although plaster may be made from various materials,
nodern usage confines the term to interior surfaces only. Many materials have
>een used in making it; all employ water for reducing the dry components to a
)lastic state. The active or setting components of plaster are commonly clay
vhich sets by drying, lime which sets by carbonation, and gypsum and pozzolana
vhich set by hydration. Inert materials, like sand and finely crushed stone, and
ibrous binders, like hair, straw, and jute, are important ingredients in plaster.
>and and other inert materials are necessary in preparing thick coats of lime
>laster since neat lime plaster shrinks and contracts on drying. This shrinkage is
ilmost entirely prevented by the sand grains when these grains are in contact
vith each other. Very fine sand does not allow a proper distribution of the lime,
ince the pores are too small to be filled properly with the particles of lime hydrate.
Hay plasters usually contain inert materials and fibrous binders to prevent undue
ihrinking. Gypsum and pozzolanic plasters are not so dependent upon these fillers
or shrinkage prevention and binding but may have them added for economy.
Plaster, since earliest times, has served as a support for painting and decoration.
The pigment may be incorporated directly in the wet plaster, as in true fresco, or
t may be applied to the dried plaster surface with an organic binding medium,
is in fresco secco.
The wall upon which plaster is laid must be specially prepared for the reception
md retention of the plastic material. A creviced structure through which the
plaster is forced and keyed was, in early times, built up with reeds and saplings
joined to joists with cords; wooden laths, either split or sawn, have been used
for hundreds of years; metal and wire lathing are now common. The durability of
plaster work very often depends upon the sturdiness of the foundation. A brick
and masonry surface is frequently rough and porous enough for the direct ap-
plication of plaster, provided the joints have been thoroughly raked out. It is
essential that the surface be not dry and porous because then water is sucked
from the plastic mass and poor setting results. The shrinkage and ultimate decay
of wood and other organic materials used as supports are often the beginning of