PAINTING MATERIALS
sawed. Coniferous woods are used throughout, however, for the cheaper ply-
woods. Animal glue is perhaps the most extensively used adhesive in making ply-
wood but fish glues, casein, blood albumin, and, in special cases, sodium silicate
and artificial resin plastics may be used. Obviously, the strength, durability of the
adhesion of the plies, and behavior to moisture and water depend largely on the
character of the adhesive.
Plywood has certain advantages over ordinary lumber. The plywood panel is
stronger, in some respects, than a single board of the same thickness. Large ply-
wood panels are not likely to check, shrink, and warp so much as solid pieces of
the same size. They do not suffer so much from the grain characteristics of a solid
piece of wood. This is important since tensile strength, compression strength,
binding strength, and stiffness along the grain of the wood are twenty times more
than that across the grain. The main shortcoming of plywood is the liability to
failure of the adhesive between the plies. There are many poorly made plywoods
that come apart, layer by layer, after a short time. When made, however, with
synthetic resins, hot hide glue, blood albumin, or lime-casein adhesives and when
special care is taken in the selection of the woods, they are very durable and
lasting supports.
Poplar (Populus). Wood from trees of this genus are light in color and light
in weight. In Europe that of the white poplar (P. canescens) and the Lombardy
poplar (P.fastigiata) are the principal sources of timber poplar. Poplar wood is
soft, weak, fine-grained and fine-textured. It is diffuse-porous and the pores are
very small. The annual rings are fairly well defined but the rays are very fine and
are not visible without a lens. Poplar was frequently used in the Italian schools
for panel painting. Although poplar is sometimes known in the trade as ' white-
wood * this name is also used for the so-called yellow or tulip poplar (JLiriodendron
tuHpifera). Tulip poplar is frequently used when panel construction is called for
today. It is stronger and tougher than ordinary poplar; it is fine-grained and fine-
textured. The annual rings are indistinct and there is little or no figure in longi-
tudinal section. The heart-wood is frequently yellowish green but it may be
streaked both white and black. The wood is diffuse-porous and the rays are fine.
Portland Cement is entirely an artificial product but it represents the most
important branch of the modern cement industry. It is a hydraulic cement ob-
tained by burning a mixture of lime and clay and pulverizing the resulting clinker.
The clinker thus produced is mixed with 2, per cent gypsum and ground to aoo
mesh. The product is a greenish gray powder that consists of basic calcium sili-
cates, calcium aluminates, and calcium ferrites. When mixed with water it
solidifies into artificial rock. It derives its name from its similarity to Portland
stone.
Pozzolana is a cement derived from volcanic ash. Deposits of it are found in
Italy, near Naples (Pozzuoli), in the islands of the Grecian archipelago, and in
Germany on the Rhine. They are easily decomposable silicates which have re-