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suited from the action of volcanic fires and need no further treatment than fine
grinding and mixing with lime. Pozzolanic cements are slow in hardening but have
a considerable ultimate strength. They have been used since the time of the
Romans who were well acquainted with their properties. The term ' pozzolanic *
has become broadened in its use to apply to those materials which may not be
cementitious but which will combine with hydrated lime at ordinary temperatures
in the presence of water to form stable, insoluble compounds of cementitious value.
Artificial pozzolanic materials may be derived from pounded bricks and tiles,
burnt clay, or granulated blast-furnace slag. Pozzolanic cements are very similar
to Portland cement in chemical composition; both set by the formation of calcium
aluminum silicates of complex chemical structure.
Ramie} Rhea, or China Grass are names given to a plant of the nettle family,
Boehmeria nivea, which grows wild within 36 degrees of the equator and has been
cultivated in China, Assam, and Java for centuries. The bast fibre has been suc-
cessfully cleaned only by hand and can not be spun so easily as wool or cotton. In
China it is made into grass cloth by hand, but is not manufactured by machinery
anywhere. It has more than three times the tensile strength of linen or cotton,
has a very pure color, a good luster, fineness, and resistance to climatic conditions.
Relining Canvas is used in the backing of an old painting on fabric. A new
piece of fabric, the relining, is attached to the reverse side of the old by means
of an adhesive (see also Fabrics, conservation and treatment). The purpose is
usually to strengthen the support, but at times this treatment is applied where
the ground film has become loosened from the old fabric.
Silk is the natural product of certain moths. There are four or five main groups
of these, some cultivated and some wild. In external appearance silk is a solid
thread resembling a glass rod, but the fibre is really composed of three layers sur-
rounding a tube (see also Fibrous Substances). The tube is filled with a fatty
matter which helps to preserve flexibility. No textile fibre has a larger proportion
of waste than silk. The fibre is a double filament and, prior to boiling off or degum-
ming in a hot solution of soap, it is harsh and relatively lacking in luster. In the
degumming about 30 per cent of the weight is lost. The microscope shows the
double filament irregularly coated with masses of sericin, but after boiling off it
appears as a shining, cylindrical, solid rod. Strong acids dissolve silk but weak
acids, such as tartaric and acetic, are absorbed and improve the luster. Dilute
alkalis do not affect it so much as they do wool, but hot concentrated solutions
rapidly dissolve it. Ammonia dissolves the sericin but attacks the fibroin very
slowly. Silk has great absorptive power for dyestuffs. The general method of
weighting silk is by means of solutions of metallic compounds, especially tin.
This results in a great loss in strength, especially upon exposure to sunlight.
The most striking physical property of silk is its luster. It is a highly absorbent
fibre and readily becomes impregnated or wetted with water. It stands a higher
temperature than wool without receiving injury. It can be heated to 110° C,