56 PAINTING MATERIALS
ithout decomposition but at 170° C. it is rapidly disintegrated. It readily ab-
3rbs dilute acids and in so doing increases in luster. Luster is diminished by dilute
[kalis, however. Ammonia and soaps have no effect beyond dissolving the silk
lue or sericin. Silk absorbs metallic salts, particularly those of tin, and this
ehavior is made use of in the weighting of silk.
It is generally accepted that the silk industry originated in China, J. M.
latthews (Textile Fibers•, p. 2.42) says that historically it dates back to about
700 B.C. Tradition has it that in early history the cultivation of the silkworms
nd the preparation of the fibres were strictly guarded secrets known only to the
Chinese royal family. Gradually the industry spread throughout China but that
ivilization monopolized it for about 3000 years. In the early period of the Chris-
ian era sericulture was introduced into Japan. The Arabs acquired a knowledge
f the silk industry in the VIII century and it spread under Moorish influence to
pain, Sicily, and the African coast. In the XII century sericulture was practiced
i Italy, and in the XIII century, in France, but it did not become important
here until the reign of Louis XIV.
There is little evidence of silk as a paint support in Europe but it is common
ti the Far East and its history, traced in the paintings found, carries it back to
T'ang times (618-907). Probably before and certainly since then it has been the
hief material on which was executed the series of great scroll paintings made in
^hina and Japan. Painted silk was probably not often pictorial in the practice of
European workshops, though Cennino Cennini speaks of using linen or silk as if
:he two were co-ordinate (ed. Thompson, p. 103).
Stone. The three general types of stone, based on method of formation, are:
sedimentary, igneous, and metamorphic. Sedimentary rock varies, in its mechani-
:al structure, according to the degree of consolidation or compactness and
iccording to the materials of which it is composed. One of its most characteristic
Matures is a stratified structure because of its origin in sediments of eroded ma-
:erial that has been deposited under water in horizontal layers. Since it is strati-
aed, sedimentary rock cleaves readily; slates and shales are good examples. The
principal types are: sandstone, shale, limestone, and gypsum. Sandstone is
composed of grains of sand, varying from microscopic to large grain, and bound
;>y a cementing medium such as silica, alumina, carbonate of lime, or oxide of
iron. The color, depending on the amount of iron, varies widely—cream, red,
blue, etc. The grains of sand may be composed of mineral quartz, granite, basalt,
limestone, etc. Shale is a soft, brittle rock, actually compact mud, silt, or clay.
Limestone is a dense, compact rock composed mostly of carbonate of lime. It
is porous and is often filled with shells or fossils. Gypsum, called alabaster in
England, is hydrated sulphate of lime, deposited by precipitation from sea water.
As a rock, gypsum is soft, usually fine-grained, and readily cleaved.
Igneous, or primary, rocks are formed by solidification of molten masses from
within the earth. They are coarse- or fine-grained, depending upon the rate of