supports	227
coast of the United States, are the best in quality; Egyptian cotton is next.
Cotton fibres, being nearly pure cellulose, are readily affected by acids and by
moderately strong oxidizing agents but alkali hypochlorites, in dilute solutions
and at ordinary temperatures, have little effect on them. In spite of its long history,
cotton has been used comparatively little as a support for paint until recent times
(cf., Linen). Particularly in the XX century has it been made into a commercially
prepared canvas. As yet it has not acquired the reputation for permanence that
linen has.
Cradle is the term applied to a wood structure fixed to the back of a panel
painting to prevent it from warping with changes of humidity. It consists of
narrow wood strips having slots and being joined with glue to the back of the panel
parallel to its grain. Transverse strips ride freely in the slots and, if conditions are
favorable, keep the panel flat but do not interfere with the normal expansion and
contraction, across the grain of the wood, with changing humidity conditions.
(For illustrations, see Thompson, Tempera Painting, pp. 11-12, and De Wild,
p. 90. See also Wood, deterioration and treatment.)
Esparto or Esparto Grass (Stipa tenacissima) is a kind of spear grass used,
particularly in England, for the manufacture of paper (see also Paper). The
plant has long leaves in which the fibres are strong and flexible. To produce a
pulp from the grass, it is boiled in a solution of caustic soda.
Fabrics. As supports for painting, fabrics may be considered according to the
weave and also according to the thread and to the origin of the fibres. Animal
fibres (see Fibrous Substances) may be divided into those which come from animal
hairs (see Wool) and those which come from insects, i.e., the silk worm (see
also Silk). The animal fibres may be identified by their continuous cylindrical
structure as seen under the microscope, by chemical identification of their protein
content, and by their characteristic odor when burned.
Among the vegetable fibres are three general categories. Vegetable hairs
include cotton—seed hairs from various species of Gossyfium—fibres of 'cotton
trees,' Bombax cottons, vegetable silks, seed hairs of Asclepiadaceae_y etc. The
second group is made up of bast fibres from the stems of dicotyledonous plants.
Further division of this group would begin with flax and flax-like fibres, hemp,
Gambo hemp, sunn hemp, and yercum fibre. After these come the Bohmeria
fibres, ramie, and China grass. Next come jute and jute-like fibres, then the coarse
bast fibres, and, finally, basts, including linden and other woody materials. The
third group includes fibre bundles of monocotyledonous plants. First are leaf
fibres, including Manilla, pita, sisal, Mauritius hemp, New Zealand flax, bow-
string hemp, and esparto. Next are stem fibres such as tillandsia, fruit fibres such
as coir, and, finally, paper-making fibres—straw, esparto, bamboo, wood, paper-
mulberry, etc. Out of the large range of these categories only a few fibres have a
marked place among the paint supports that have been used or are used now.
These are the animal fibre, silk, the bast fibre, flax, and the seed hair, cotton.