226 PAINTING MATERIALS
gradually acquires a thin, green patina composed of the basic salts of copper.
These are formed as a result of the corrosive action of atmospheric carbon dioxide
and sulphur dioxide. The patina may last for centuries, under these conditions,
even when the metal is in sheet form. When it is buried in the ground, however,
corrosion is fairly rapid, particularly if the metal comes into contact with saline
waters. Although copper is not acted upon quickly by dilute mineral acids, since
it is below hydrogen in the electromotive series, it is acted upon by acids under
oxidizing conditions. Some organic acids, like acetic acid, corrode it slowly. The
fatty oils and fats also attack copper slowly but the action does not bear any
simple relation to the acid content of the oil. C. W. Volney (Mellor, Comprehen-
sive Treatise, III, p. 102) found that several natural fatty oils, including linseed
oil when in contact with copper, dissolved fairly large proportions of the metal.
Individual oils have different effects on the copper surface: some leave it bright;
others tarnish it. The oil itself turns green. It has been observed that spirit-resin
varnishes in contact with copper or brass turn green (' green drip '). Painting on a
copper panel has been observed to have a green-stained, oil-resin film between
the metal and white ground. This indicates a slow chemical reaction between the
copper and the oil or resin while the latter was still plastic or semi-fluid.
Known in prehistoric times, having a continuous history which is much in-
volved with that of bronze (a copper-tin alloy), copper itself was not much used as
a support for paint until it became cheap and plentiful in sheet form. That was
probably in the XVI century (see also Metals). At about that time, also, its use
for intaglio printing began to be exploited and it has since been the principal
plate material of etchers and engravers.
By the XVII century mention of the use of copper as a paint support had got
into the literature. The voluminous MSS of De Mayerne (Berger, IV, 416)
include a reference to painting with oil on copper and other metals. The Spaniard,
Palomino, writing in 1724 (Museo Pictorico, II, 44; see Berger, IV, 83) says that
copper panels are to be grounded with the same oil preparation as that used for
panels of wood. He points out, also, that the smooth surface of the metal will not
give a good bond unless it is first rubbed with garlic.
Cotton is the seed hair of the cotton plant (Gossypium); it consists of a single
hair-like cell which, when fully ripe, is flattened and twisted. The length of the
cotton fibre varies from 2 to 5.6 cm, and the diameter from 0.0163 to 0.0215 mm.
The walls are thick and the lumen or central canal is broad, giving the fibre the
appearance of a collapsed, twisted tube (see figure 2, p. 231). Cotton is over 90
per cent cellulose and is one of the most important sources of that material.
Like wool and linen, cotton was being made into clothing in prehistoric times.
It has been for thousands of years the staple fabric of the Orient. The plant grows
generally in tropical and sub-tropical regions, more than half the world's normal
supply coming from the United States. It is also grown in British India, Egypt,
Russia, and Brazil. Sea Island cottons, grown on islands off the southeastern