236 PAINTING MATERIALS
to oxidize or to rust is also well known. Although the art of coating copper vessels
with tin reached a high standard in Roman times (E. S. Hedges, p. 207), the prac-
tice of coating sheet iron with tin (tin plate) was not general before the XVII
century. The coating of iron with zinc (galvanizing) probably dates from experi-
ments of Melouin in 1741 (see Hedges, he. cit.}. Sheet iron and tin plate appear
to have been used only to a limited extent as a support for paintings. It may be
seen more frequently, however, as a support for miniatures, coats of arms, and
signs. Cennino Cennini's painting on iron with oil (ed. Thompson, p. 57) is prob-
ably not painting of an iron panel but painted design on an iron object.
Ivory. For commercial purposes the tusks of the walrus, the hippopotamus, and
other animals are considered to be ivory but that, in its strict sense, means the
material of the tusk of an elephant. This is a tooth and the two tusks are the upper
incisors. They grow during the life of the animal, forming themselves out of phos-
phates and similar substances. Structurally, they lie in layers, the one inside being
the last produced. Ivory is very dense; its pores are close and compact and they
are filled with a gelatinous solution. Arcs of slightly different color show in cross
sections of the tusk. These arcs intersect to make lozenge-shaped spaces in which
lie innumerable minute tubes closely placed and radiating outward in all direc-
tions. The marking in cross sections is characteristic and is often used to distin-
guish ivory from materials used for its imitation. Most ivory comes from Africa
but some is Asiatic and a certain amount is still derived from the remains of pre-
historic animals in Siberia. Unlike bone, ivory can be worked immediately.
This has made it a favorite material for carving from neolithic times. It was
cut and fashioned and often stained long before it came into use as a support for
paint. Lucas (p. 310) reports that in ancient Egypt it was used as a ground for
writing. By the days of Greek ascendancy it was evidently a common practice to
paint on ivory, for Pliny (XXXV, 149) mentions it quite definitely in that con-
nection. It is probable that the practice continued, though mention of it is rare.
Theophilus (chap. CX) speaks of ornamenting ivory with gold leaf, and Pierre
Lebrun, Recueil des Essaies des MerveilZes de la Peinture, 1635, chap. XXXVII
(Merrifield, II, 820-821) says, * Ivory must be washed with the liquid which is
found under horse dung, for the colors can not be applied without this secret and
invention.*
Ivory, cut into thin slices, has been much used as a support for miniature
painting. It became popular in England and France for that purpose in the XVIII
century because it was the most suitable material for painting with transparent
colors. Sometimes curved slices of ivory were flattened out by hydraulic pressure,
making unusually large sheets. Some of these, however, have curled and cracked
with age (see Encyclopaedia Britannica, I4th ed., on Miniature Painting).
Jute is derived from several species of Corchorus, especially in India and China,
where it has been cultivated for centuries. It was first introduced into Europe in
the latter part of the XVIII century. The commercial fibre is 4 to 8 feet long but