160 PAINTING MATERIALS
times takes on a greenish tone (reduction to chromic oxide) when exposed to
strong sunlight (see Eibner, Malmatenalienkunde^ p. 165). Like barium chromate,
this alkaline earth chromate is also sold as 'lemon yellow.'
Synthetic Pigments are those made by processes of chemical synthesis from
chemical elements or compounds. They may be inorganic, compounds of the
metals, or they may be organic, complex compounds of carbon, like the dyestuffs
of coal-tar origin. Some synthetic or artificial pigments, like Egyptian blue, white
lead, and verdigris, have been known since classical times or earlier.
Talc (soapstone, steatite) is a natural hydrous magnesium silicate, 3MgO-
4Si02*H2O, and is found as a soft stone. It is smooth and unctuous to the touch
because the cleavage is highly perfect and it powders to form thin, laminar
particles. It has properties like those of China clay, and in the arts is used for
similar purposes. Talc is white to grayish white in color. It is very inert and is used
commonly as a filler in paints and paper, in colored crayons, and for dusting.
Steatite or soapstone, which is a massive variety of talc, serves as a sculptor's
medium and for certain ornamental purposes. A fibrous form of talc from New
York State called 'asbestine' is widely used in outside paint films to increase
strength and weathering properties; it contains about 92 per cent magnesium
silicate (Gardner, p. 1250).
Terra Alba (see Gypsum).
Terre-Verte (see Green Earth).
Th&nard's Blue (see Cobalt Blue).
Tin Leaf and, to a smaller extent, Tin Powder were very early used to embellish
paintings. The metal was used in its own right or for imitating silver, or it was
lacquered yellow to imitate gold. Tin, which was one of the metals known to the
ancients, is soft and malleable; hence, it is easily beaten into leaf or foil. It is
superior to silver in that it does not tarnish and blacken with time. Mediaeval
recipes for the use of tin in imitation of gold are numerous. With a yellow varnish
to give it a golden glint, it was named * auripetrum! Many of the recipes for this
call for saffron (see Saffron) or other transparent yellow or red vegetable coloring
matters. Theophilus (pp. 31-33) tells how to beat out the tin foil on an anvil,
how to polish it, how to ornament letters and pictures in books with it (p. 41), and
how to imitate gold by coating it with glair mixed with saffron. Jehan Le Begue
(see Merrifield, I> 304) mentions that tin was used in powder form as well as foil.
Laurie says (Materials of the Painter's Craft, p. 203) that the famous Spanish
leather hangings were decorated with tin foil to give both the silver and the gold
effects. Much of the brassy gilding observed so frequently on Russian icons is
probably tin foil coated with yellow varnish. Thick tin foil has occasionally been
put on the backs of panel paintings and varnished over to make the wood im-
pervious to moisture and to prevent warping.
Titanium Dioxide (titanium white,' titanox'), TiO2, is the whitest and has the
greatest hiding power of any of the white pigments. The principal titanium ore.