PAINTING MATERIALS
Iron Gall Ink is made from tannin or gallotannic acid which is derived from
oak galls. When this is combined with ferrous sulphate, a colorless compound,
ferrous gallotannate, is formed which develops a black color on exposure to air
because of oxidation to ferric gallotannate. Because 7 to 10 days are required for
complete oxidation, a dye or other provisional coloring matter is added to the
ink to give it immediate color. The ink ingredients are suspended in a solution
of gum and water (see Thorpe's Dictionary, Ink). It is uncertain just when iron
gall inks came into use, but apparently it was some time in the Dark Ages or in
very early mediaeval times (see Mitchell, p. 11).
Iron Oxide Red (see also Haematite, Indian Red, Light Red, Mars Colors,
Tuscan Red, Venetian Red). Ferric oxide in both its anhydrous (Fe2O3) and
hydrous (Fe2O3-«H20) forms has been used as a coloring material since pre-
historic times. It was formerly all derived from natural sources, but at present
much that is used is artificial in origin. The extensive deposits of iron oxide which
occur all over the earth vary widely in hue, depending upon the degree of hydra-
tion and subdivision. The anhydrous oxide is dark purple-red or maroon while
the hydrated varieties range from warm red to dull yellow, as in yellow ochre.
Iron oxide is a very stable compound; it is unaffected by light and by alkalis;
it is soluble only in hot concentrated acids; and the only effect of heat is to darken
the lighter colored varieties. Microscopically, it is moderately characteristic. The
natural forms are heterogeneous in composition and in particle size; in the darker
varieties, elongated and splintery, dark brown, lustrous particles of haematite
can be seen. In some varieties, the smaller particles are ruby-red by transmitted
light, similar to vermilion, but usually they are opaque and dense. The artificial
varieties are finely divided and homogeneous and have no very characteristic
optical properties. Distinction is difficult, even microscopically, between the finer
grades of natural iron oxide and the artificial varieties.
The iron oxide pigments have had such continuous use in all periods of paint-
ing and in all parts of the world that it is unnecessary to go into details concerning
their history and occurrence in paintings. Even today they are commercially
among the most important pigments. Both the natural and artificial varieties of
iron oxide are known by numerous names. Some names show the source; some
originally were applied to natural products but are now used for artificial ones;
others indicate some very special kind of preparation.
An excellent natural red oxide comes from Ormuz in the Persian Gulf and is
sold in large quantities under the name, 'Persian Gulf Oxide.* It contains about
70 per cent of Fe2O3 and 25 per cent of silica. The well known Spanish red oxide
contains, usually, more than 85 per cent of Fe2O3. These crude natural oxides
require only grinding and sieving to convert them into pigments. Finer products
are obtained by washing and levigation.
Italian Pink (see Pink).
Ivory Black, strictly speaking, is made by charring waste cuttings of ivory in
closed vessels and then grinding, washing, and drying the black residue. It is the