PIGMENTS 99
Bone Black (animal black, drop black) (see also Carbon Black and Ivory
Black) is made by charring animal bones in closed retorts; usually bones from
glue stock, boiled to remove fat and glue, are used. Bone black is blue-black in
color and is fairly smooth in texture (see Beam, p. 131). It is denser than carbon
or lamp black. It contains about 10 per cent carbon, 84 per cent calcium phos-
phate, and 6 per cent calcium carbonate. Although the calcium compounds (ash)
in it have no color value, they improve the working quality and give a superior
black. Microscopically, it is also quite different from lamp black; the particles are
coarser and more irregular in shape and size, many of them being transparent
and some brown. Merwin, who has described some optical properties of ivory
and bone black, says (p. 514): 'In charred bones a small amount of dark carbo-
naceous matter is held in the most minute subdivision throughout a large amount
of calcium phosphate. Grains 5 fj, in diameter transmit an appreciable brownish
color and appear practically homogeneous optically. Their apparent refractive
index, when their pores are filled with oil, ranges from about 1.65 to 1.70; thus
the desirable characteristics of slight diffusing power and effective absorbing
power are combined.' Bone black sold under the name, 'ivory black,' is a favorite
among artists today. It works well in water color.
Bone White (bone ash), which is made by calcining animal bones, is composed
chiefly of tricalcium phosphate, Ca3(P04)2 (85 to 90 per cent); calcium carbo-
nate and minor constituents make up the rest (see Thorpe). Ash from the bones
of different animals varies little in composition. Bone white is a grayish white
and slightly gritty powder. In mediaeval times, it was used on paper or parch-
ment to give it tooth or abrasive quality to receive the streak of silver point
(Thompson, The Materials of Medieval Painting, pp. 94-95).
Brazil-Wood is a natural red dye from the wood of Caesalpinia braziliensis.
That used in the Middle Ages came from Ceylon. It was known and called
'brazil' many centuries before the discovery of the country, Brazil (Thompson,
The Materials of Medieval Painting^ p. 116). Pernambuco wood, Caesalpinia
crista, from Jamaica and also from Brazil has about twice as much coloring
matter, and various other trees of the family, Leguminosae, yield brilliant dye-
stuffs. The interior of the live wood is light yellow, but changes rapidly to deep
red on exposure. Brazil-wood extract is made by boiling the finely chipped wood
with water and by concentration of the liquor in vacua. The leuco compound is
brazilin, CieHuOg, which forms, on exposure to air, brazilein, CiGH^Os, which is
deep red to brown in color (Co/our Index ^ p. 297).
Brazil-wood lakes, which are prepared with different mordants, vary in color
from bright cherry to deep red and are sold under a variety of names. One method
formerly used was to extract the chips with hot alum solution, followed by pre-
cipitation with lye (Thompson, The Materials of Medieval Painting^ p. 118; see
also Perkin and Everest, pp. 627-628). These lakes are insoluble in water and in
alcohol but are partially soluble in alkalis, giving them a brownish red color.