PAINTING MATERIALS

Bitumen (see Asphaltum).
Black Chalk, a bluish black clay containing carbon. (See also p. 285.)
Black Lead, the material used for 'lead' pencils, has no relation to lead metal,
nor is it a lead compound; it is a common term for the mineral, graphite (see
Graphite), or a mixture of clay and graphite which is more useful for writing
purposes. The confusion in terminology arose from the color similarity of graphite
and lead for marking purposes.
Blanc Fixe (see Barium White).
Blue Bice (see Blue Verditer).
Blue Verditer (blue bice) is a name now given to an artificial basic copper
carbonate, 2CuC(VCu(OH)2, which is similar in chemical composition to the
mineral, azurite. This pale greenish blue pigment is little used today, but can
still be obtained from some artists' colormen. Recipes for making artificial copper
blues or 'azures' are very old (see Thompson, 'Trial Index for Mediaeval Crafts-
manship,' p. 415, f.n. 7). The more practical of these call for the addition of lime
or potash and sal ammoniac to a soluble copper salt like blue vitriol (copper
sulphate). Microscopically, blue verditer may be seen as tiny, rounded, fibrous
aggregates, even in size, highly birefracting, and blue by transmitted light. It is
similar in color to finely ground azurite. The artificial copper blues have not
been credited with great permanence, and Thompson says (The Materials of
Medieval Painting, p. 151) that they had a tendency to revert to green through
the loss of their ammonia content (see also Beam, p. 93). According to Laurie
(The Pigments and Mediums of the Old Masters, p. 43), the manufacture of blue
verditer seems to have been carried on in England in large quantities at one time.
Thompson states (loc. cit.} that 'the artificial blues from copper are probably
more significant in medieval painting than all the rest (of the blue pigments) put
together.' They were the best cheap substitutes for the more expensive azurite
and ultramarine. Laurie (op. cit., p. 122) identified this pigment in various English
illuminated manuscripts (Coram Rege Rolls) of the early XVII century. He re-
cords, in another place (New Light on Old Masters, p. 42) that it was used through-
out the XVIII century and that the 'Madame de Pompadour' by Boucher, in the
National Gallery, Edinburgh, is painted with it.
Bole (Armenian bole, red bole) is the name frequently given in the arts to
clay, either white or colored. White bole is about identical with kaolin (see China
Clay). Red bole is a natural, ferruginous aluminum silicate which was originally
found in Armenia but now elsewhere in Europe. It is similar to ochre in compo-
sition but is softer and more unctuous, and because it is capable of receiving a
high- polish, it has served since early mediaeval times as a ground for gilding.
It is obtainable today under various names such as 'gilders red clay* or *red
burnish gold size.'
Bone Ash (see Bone White),