Asphalt or Bitumen.—These names are applied to a variety of black or brown resinous matters found in many parts of the world in a mineralised state, though derived originally from organic sources. The “Bitumen of Judæa” is supposed to be found on and around the Dead Sea, but the bulk of the product going by that name really comes from Trinidad. All kinds of asphalt have a pungent and peculiar smell, melt at a low temperature, are very combustible, and while dissolving in turpentine, and more readily in coal tar naphtha, are insoluble in water and in alcohol. Very little asphalt is now used as a pigment, but it continues to find a limited application in varnish making, notwithstanding the tendency of varnishes containing it to suffer from minute cracks with the lapse of time.
Bistre.—This pigment is used exclusively in water-colour painting, for which purpose it affords a fine warm yellowish tinted brown. It is of vegetable origin, being prepared from the soot which is deposited in the flues leading from fireplaces which consume wood fuel. Every wood, however, does not afford an equally good sample of bistre, and beech occupies the foremost rank in this respect. The brightest and blackest soot is selected, and after careful grinding and sifting through a very fine sieve, it is repeatedly stirred up, for several hours at a time, in a series of clean hot waters, the object of which is to dissolve out all traces of tarry and other soluble matters, which very seriously affect its permanence, being oxidised on exposure to air and light, and thus weakening the tint. The washing is therefore a matter of the very first importance. The solid pigment is allowed to settle out of each wash water, and is collected and dried, being mixed with a small proportion of gum water to give cohesion. The drying is effected in a stove room.
Bone Brown.—This unimportant pigment is simply underdone bone black (see [p. 6]), and is obtained by stopping the calcination of the bones at a point which falls short of thorough charring. In consequence it contains a proportion of unaltered animal matters, which sooner or later may undergo decomposition, and prejudicially affect the painting.
Cappagh Brown.—A mineral pigment which is only a variety of umber, and may best be described under that head (see [p. 105]).
Cassel Earth.—Another name for Cologne earth, q. v.
Chicory Brown.—This vegetable pigment is rich-coloured but lacks permanence. It is prepared by calcining roots, such as those of chicory, in vessels to which air is not admitted, from which then results a fine brown powder. This is boiled in water, and the solution is evaporated to dryness, yielding a brown pigment, which, being soluble in water, is sometimes employed by water-colour artists.
Cologne Earth.—This material, which is also known as Cassel earth or Rubens brown, is an earthy carbonaceous substance, probably derived from the decomposition of lignite or brown coal, readily undergoing combustion without emitting flame or smoke. Large deposits of it are worked in the vicinity of Cologne, whence its name. It is blackish-brown in colour, smooth and crumbling to the touch, and very light. To remove soluble impurities it is subjected to several washings in water, and then collected, mixed with a little gum-water, and dried in small moulds. The colour is used by artists, but is very variable in composition and uncertain in durability.
Manganese Brown.—One of the most durable brown pigments used by the Romans is found to be oxide of manganese, which discovery has led to the proposal to prepare the binoxide of that metal as a brown pigment. The method suggested is as follows:—
The protochloride of manganese, derived from the manufacture of chlorine, or the protosulphate resulting from the calcination of the protoxide with iron sulphate, is dissolved in warm water (85°-105° F.); to this is added a sodium hypochlorite solution, or a solution of potassium hypochlorite containing a small proportion of carbonate of soda, the addition being continued until the precipitated manganese binoxide ceases to change colour, marking the completion of the oxidation. The supernatant clear liquor is drawn off, and the precipitate is washed first with acidulated water (containing 2 per cent. of sulphuric acid), and then with pure water till all trace of the acid is removed. The dark-brown impalpable powder of manganese binoxide is stove dried, and forms a permanent and safe pigment with good covering power.
Mars Brown.—One of the products obtained by the calcination of Mars yellow (q. v.) at various temperatures and under different conditions is a full-tinted and durable brown due to sesquioxide of iron. Another method of preparing it is from alum, sulphate of iron, and chloride of manganese. In either case the pigment is not superior to umber or oxide of iron, while it cannot be produced as cheaply.