Brown colouring matters are obtained from all three kingdoms—the animal, the vegetable, and the mineral—but in greatest abundance from the last named. The natural mineral brown pigments afford almost every variety of tint, and being largely composed of silica and metallic oxides they are remarkably permanent.

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.

Prussian Brown.—An artists’ colour known by this name is prepared from Prussian blue, but as it has no superiority over Vandyke brown or umber, and is higher priced, it is not in general use. It consists essentially of carbon and ferric oxide, resembles bistre in tone, and possesses durability and good covering powers. The operation of calcining the Prussian blue should be conducted slowly, and is best performed in a closed vessel, though it may also be done in the open. The pieces of blue should not be larger than a hazel nut. They soon split, scale off, and become red, when the heating should cease. On breaking the cooled particles they will show a patchy coloration varying from yellowish-brown to black. On grinding, the mass assumes the desired brown hue.

Rubens Brown.—Another name for Cassel brown or Cologne earth (q. v.).

Sepia.—This is one of the few pigments derived from the animal kingdom. It is produced by several sea-inhabiting creatures belonging to the class called Cephalopoda, and more particularly by two members of the genus Sepia, known respectively as Sepia officinalis and Sepia loligo. A peculiarity of these cephalopods is that they are provided with what is commonly called an ink bag, in other words a gland or sac filled with a blackish-brown liquid, which possesses intense colouring power. The object of the secretion is the protection of the creature from pursuit by its enemies, a portion of fluid being discharged at will, and so obscuring the surrounding water that escape is facilitated.

For the sake of this pigment the cuttle-fish are sought after by fishermen in the localities frequented by the animals, notably in the warm waters of the Mediterranean. When the creatures are captured, their glands are carefully extracted and sun-dried so as to solidify the contents. In this state ink bags are sent into commerce. The colourman subjects the sacs to boiling in a solution of soda or potash, whereby the colour is dissolved out of the receptacle, and being filtered clear of all fragments of the animal tissue, is next precipitated by the addition of acid, collected on a filter, washed, and dried. It then forms an exceedingly useful pigment, having, according to Prout, the following average composition:—

It is remarkably permanent for an organic substance, suffering no alteration on being combined with other pigments, and withstanding the effects of exposure to air and light. Though slightly transparent, and not quite constant in tint, it possesses very great colouring power. Being of extremely fine texture it can be worked up equally well as an oil colour or as a water colour, but it is especially in the latter capacity that it forms an indispensable artists’ colour, and permits the production of a great range of shades and tints.

Ulmin.—The pigments grouped under this name are also of organic origin; but though they possess good colour, mix well, and flow readily from the brush, they lack the durability which is essential to their successful use. The following methods have been employed in their preparation:—(1) Fused caustic potash is digested in alcohol, and the liquor filtered and heated till a brown powder is thrown down, which is filtered and washed with acidulated water; (2) Waste cotton, peat, or brown coal, heated with an alkali; (3) Farinaceous matters carbonised by mineral acids.

Umbers.—These form a large class of natural earths of a brown colour, differing widely in the proportions of their chief constituents, but closely allied to the ochres and siennas in general composition, and owing their colour mainly to the presence of hydrated oxides of iron and manganese, the latter prevailing in the umbers to a greater degree than in the ochres and siennas, which consequently belong to the yellow group (q. v.).

Beds or veins of umber of varying thickness and extent are found in many places, especially in connection with magnesian limestone (dolomite). Apparently they are often derived from decomposition of this rock, perhaps due to the infiltration of carbonated water, which has acted upon the calcium and magnesium carbonates in the dolomite, and left the silica and the iron and manganese as oxides, forming the bulk of the umber. Usually these beds of umber are near the surface, though covered by an overburden of vegetable soil, and the operation of working them may be called quarrying rather than mining, being of a superficial and simple character, often only amounting to small pits.

As no umber is a definite body, but rather a mixture of various substances, so the composition of every kind is peculiar to itself, and very wide differences are noticeable. Even the same bed will not necessarily produce always the same class of umber. The following figures show the extent to which the proportions of the several ingredients may vary:—

Calcium carbonate is sometimes present to the extent of 2½ to 6 per cent., and at other times is quite absent, its place being taken by ½ to 1 per cent. of lime (calcium oxide); some of the English umbers contain about 2 per cent. of calcium sulphate (gypsum) in addition to the carbonate. Alumina may occur to the amount of 2½ to 12½ per cent., or may be wanting altogether. In a sample of Derbyshire umber analysed by Hurst there appears to have been over 30 per cent. of barium sulphate (barytes), which looks suspiciously like adulteration.

Almost every variety of shade may be found in umbers. The darkest and richest in colour—a warm violet-brown—is the so-called Turkey umber, mined in Cyprus, and formerly shipped vi Constantinople; this is of very fine quality and commands the higher price in the market. A reddish-brown Irish umber, known as Cappagh brown, obtained from the Cappagh mines in Cork county, is much esteemed among artists, both for water-colour and oil painting, and especially for the latter when it has been subjected to a preliminary desiccation at a temperature of about 170° F. Heated to the boiling point its colour changes to a rich red, resembling burnt sienna. Cornish umbers are of fairly good quality. Derbyshire umbers are poor, and incline to a reddish tint, besides being gritty. Sometimes they are adulterated with a little lamp black, which renders the tone more like that of Turkey umber, and thus deceives the unwary buyer.

There are three conditions in which umbers come into commerce: (1) as raw lump, being the mineral just as it is mined; (2) as raw powdered, when it has been ground very fine and levigated or washed in flowing water, whereby the particles get assorted according to their several degrees of fineness; and (3) as burnt, being the powder after it has been subjected to calcination in a closed furnace. Some umbers are so soft that they can be washed without any previous grinding, but this is not generally the case. The apparatus used in grinding and levigating is common to all pigments where these processes are employed, and will therefore be described once for all in a later chapter. The calcination is conducted at a red heat, and by this process the tint is made darker and warmer, but it must not be pushed too far or the pigment will blacken.

While different samples of umber present differences of tone and shade, from a yellowish to a violet brown, they are alike in being very durable and proof against the injurious influences of air, light, and impure atmospheres; ordinary acids and caustic soda have no appreciable effect. They mix well with other pigments without provoking any change, and are equally satisfactory as oil or water colours. They do not admit of much adulteration, except in the substitution of an inferior grade for a superior one, and possibly the addition of barytes as a make-weight.

Vandyke Brown.—What the original brown used so much by the great Van Dyke was no one can tell. The pigments now sold under the name of Vandyke brown are of varying composition, some being simply mixtures of red oxide of iron and lamp black, others are natural earthy substances after the character of Cologne earth, and others again are artificial products of the partial carbonisation of vegetable matters, such as cork waste. As it is uncertain what was the composition of the original Vandyke brown, no standard of chemical purity can be established.

Probably the most general sources of Vandyke brown are red oxide and lamp black, and the quality of such a pigment will chiefly depend on securing a good black, as any traces of unburned oily matter will make the paint difficult to dry. Almost any variety of shade can be produced by adjusting the proportions of lamp black and red oxide, with sometimes the addition of a little ochre. The pigment made in this manner forms the staple brown paint for industrial application, mixing well with oil, and being of a durable character, but it does not mix so well with water.

Vandyke browns of the Cologne earth type, from earthy lignites and peaty matter, are much used in and around the localities where they are produced, and entail nothing more than grinding and levigation to fit them for the market. They are in general best adapted to water-colour painting.

Warm and slightly reddish tints of Vandyke brown are obtained by the partial carbonisation of ligneous material, in other words by subjecting cork and bark waste to moderate calcination in closed retorts. These mix equally well in water or oil.

All varieties of Vandyke brown are stable pigments, without any disturbing influence when used in admixture with other colours, and quite proof against any change on exposure to air and light. Next to the umbers they are the most generally useful browns.