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:—
| Per cent. | |||
| Water given off at 212° F. | 4 | to | 65 |
| Water in combination | 5 | to | 11½ |
| Silica | 4½ | to | 29½ |
| Manganese dioxide | 7 | to | 27 |
| Ferric oxide | 6 | to | 36 |
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 viâ 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.