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He considers it to be a double silicate of copper and calcium, having the formula 4 Si O2, Ca O Cu O; the iron being, as might be expected from its small amount, an accidental impurity. ancients, he considers, obtained it by fusing together roasted copper ore with sand and lime.

The

Fouque states that he has obtained a blue colour of a similar composition in a crystalline condition; the crystals were dichroic, appearing of a deep sky-blue tint when seen by surface reflection, and of a pale rose tint when seen through the edges. There are a few practical difficulties in its preparation, which consist in heating to a bright red heat a mixture of its component parts; if the heat becomes too high, then the blue colour disappears and only a green coloured glass is obtained. The blue pigment thus obtained is said to be quite permanent; it remains unaffected when boiled with sulphuric acid, soda, or lime, or any other alkali or acid; and it is unaffected by sulphuretted hydrogen.

Peligot's Blue is a similar preparation made by fusing together 73 parts of silica, 16 parts of copper oxide, 8 parts of lime, and 3 parts of soda; the temperature must not be allowed to exceed about 800° F., or the colour will be changed from a blue to a black; if the soda was omitted in this process, the results would be nearer those of the original blue. Peligot's blue has not come into use.

CÆRULEUM (2).-Messrs. Rowney & Co. have offered to artists a fine light blue of a greenish tone, for which they adopt the name cæruleum; it is a compound of the oxides of tin and cobalt and is a fairly permanent pigment. The method of producing it has never been published, but, possibly, it is made by preparing a solution of stannate of soda and precipitating this with a solution of cobalt nitrate; the precipitate will consist of a mixture of the oxides of cobalt and tin; the precipitate is heated to a bright red heat, when the blue pigment will result. Another method is to mix together solutions of tin and cobalt, and precipitate with soda; after washing free from the alkali, the precipitate is heated as before. By using silicate of soda as the precipitating agent, so as to obtain a precipitate containing silica, tin, and cobalt, a fine blue could be obtained.

Several other bodies of a blue colour have been suggested for use as pigments, but, partly on account of their greater expense, they have not been able to compete with ultramarine or Prussian blue as pigments; hence, their use has been either limited or abandoned. One may be mentioned.

Manganese Blue.-Some years ago Bong gave a description of the process of making manganese blue, which resembles very

much that of ultramarine. He gives the following mixtures which may be used:-(1) 3 parts of silica, 6 parts of soda ash, 5 parts of calcium carbonate, and 3 parts of manganese oxide. (2) 3 parts of silica, 3 parts of manganese oxide, and 8 parts of barium nitrate. (3) 2 parts of kaolin, 3 parts of manganese oxide, and 8 parts of barium nitrate. In each case the mixture is heated to a red heat in an oxidising atmosphere. Iron must not be present in the ingredients. By varying the proportion of manganese the intensity of the blue can be varied, but not its tint; on the other hand, by increasing the proportion of alkali and silica the blue becomes more violet or green.

ANTIMONY BLUE.-Boettger discovered in 1871 a blue compound of antimony. Sebor has lately described a method for its preparation. Sulphide of antimony is dissolved in strong hydrochloric acid, and the solution filtered through asbestos. The solution is heated to the boiling point, and then mixed with a concentrated solution of potassium ferrocyanide. A small quantity of potassium chlorate or of nitric acid is added, and the mixture boiled. The blue pigment formed is filtered off, well washed, and dried. In this way a very deep fine blue pigment is obtained. A pale blue pigment can be prepared by mixing solutions of antimony chloride and potassium ferrocyanide and then adding a large excess of water. Antimony blue contains

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It resists the action of acids, in which respect it resembles the Prussian blues; cold solutions of caustic soda, or caustic potash, or ammonia, have no action on it, but hot solutions decompose it. It is insoluble in oxalate or tartrate of ammonia, in which respect it differs from Prussian blue.

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CHAPTER VII.

BROWN PIGMENTS.

THIS group of pigments is a small one-umber, vandyke-brown, sepia, manganese brown, Cappagh brown, and one or two others of little importance complete the list. Most of these browns are natural pigments.

UMBER.

Probably the most important of the brown pigments is umber. It is an earthy pigment closely resembling the ochres and siennas in its composition and properties; in fact, these three pigments form a natural group of yellow to brown colours having the ochres at one end of the scale and the umbers at the other, while between the two extremes it is possible to find all or nearly all the intermediate tints or shades.

Umber is found native in many places; as in Derbyshire, Devonshire, Cornwall, Wales, &c., in this country; in France; in Italy; and in many localities in America. The finest umber comes from Cyprus.

It is found in veins and layers of varying thickness in rocks of all geological ages, and from which it has, in some cases, been derived by decomposition. At Ashburton, near Dartmoor, the umber is found in a layer of from 20 to 30 feet in thickness, overlying the bed rock, which is a dolomitic limestone containing some manganese and iron and from which it has been formed; above the umber there is an overburden of soil.

Umber varies somewhat in hue from a reddish-brown to a violet-brown, the former hue being characteristic of the Derbyshire umbers, while Turkey umber (which comes from Cyprus) has a warm violet-brown hue. This umber owes its trade name to the fact that it was imported through Constantinople, and its real source was at one time not properly known.

Umber is sold in three forms-raw lump umber, raw powdered umber, and burnt umber.

Raw lump umber is the pigment just as it is obtained from the

mines. The method of mining will vary somewhat according to the varied conditions under which the umber is found. At Ashburton it is mined by taking off the overburden of soil, digging square pits until the bed rock is reached, and lining the pits with timber as the material is removed. When the bottom is reached these timbers are removed and fresh pits sunk. A similar method is probably in use for mining other deposits of umber.

Powdered raw umber is the lump umber ground and levigated in the same manner as ochres are treated.

Burnt umbers are the raw umbers calcined at a red heat in a furnace, by which treatment the colour becomes darker and warmer; the change which occurs is similar to that which ensues when ochres and siennas are calcined (see pp. 112, 144, 150).

COMPOSITION AND PROPERTIES OF UMBERS.— Umbers have, as has already been stated, a great resemblance to the ochres and siennas in their composition; but they contain more manganese, which probably accounts for their darker colour. The following analyses of various colours will serve to show the average composition of these pigments:—

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No. 1 is an analysis of Cyprus umber; this sample had a dark, warm brown tint.

No. 2 is an analysis of crude Derbyshire ochre; this sample was of a soft character and of a reddish-brown tint.

No. 3 is an analysis of a sample of umber, probably of English origin. This sample had a yellowish-brown tint. The above analyses were made by the author.

No. 4 is an analysis made by J. J. Beringer (quoted by Frecheville*) of the umber from Ashburton, already referred to; and the next shows the composition of the dolomitic limestone from which it is derived.

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The decomposition has probably been brought about by the dissolving action of carbonated water on the calcium and magnesium carbonates, so as to leave the iron, manganese, and silica behind to form the umber; the amount of limestone which must have been disintegrated to form deposits of umber, 20 to 30 feet in thickness, must have been enormous, as but little of the calcium and magnesium remains behind in the umber. The umber found at Veryan (near Truro), Milton Abbot, and other places in Cornwall is undoubtedly formed from limestone rocks in a similar manner.

Umbers are pigments of a warm brown colour, varying in hue' from yellowish to violet-brown. By calcining, the colour is rendered darker and warmer. As pigments, they work well in both oil and water, and they can be mixed with all other pigments without any change occurring. They are perfectly permanent, being unaffected by all the ordinary conditions to which pigments are exposed. Umbers, therefore, meet with extensive use among all classes of painters.

Umbers are not readily attacked by acids, but prolonged digestion with strong hydrochloric acid dissolves the larger proportion of the umber, forming a brownish-yellow solution containing iron, alumina, manganese, and lime; the silica and the barium sulphate remain undissolved. The metals may be tested for by the usual analytical methods. Caustic soda has no action on umbers.

ASSAY AND ANALYSIS OF UMBERS.-Umbers may be assayed for colour or hue, colouring power, covering power, and similar properties by the usual methods.

A chemical analysis of umbers is rarely required, as they are rarely, if ever, adulterated, except possibly a dearer umber by a Trans. Roy. Geolog. Soc., Cornwall, xvii., p. 217.

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