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Ammonia usually has a solvent action on the copper blues, forming a deep blue solution; by the tests here given copper blues can be readily distinguished from other blues.
The copper blues are rarely met with, their place having been taken by ultramarine and cobalt blue.
CÆRULEUM. Under the name of Cæruleum two pigments are known; one of these is, or rather was, an old one used by the ancient Egyptians in the decorations of their temples and tombs; this pigment was a very permanent one and of a fine tint of blue, but nothing is now known as to how it was made, although, lately, a French chemist has announced that he has made this colour in all its original properties.
The other pigment is a more modern one, of a fine tint, but of no great permanence. The two pigments will now be described as fully as is necessary.
CÆRULEUM.—The fine blue pigment found on the paintings and decorations of the ancient temples of Pompeii, Alexandria, Cairo, and other old cities testifies how largely it was used by the ancient Egyptians, who were, in all probability, the discovererg of it. So far as can be discerned, the paintings on these ancient temples are of as bright a blue colour now as on the day when they were painted, although they have been exposed to the weather for more than 1,000 years. There is no trace of its manufacture and use after the barbarian invasion of Italy.
The blue has been examined by Chaptal, Sir Humphrey Davy, Girardin, and others, but without any result, so far as its preparation was concerned; and, yet, if it could be produced at a low price it would be very extensively used by all classes of painters.
A more recent observer, Fouque, has examined this pigment, and in a memoir communicated to the Academy of Sciences of Paris, and published in the Comptes Rendus, 1887, pp. 108, 325, he describes the results of his analyses of the blue, and of his experiments for its production. Fouque gives the composition of this old pigment as Silica, Si 02,
63.7 per cent. Calcium oxide, Ca 0,
14:3 Copper oxide, Cu 0,
21:3 Ferric oxide, Fe, 03,
He considers it to be a double silicate of copper and calcium, having the formula 4 Si 0,, Ca 0 Cu 0; the iron being, as might be expected from its small amount, an accidental impurity. The ancients, he considers, obtained it by fusing together roasted copper ore with sand and lime.
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
30.28 per cent.
It resists the action of acids, in which respect it resenibles 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.
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 :
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.