« VorigeDoorgaan »
when exposed to light and air, and hence is largely used by artists, especially water-colour artists, as it works better in water than in oil. Cobalt blue can be mixed with all other pigments without affecting them in any way, or being altered itself; this is a feature of some considerable importance. It is unaffected by treatment with either acids or alkalies.
It is a compound of the oxides of alumina and cobalt, with, occasionally, some phosphoric acid. The following analysis of a sample, made by the author, will serve to show the average composition of cobalt blue :
3.075 per cent.
ASSAY AND ANALYSIS OF COBALT BLUE.-Cobalt blue is rarely subjected to any tests for purity or other properties. Its tint, colouring power, &c., can be examined by the usual methods. Its tint and unalterability by treatment with alkalies and acids serves to distinguish cobalt blue from other blues.
By strongly heating cobalt blue with strong sulphuric acid for some time it is decomposed, a violet solution and a white powder being obtained ; on diluting with water, the latter dissolves, and a clear blue solution is obtained, which can be examined by the usual methods of metal analysis.
Cobalt blue has been sold under a variety of names, such as Gahn's ultramarine, Thenard's blue, cobalt ultramarine, azure blue, &c.
There are a few blue pigments which owe their colour to copper. At one time some of these were very largely used for painting of all kinds, but they have had to give place to ultramarine, which is at once a more powerful colour and more permanent. They are not expensive colours, but are only used now to a very limited extent by artists, and scarcely, if at all, by house-painters. The copper blues are known under a variety of names :-mountain blue, Bremen blue, lime blue, blue verditer, &c.
All these blues have a similar composition and very similar properties.
MOUNTAIN BLUE.—This blue pigment (the azurite of
mineralogists) is found naturally, and is essentially a basic
which corresponds to the formula 2 Cu C 02 + Cu H, 02.
For use as a pigment the mineral is ground up very fine; it is of a fine tint of blue, and is much more permanent than any of the other copper blues. It is not much used as a pigment.
BREMEN BLUE.—Bremen blue is a pigment of a pale greenish-blue tint without much colouring power; at one time it was made on a large scale, but is now replaced by ultramarine and cobalt blue, so that now it is only made on a limited scale. There are several ways by which it can be made.
1. 1254 lbs. of common salt and 111 lbs. of copper sulphate are ground together into a paste, with water; this results in the formation of chloride of copper and sulphate of soda. With the paste is mixed about 1 cwt. of clean copper, in small pieces about 1 cubic inch in size. All these are thoroughly mixed together and kept in wooden boxes or tubs; at intervals of two or three days the mass is turned over with a wooden spade, so as to ensure that the metal and the paste are brought into intimate contact with one another. In about three months all the copper will have been converted into a green basic oxychloride of copper, which was at one time sent out as a pigment under the name of Brunswick green; this basic oxychloride is insoluble in water, and, after it has been formed, the mass is thrown into tubs and thoroughly washed with water, by which means all the soluble alkaline compounds are washed out. The green is now ready for being converted into the blue; to effect this the green is mixed with a small quantity of hydrochloric acid and allowed to stand for 24 hours; to the pasty mass is then added about 2 times its volume of caustic soda, at 40° Tw., which is thoroughly mixed with it; and then the mass is allowed to stand for 36 to 48 hours, by which time it will have been converted into the required blue; it is now thoroughly washed with water to free it from soda, and dried, when it is ready for use.
2. 50 lbs. of sulphate of copper and 26 lbs. of common salt are dissolved with a small quantity of water, heat being used to facilitate the operation. The solution is gradually poured into a solution of 50 lbs. of soda crystals, when a precipitate of copper
carbonate is formed ; on allowing this to stand for some time it is gradually changed into basic chloride of copper, and as a considerable effervescence occurs, owing to the liberation of carbonic acid, large vessels must be used. After being washed, the green chloride is transformed into the required blue by the same process as described above.
3. A cheap method of making Bremen blue consists in making a solution of sulphate of copper, to which is added a solution of chloride of calcium or of chloride of barium as long as a white precipitate falls; this is allowed to settle, and the clear blue liquor obtained is mixed with a quantity of freshly-prepared milk of lime until all the copper has been precipitated. This is known by allowing the precipitate to settle and noting the colour of the liquor; if this is blue, then more lime is required. As a rule, 20 lbs. of quicklime is sufficient, when converted into milk of lime, to form 100 lbs. of sulphate of copper. The precipitate is allowed to settle, washed, and dried ; while the clear liquor, which is a solution of chloride of calcium, may be used to precipitate fresh sulphate of copper. The results are not so good as with the methods described above.
Bremen blue consists mostly of hydroxide of copper, Cu H, 02, with small quantities of carbonate of copper.
BLUE VERDITER.—Blue verditer is a pigment of a skyblue tint. It is very similar to Bremen blue in its composition and mode of preparation.
1. A solution of copper sulphate of 1.312 (621° Tw.) specific gravity is prepared and heated, and a hot solution of calcium chloride added until no further precipitate is obtained. The mixture is filtered, and the liquor, which consists of a solution of copper chloride, is diluted with water until it has a specific gravity of 1.157. Slaked lime is thoroughly ground with water to a great degree of fineness, and added to the copper solution in small quantities at a time, until all the copper has been precipitated. The mixture is now filtered, drained, and washed, and a small portion of the paste weighed and dried as rapidly as possible to ascertain the amount of actual dry colour it contains. The green paste thus obtained is placed in wooden tubs, and for every 35 lbs. of dry colour it contains, 4 lbs, of the lime paste, made as above described, and 21 pints of a solution of carbonate of potash of 1.116 (251° Tw.) specific gravity is added, and thoroughly stirred with it. The mass is allowed to stand, and, when the proper shade has been developed, it is washed with water, filtered, and dried, when it is ready for use.
2. Another method, which is really an extension of the last
method, is used in some works in Germany. The process is, up to the stage of producing the green paste and mixing it with lime and carbonate of potash, identical with the last method ; but, now, the paste is placed in vessels which can be hermetically sealed, and to every 35 lbs. of dry colour there is added a solution of 1 lb. of ammonium chloride, and 2 lbs. of copper sulphate, in 31 gallons of water; when all are mixed together, as thoroughly as possible, the vessels are closed up, and left for four to five days, after which they are opened, and the colour washed and dried in the usual way.
3. A solution of copper sulphate or nitrate is prepared, and to it is added a solution of either the carbonate of potash or the carbonate of soda as long as a precipitate falls down; this is collected and washed, then treated with a weak solution of caustic soda to turn it blue (as is done in making Bremen blue).
Any copper solution may be used, but the nitrate or chloride gives the best results.
What is called “refiners' blue verditer” is prepared from the copper solution obtained in refining gold or silver. It differs in no way from the verditer prepared from other copper solutions.
A sample of refiners' blue verditer, examined by the author, had the following composition :Copper carbonate, Cu C 03,
77.797 per cent. Copper sulphate, Cu S 04, .
9.426 Copper oxide, Cu 0,
12:350 Water, hygroscopic,
Another sample of blue verditer had the composition :-
62:45 per cent.
31.19 Water, hygroscopic,
3.29 Calcium sulphate, taso.
100.00 Blue verditer is used to a small extent by artists, especially in water-colours; but it is not a permanent pigment.
LIME BLUE.—Before the introduction of artificial ultramarine lime blue was very largely used for fresco-painting, and common lime-washing and distemper work, on account of its resisting the action of alkaline vehicles and pigments. It has, however, gone out of use, so much so that it is extremely difficult to procure a genuine sample of lime blue, ultramarine being sold for it under the name of lime blue.
Lime blue can be made by the following methods :
1. 125 lbs. of copper sulphate are dissolved in water, and to the solution is added 121 lbs. of sal-ammoniac dissolved in warm water; 30 lbs. of good clean quicklime are carefully slaked with water, and the slaked lime ground into a fine paste with water, after which it is made into a milk by adding more water. The milk of lime is poured into the copper solution, both being well mixed by constant stirring; when all the lime has been added a blue precipitate and a blue solution will be obtained; this mixture is allowed to stand until the solution has become colourless, taking care to stir it up from time to time while the decoloration is proceeding. The blue pigment formed is filtered, washed with water, and dried.
2. A strong solution of sulphate of copper is prepared, and to this is added sufficient ammonia to redissolve the precipitate first obtained. The solution is heated slightly, and to it is added milk of lime, prepared as in the preceding method; the blue gradually precipitates, and is collected, washed, and dried.
Lime blue is essentially a mixture of copper hydroxide and calcium sulphate.
In preparing all these copper blues care must be taken not to have the liquors too hot, as, if so, there is a liability for the copper precipitate to be decomposed and the black oxide of copper to be formed.
PROPERTIES OF THE COPPER BLUES.—As the properties of the copper blues are so similar a general description will suffice. All the copper blues are characterised by being of a pale greenish-blue tint, varying a little in shade and colouring power; being opaque, they are good pigments, especially in water, but in oil they lose some of their opacity. Although not quite permanent, yet they resist a considerable amount of exposure to light and air; they are blackened by sulphuretted hydrogen or by sulphur, owing to the formation of the black sulphide of copper; on this account they cannot be used in places where they are likely to come in contact with sulphur or sulphur gases, nor can they be mixed with other pigments containing sulphur. Exposed to heat they blacken, owing to the loss of water and of carbonic acid and the formation of the black oxide of copper. Acids dissolve them, forming blue solutions which give the characteristic tests for copper, such as the deep blue solution with ammonia and the brown precipitate with ferrocyanide of potassium. Alkalies have little action in the cold; but when heated with them they turn black, owing to the formation of the black oxide of copper.