Pagina-afbeeldingen
PDF
ePub

.

+

Cadmium
chloride.

water, and freely in ether and alcohol. On being subjected to dry distillation a yellow body, called euxanthone, sublimes. The salts of euxanthic acid are all yellow-coloured bodies; those of the alkalies are soluble in water, while those of most of the metals are insoluble and may, therefore, be used as pigments.

CADMIUM YELLOW. This important yellow pigment, so much used by artists on account of the brilliance of its colour and its permanence, is the sulphide of the metal cadmium and is composed of Cadmium,

77*78 per cent. Sulphur,

22.22 and has the formula Cd S. It is made by passing a current of sulphuretted hydrogen gas through solutions of cadmium salts, as shown in the equation :

Cd Cl2 + H2S CAS 2 HCI

Sulphuretted Cadmium

hydrogen. sulphide. G. Buchner has investigated the properties of cadmium yellow more thoroughly than any other chemist. He describes * four modifications of cadmium sulphide which he distinguishes as

1. A-modification, obtained by passing a current of sulphuretted hydrogen gas through a slightly acid solution of a cadmium salt. The colour is a very bright and pure citron-yellow, has a good body, and works well as an oil-colour. By various means it can be converted into the B-modification. When used as an oil-colour it is quite permanent, but when used in water, or kept in a moist atmosphere, it gradually undergoes oxidation, passing into sulphate, this change being accompanied by a loss of colour.

2. B-modification.- This has a bright vermilion-red colour, and is obtained by passing sulphuretted hydrogen gas through a strongly acid solution of a cadmium salt. It is the most permanent form of cadmium sulphide and is unaffected by exposure to light and air.

The author has been unable to prepare this red variety of cadmium yellow. Although Buchner does not give any clear description of the method of obtaining it, yet from the remarks as to the conditions under which this red variety is formed, it is evident that it cannot be obtained by precipitation free from the yellow variety, and that the process of separation consists in

* Chemiker Zeitung quoted in Journal Socy. Chem. Ind., VI. 665.

exposing the mixture of yellow and red sulphides to light and air for some considerable period. It is not practicable to prepare this variety as a commercial article.

3. C-modification.—A variety soluble in water, of no practical interest, and prepared by a process of dialysis.

4. D-modification. This variety has a pale yellowish colour and little or no body. It is prepared by passing sulphuretted hydrogen gas through an ammoniacal solution of a cadmium salt. It is of no practical use.

Cadmium yellow is made commercially in various shades of yellow and orange, the processes for the production of which are described below.

Preparation of Yellow Cadmium.—This is prepared in several ways. 1. A slightly acid solution of any cadmium salt is prepared and through it is passed a current of sulphuretted hydrogen gas; the apparatus shown in Fig. 17, p. 118 may be used. This has a pure chrome-yellow shade. 2. A lemon-yellow shade is obtained by dissolving 1 lb. of cadmium sulphate in 4 gallons of water and adding 11 gallons of the ordinary yellow ammonium sulphide. 3. Or a solution of cadmium sulphate is made, to which is added a solution of sodium thiosulphate and a little sulphuric acid and the mixture boiled for an hour. This variety contains much free sulphur, and is, hence, liable to undergo oxidation to sulphuric acid, which destroys the yellow.

Preparation of Orange Cadmium.-1. A solution of cadmium sulphate or chloride is prepared. It is made strongly acid by the addition of excess of hydrochloric acid, and a current of sulphuretted hydrogen gas is passed through it. 2. 1 lb. of cadmium sulphate is dissolved in 4 gallons of water; the solution is boiled, and while boiling, yellow ammonium sulphide is added. All the precipitates of yellow obtained in the various ways just described must be well washed in water, especially those obtained with the ammonium sulphide, to free them from any trace of acid or sulphide which, if left in, would ultimately lead to the destruction of the colour. After being washed they should be thoroughly dried at as low a temperature as possible, not exceeding about 150° to 160° F.; too high a temperature causes the shade to become brown while hot and although the colour comes back on cooling, yet it never quite regains the original brilliancy.

PROPERTIES OF CADMIUM YELLOW.-Pure cadmium yellow is one of the most permanent pigments known; it is unaffected by exposure to light and air. It mixes with any vehicle used in painting. When heated strongly the colour darkens, changing to a dark violet-red; on cooling, the original

colour comes back, not, however, always in its original brilliance, but with a brownish tone. The impure yellows, those which are made with yellow ammonium sulphide or sodium thiosulphate, are not permanent pigments. When they are exposed to the combined action of air and moisture, the free sulphur they contain becomes oxidised to sulphuric acid, and this, acting on the yellow cadmium, changes it to sulphate, which change is shown by a bleaching of the colour, and occurs whether the pigment be ground or used in oil or water.

Cadmium yellow can be mixed with almost all the other pigments without affecting them or being affected by them; the only exceptions are those pigments which, like white lead, emeraldgreen, and the chrome-yellows, contain lead or copper as their basis. When such piginents are mixed with cadmium yellow double decomposition sets in, resulting in the formation of black sulphide of lead or copper as the case may be; the production of either compounds causes the mixture to acquire a greyish or brownish tint.

ASSAY AND ANALYSIS OF CADMIUM YELLOW. -Besides the usual tests for colour and body, cadmium yellow should satisfy the following tests :-Strong hydrochloric acid should completely dissolve the yellow with evolution of sulphuretted hydrogen to a clear colourless solution, from which, on dilution with water and passing sulphuretted hydrogen gas, a yellow precipitate only should be obtained. The filtrate from this precipitate should give no precipitates on adding ammonia and ammonium sulphide. The addition of barium chloride to the solution should produce no turbidity. On boiling with caustic soda, filtering off the residue and adding hydrochloric acid to the filtrate no yellow precipitate, indicating the presence of arsenic yellow, should be obtained. Carbon bisulphide should extract no sulphur from it. Samples should not yield anything to water when boiled with it. The aqueous liquor should not give any precipitates with silver nitrate or barium chloride nor any acid or alkaline reactions to test papers.

Cadmium yellow is rarely adulterated; the common adulterants are arsenic yellow, zinc chrome, and the chrome-yellows, the presence of which can be distinguished by the application of the characteristic tests, which are given under the respective pigments.

AUREOLIN. This pigment is a double nitrite of potassium and cobalt prepared by precipitating cobalt nitrate with sodium carbonate,

dissolving the precipitate in acetic acid and adding a strong solution of potassium nitrite. On allowing the mixture to stand for some time the colour is gradually precipitated, and is collected and washed; after being dried it is ready for use.

Aureolin is of a bright yellow colour, but is not permanent, being affected by exposure to light and air ; acids dissolve it, while alkalies have no action.

[ocr errors][ocr errors]

160

OHAPTER V.

GREEN PIGMENTS.

The green pigments in common use by the painter and the artist are derived from both natural and artificial sources, but usually from the latter. The green pigments are valuable, largely made and used, and are fairly numerous.

The commonest are those known as Brunswick greens, which are made in very large quantities for common painting ; next is emerald-green, although this colour, owing to its poisonous nature, is gradually being displaced by substitutes made from the coal-tar greens; then comes the true chrome-green; then some of the other copper-greens; while the rest are only used on a limited scale.

BRUNSWICK GREEN.

wick green,

Under the names of "
pale Brunswick green,

" "middle Bruns," "deep Brunswick green,” are sent out several green pigments, varying in shade or tint from a pale yellowish-green toa very deep blue-green. These pigments are made in very large quantities, and are mixtures, in various proportions, of chromeyellow, Prussian blue, and barytes. They must not be confounded with the pigment originally known under the same name, which was a compound of copper, and which has almost completely gone out of use. Brunswick greens can be made in various

ways,
almost

every colour maker having his own favourite manner of mixing the various ingredients together.

DRY METHOD.-In this method the materials composing the green are thrown into the pan of an edge-runner grinding mill or, better, into a mixing and sifting mill; in the former the materials are ground as well as mixed, and this probably has some influence in developing the tint of the green. The main advantage in this method is considered to be that the shade of green which is being produced is visible while in the mill, and

« VorigeDoorgaan »