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 pigments 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.

151

CHAPTER 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.

Under the names of "pale Brunswick green," ""middle Brunswick 99.66 green,' deep Brunswick green," are sent out several green pigments, varying in shade or tint from a pale yellowish-green to a 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 into a mixing mill; the former is preferable, as the materials are ground as well as mixed, and this 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 that if too

much yellowing is being produced as compared with a standard sample, then, by throwing a little more blue into the mill, the fault can be remedied at once; but against this advantage must be set off the disadvantage that the tints of green are not so fine as those which are obtainable by wet methods. The following proportions can be taken as guides in making the various shades of Brunswick green by this dry method:

Pale Brunswick Green.-1 cwt. of barytes, 1 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.

Middle Brunswick Green.-1 cwt. of barytes, 2 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.

Deep Brunswick Green.-1 cwt. of barytes, 5 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.

Extra Deep Brunswick Green.-1 cwt. of barytes, 8 lbs. of Prussian blue, and 35 lbs. of chrome-yellow.

The quality and tint of the chrome-yellow used will be found to have some influence upon the tint of the green produced; for the pale shades lemon-chromes should be used; while for the deep shades the middle shades of chrome can be used. Before making up a large batch of green with a new batch of either chrome-yellow or Prussian blue a small trial lot should be made to see if the two pigments will produce the required shade, as experience shows that different makes of chrome-yellows, even if of the same shade, do not always give the same results in making these greens; while the difference between two makes of Prussian blue as green-producers is greater than that between two makes of chrome-yellow.

For making the palest shades of Brunswick greens, only the best and brightest Prussian blues should be used; for the darker shades of green the quality of the blue is not of so much consequence.

The barytes may be replaced with gypsum, if thought fit; less gypsum is required to produce a given tint of green than barytes, the proportion being about 1 cwt. of gypsum to 24 cwts. of barytes.

WET METHODS.-The wet methods are those commonly adopted by makers of Brunswick greens, partly because they are the oldest (for colour makers are so conservative that it is difficult to induce them to alter their methods), and, partly, because the wet methods produce the finest greens; but they are very much more troublesome to carry out and require no little practical experience on the part of a colour maker to produce the best results. From the recipes which have been given above it will be seen that barytes is the principal ingredient in these greens; hence the principle which should underlie all wet pro

cesses is to precipitate the yellow and the blue constituents on the barytes simultaneously. This is by no means easy to do, and yet much of the brilliancy of the green depends upon this being done as successfully as possible. The best way to ensure this result would be to mix solutions of the acetates of lead and iron or of the nitrates of those metals together and to add the barytes and precipitate with a mixture of the bichromate and ferrocyanide of potash, but, unfortunately, this course is not at present available, for the reason that while the acetate or nitrate of lead can be purchased on a commercial scale of sufficient purity, the acetate or nitrate of iron is not so purchasable; the common iron liquor is too impure for use in making greens, while the nitrate of iron so-called is of too variable a composition to be recommended for use in colour making.

The materials which are used in the preparation of the greens are copperas (ferrous sulphate), which should be used as pure and as fresh as possible, the variety known as green copperas is the one required, acetate of lead, bichromate of potash, ferrocyanide of potassium (yellow prussiate of potash), and barytes. The red prussiate of potash would give rather better results than the yellow, only that its extra cost is against its use for making greens for common use; but when a good price is obtainable its use is to be recommended, as the green is much easier to make with it than with the yellow prussiate.

For producing the various shades of Brunswick greens the following proportions may be used which, as well as those given above, may be varied so as to suit the special requirements of each individual maker. The following points are, however, well worth attending to in making alterations in the proportions. 1st. That equal weights of prussiate and copperas must be used. 2nd. That the proportion of acetate of lead to the bichromate of potash should be, as nearly as possible, 10 to 3.

Pale Brunswick Green.-1 cwt. of barytes, 13 lbs. of acetate of lead, 1 lb. of copperas, 1 lb. of yellow prussiate of potash, and 4 lbs. of bichromate of potash.

Middle Brunswick Green.-1 cwt. of barytes, 13 lbs. of acetate of lead, 1 lbs. of copperas, 1 lbs. of yellow prussiate of potash, and 44 lbs. of bichromate of potash.

Deep Brunswick Green.-1 cwt. of barytes, 14 lbs. of acetate of lead, 2 lbs. of copperas, 2 lbs. of yellow prussiate of potash, and 4 lbs. of bichromate of potash.

Extra Deep Brunswick Green.-1 cwt. of barytes, 16 lbs. of acetate of lead, 4 lbs. of copperas, 4 lbs. of yellow prussiate of potash, and 5 lbs. of bichromate of potash.