between this amount and that found in the first instance represents the lead which is present in the green in other forms.

A qualitative analysis should always precede a quantitative one; and the above scheme should be modified if such an analysis shows that other constituents, besides those mentioned above, are present in the greens. The notes on the analysis of chrome-yellows (p. 135) will be found useful.

BRUNSWICK GREEN.

The modern Brunswick greens must not be confused with the pigment which was made and sold at one time under this name and which has now become quite obsolete. This old Brunswick green was a basic chloride of copper, sometimes called an oxychloride, Cu, O Cl. It can be prepared by several methods.

1. 20 lbs. of copper turnings are placed in a vessel capable of being closed; over them is poured a solution of 30 lbs. of ammonium chloride in 6 gallons of water; the vessel is closed up and the contents well mixed by shaking; the vessel is kept in a warm place for about two months, and at intervals the contents are mixed by shaking up; at the end of the time the vessel is opened, when it will be found that most of the copper has been converted into the green oxychloride.

2. Into a wooden tub is placed about 1 cwt. of old sheet copper cut into small pieces; over them is poured a solution of 105 lbs. of sulphate of potash and 1 cwt. of common salt; the mass is allowed to react together for some time, the length of which depends upon the temperature, and is longer in winter than in summer. The green gradually forms, and when it is seen that most of the copper has been converted, the green is separated from the undecomposed copper by sieving and washing.

3. 1 cwt. of copper is mixed with 67 lbs. of salt and 34 lbs. of sulphuric acid mixed with 3 times its volume of water; after standing some time the green is formed, when it is treated as before.

4. Metallic copper is taken and just covered with a strong solution of chloride of copper and left until it is changed into the basic chloride, when it is finished as described under method 2.

The preparation of Brunswick green is a very slow operation, extending over 2 to 4 months as a rule; in all cases the green is collected by washing it with water to free it from any alkaline bodies, sieving to free it from unchanged copper, drying slowly at a low temperature, since high temperatures tend to decompose it; necessarily it is somewhat costly.

As a pigment it is fairly good, working well both in oil and water, and having a good covering power; in tint it has somewhat of a bluish-green cast of no great depth of colour. It is not quite permanent, although it resists some considerable amount of exposure to light and air. In its general properties it closely resembles the Bremen blues and the Bremen greens, which see.

CHROME-GREEN.

True chrome-green is a most valuable pigment, not only on account of the brilliance of its colour, but also on account of its great permanence, being in fact the most permanent green pigment known.

In its chemical composition chrome-green varies somewhat according to the method of making; in some cases it consists entirely of the oxide of chrome, Cr2 Og; in others of the phosphate of chrome, Cr, 2 PO4; while in yet others it is a mixture of these bodies. It should not be confused with the mixture of chromeyellow and Prussian blue which is sometimes sold as chromegreen.

Various methods are in use for preparing chrome-green.

1. Guignet's Process.-Guignet was one of the first to prepare chrome-green, if not the very first; hence the pigment is frequently sold under the name of "Guignet's green," mostly to calico-printers in the form of paste. Guignet uses boric acid (boracic acid) and bichromate of potash. As a rule, the commercial articles are of sufficient purity to prepare good pigments with. But if very good results are required it is advisable to purify them by re-crystallisation.

88 lbs. of potassium bichromate and 33 lbs. of boracic acid are ground into a stiff paste with water; the mixture is then put into a furnace where it is heated to a dark red heat for 4 hours. A form of reverberatory furnace is the best that can be used. The fused mass is thrown into water and repeatedly washed by decantation; the washed pigment is ground whilst still wet under an edge-runner mill, again washed, filtered, and dried.

The first wash-waters contain a good deal of the boracic acid in the form of potassium borate; this acid may be recovered and used over again; the waters are boiled down a little and to the liquor is added hydrochloric acid; this throws out the boracic acid, which gradually collects in the form of crystals on standing; these crystals can be collected and used for making another batch of green; in this way at least 70 to 75 per cent. of the boracic acid originally used is recovered.

The reaction which takes place between the boracic acid and the bichromate is expressed in the following equation:—

=

3 Cr2 H6 06 + 2K3 BO3 + 90

3 K2 Cr2 07+ 2 H3 BO3 from which it can be calculated that a considerable excess of boracic acid has been used in the process of making the colour; this excess is not wasted, since part is recovered, and, moreover, an excess is necessary for the production of a fine quality of the pigment. Borax cannot be substituted for the boracic acid.

Chrome-green made by this process has a fine yellow-green tint. 2. 3 lbs. of bichromate of potash and 2 lbs. of ammonium chloride are thoroughly mixed together into a paste with water; this is dried and then calcined at a red heat in a furnace; the calcined mass is well washed in water, and the pigment thus obtained is ground. This process gives a fine quality of green, but it is not quite equal to that obtained by the last process.

3. When solutions of ammonia or caustic soda or carbonate of soda are added to solutions of the basic chromium salts a precipitate of the hydroxide, Cr, H6 O, is obtained; when this is heated to redness it loses water and passes into the oxide, Cr, Ogi the tint of green obtained in this way is not good, being of a greyish hue; by mixing with the precipitate some salt before calcining, and afterwards thoroughly washing with water, the tint of the green is materially improved.

4. A solution of chromium chloride is prepared by heating a strong solution of potassium bichromate with hydrochloric acid and a little methylated spirit; to this solution is added, first, sufficient soda to neutralise the acid, and, then, a solution of sodium phosphate; the precipitate of chromium phosphate is collected, dried, and calcined; the green is then finished by washing and grinding in the usual way.

5. A cheaper method of producing the phosphate consists in preparing a solution of 10 lbs. of potassium bichromate and 18 lbs. of sodium phosphate; the mixture is boiled, and, while boiling, a solution of 10 lbs. of sodium thiosulphate is added, and then a little hydrochloric acid. On continuing the boiling the chromium phosphate is slowly precipitated; when the precipitation is complete, the green is treated as in the last process. The pigment obtained by this process is apt to contain a trace of sulphur, which introduces into it an element of change.

The phosphate-of-chrome greens given by the two last processes are by no means equal to the oxide-of-chrome greens for brilliancy of tint.

PROPERTIES OF CHROME-GREEN.-Chrome-green forms a fine green pigment of a slightly yellowish tone; it mixes well with either oil or water, has good body or covering power, and is quite permanent, being one of the best pigments which the painter can use, on which account it is much used by artists. It mixes with all other pigments without being affected by them or altering them in any way.

When properly made it is quite insoluble in either acids or alkalies. The solubility of oxide of chromium depends upon the temperature and length of time to which it has been heated; the greater these two factors are the more insoluble becomes the oxide, so that well prepared oxides are very insoluble owing to the fact that they have to be heated to a high temperature for some time.

Very nearly the same property is found in the phosphate greens.

ASSAY AND ANALYSIS OF CHROME-GREENS.Chrome-greens should be assayed for colour, brilliance, covering power, and similar properties in the usual way. When pure,

chrome-green should not impart a yellow colour to dilute hydrochloric acid when boiled with that reagent, such yellow colour would indicate adulteration with chrome-yellow. When boiled with caustic soda chrome-green should remain unacted upon. The liquor should be divided into two portions—to the one acetic acid should be added, when no yellow precipitate. indicating chrome-yellow should be obtained; to the other hydrochloric acid and ferric chloride should be added, when no blue precipitate should be obtained, such precipitate would indicate the presence of Prussian blue. Chrome-greens are usually adulterated with the Brunswick greens, which adulteration is detected by the application of the two tests just given. For use by calico-printers, Guignet's green is supplied in the form of a paste, containing usually 30 per cent. of actual colour.

COPPER GREENS.

Copper forms the base of a number of greens, some of which are of value, although the bulk are of but minor importance, and their use is gradually decreasing. The variety of names under which the copper-greens have been offered from time to time is very great; very few are now in use, and it is rather difficult to know exactly to what copper compound any particular name one meets with in old books and papers belongs. One of these greens has already been described.

Three of these copper-greens-verdigris, Scheele's green, and emerald-green-are very closely related to one another, and form, as it were, a group of colours. Verdigris is the basic acetate of copper. Scheele's green is the arsenite of copper, while emerald green is the aceto-arsenite of copper, and may be viewed as a compound of the other two colours.

VERDIGRIS.

The chemical composition of verdigris has been already stated. It is made in two forms, known as "distilled" and "common" verdigris. The first, being somewhat of a crystalline nature, is rarely used as a pigment, and finds its chief use in medicine; the latter is of the most importance from a painter's point of

view.

Preparation of Verdigris.-Distilled verdigris is prepared by dissolving copper or oxide of copper in the acid obtained during the distillation of wood, from which circumstance arises the name "distilled verdigris;" for the product itself is not distilled. Another method of manufacturing this variety is to mix together solutions of sulphate of copper and acetate of lime, or of the acetate of lead; the sulphate of lime or lead, as the case may be, is precipitated, and a solution of acetate of copper is obtained. From the solution of acetate of copper, obtained by either of the above methods, the verdigris is obtained by concentrating down to the crystallising point, and allowing the salt to crystallise out; this gives the best product. Or, the solution may be cautiously evaporated to dryness; this is costly and there is a risk of decomposing the green, causing it to lose its brilliancy of tint.

Distilled verdigris occurs in the form of dark green crystals, soluble in water and in acetic acid; as a pigment it is of little use, being too transparent; then, again, its solubility is against its being a good pigment.

Common verdigris is prepared in several ways.

1. French Process.-The skins and marc of grapes, left after the juice has been pressed out for making wine, are used in France for making verdigris; the material is placed in large tubs, loosely covered over with netting, in which it remains for a few days, when acetic fermentation sets in; when this has commenced sheets of copper (averaging about 8 inches by 4 inches) are thrown in among the fermenting mass-generally old scrap copper is used. They are left in the tub among the grape skins for from 18 to 20 days, the period varying according to the weather;