The ordinary pigment is made to contain one equivalent of Zn S to one of BaSO4, but it is to be borne in mind that this large proportion of barium sulphate exists in the pigment in a finely-divided condition, being precipitated simultaneously with the zinc sulphide. A mixture of natural barytes with zinc sulphide or zinc white gives a very inferior product.

In a later patent Orr describes a similar process, but the materials used are zinc sulphate and strontium sulphide. The patentee proposes to make the zinc sulphate by slightly calcining poor ores of zinc in such a way that the sulphate is formed; the calcined mass is lixiviated with water in such a way that a solution of 1.150 specific gravity is obtained. The strontium sulphide is prepared by calcining celestine (natural strontium sulphate) with charcoal; the calcined mass on lixiviation with water yields a solution of strontium sulphide of specific gravity 1.060. These two solutions are mixed in equivalent proportions when a combined precipitate of zinc sulphide and strontium sulphate is obtained, as shown in the following equation:

This precipitate is washed, dried, and calcined as above. The resulting pigment is a combination of zinc sulphide, zinc oxide, and strontium sulphate. The patentee specifies that the calcination shall be continued until all sulphur vapours are given off. The white pigment so obtained is described as being equal in body to white lead, while it possesses the merit of not being discoloured by sulphuretted hydrogen.

Owing to certain practical difficulties in preparing strontium sulphide this process is not worked.

Charlton white is an excellent substitute for white lead; its colour is good, its body is superior to that of white lead, in well-made samples it has about twice the colouring power,

while it possesses the advantages of not being discoloured by sulphuretted hydrogen, by sulphur gases or pigments, and of being non-poisonous. It mixes well with all vehicles, so that it may be used for all kinds of painting with good results. It mixes with all pigments except those having lead or copper as their base; these it is liable to discolour on account of the sulphur it contains, but with any other pigments there is no change produced on mixing.

GRIFFITH'S PATENT ZINC WHITE

was introduced about 1876, and is made under a patent dated 1875. The process of manufacture consists in taking vat waste," a residue of the Leblanc process of alkali-making and exposing it to the oxidising action of the atmosphere for some time; after which it is lixiviated with water, and a solution, consisting largely of calcium sulphide, with some sodium sulphide, obtained. To this solution an equivalent quantity of a solution of zinc sulphate is added, when a precipitate of zinc sulphide, along with some calcium sulphate, is obtained. Barium sulphate, in a state of fine division, is prepared by adding a solution of barium chloride to one of magnesium sulphate. The zinc sulphide and barium sulphate are now mixed together, and, after being dried, are calcined until no further vapours are given off; the calcined mass is then ground with water, levigated, and dried, when it is ready for use.

Another method of making this white is to mix together solutions of barium chloride and zinc sulphate, when barium sulphate is thrown down; to the mixture is now added the "vat waste" liquor as before, which precipitates zinc sulphide. The two compounds are thus obtained in a much better state of admixture than is the case when made according to the last process. The mixture is calcined as above. The best precipitating agent is said to be "pentathionide of calcium," prepared by boiling 65 parts of sulphur with 35 parts of calcium sulphide in the state of solution, when a red liquor is obtained, which can be used to precipitate the zinc salts.

A dry method of preparing the white consists in mixing 2 parts of barium sulphate with 1 part of zinc oxide and 10 per cent. of sulphur and 5 per cent. of sodium carbonate; this mixture is calcined, levigated, and dried.

A somewhat different process, one more on the lines of Orr's patent, is described in a subsequent patent, taken out in 1877

by Griffith. According to this, barium sulphide is prepared by calcining barytes with charcoal, and a solution is obtained by lixiviation in the usual way; to this is added an equivalent quantity of a solution of zinc sulphate, when a precipitate, consisting of a mixture of barium sulphate and zinc sulphide, is obtained. This is calcined in crucibles, without air being admitted, so that very little, if any, oxidation takes place; the calcined mass is ground with water, and, after being washed, is dried and mixed with a small quantity of magnesia, which imparts to it the property of more thoroughly mixing with oil, and gives it a softness in working which it would not otherwise have.

In the preparation of zinc sulphide whites, one difficulty which is encountered is that the product sometimes comes from the calcining furnace with a yellow tint; this, perhaps, is due to oxidation, which must be avoided. For this purpose Griffith mixes the mass, before calcining, with from to 25 per cent. of ammonium chloride or ammonium sulphate, which, by being volatile, produces an atmosphere of ammonia round the pigment, thereby preventing oxidation and the discolouration of the pigment.

A rather novel method of preparing sulphide of zinc was patented by Griffith & Cawley in 1879. The principle of this method lies in the fact that when zinc and sulphur are brought together, in the state of vapour, they will combine to form the sulphide. The process is carried out in the plant shown in

固

E

Fig. 10.- Zinc sulphide white plant.

F

Fig.10. Sulphur is placed in the vessel, A, where it is melted; it flows into the egg-shaped still, B, contained in the furnace, G, in which it is converted into vapour; the zinc vapour passes into another chamber, where it comes into contact with vapour of metallic zinc from the still, C; the two bodies unite to form

sulphide, which passes along into the chambers, E, where it collects. These chambers are constructed similarly to those used in making ordinary zinc white; but they are kept hot, so that any excess of sulphur vapour can pass through them and be collected in the chamber, F. The zinc sulphide which collects in the chamber, E, is ready for use as a pigment, or it can be mixed with barytes, &c. D is a revolving scraper, for the purpose of keeping the mouth of the zinc still free from any deposit of sulphide which might tend to cause it to become chocked up.

Griffith's patent zinc white is very largely used as a pigment; its colour is good, in body it is nearly, if not quite, equal to that of white lead, while it is far superior to that of ordinary zinc white. It does not become discoloured by exposure to sulphuretted hydrogen, or to any other sulphur compounds; it resists exposure to all atmospheric agencies, and is one of the most permanent white pigments known. It mixes well with oil, working very freely under the brush; in this respect it is quite equal to white lead. It mixes with all pigments, excepting those containing lead or copper, without being changed by them, or changing them in any way.

A sample of Griffith's zinc white, analysed by the author, had the following composition:

KNIGHT'S PATENT ZINC WHITE.

This zinc white is prepared, according to the patent of 1876, by adding sulphuretted hydrogen or a solution of calcium sulphide or calcium pentasulphide to hot solutions of either zinc chloride or zinc sulphate; the precipitate of zinc sulphide obtained is collected, washed, dried, and calcined. To a mixed solution of zinc sulphate and kieserite, which is crude magnesium sulphate, a solution of sodium carbonate is added, whereby a mixed precipitate of the carbonates of zinc and magnesia is obtained; this, after washing, drying, and calcining, is mixed with the calcined zinc sulphide obtained in the first instance.

The best precipitant for the zinc is said to be calcium pentasulphide prepared by boiling vat-waste liquor with sulphur in such a way that for every 40 parts of calcium there are 160 parts

of sulphur; a red liquor is obtained which may be used for precipitating the solutions of zinc.

In a patent taken out in 1882 a process of preparing zinc sulphide white is described which is rather more complicated. The process consists in preparing three solutions. No. 1 is a solution of sodium sulphide, obtained by boiling caustic soda with sulphur in the usual way. No. 2 is a solution of zinc sulphate. No. 3 is a solution of strontium chloride prepared by dissolving the carbonate in hydrochloric acid.

Nos. 1 and 2 are mixed in equivalent proportions, whereby there is formed a precipitate of zinc sulphide and a solution of sodium sulphate. To the mixture is now added an equivalent quantity of solution No. 3, when a precipitate of strontium sulphate and a solution of sodium chloride are formed. A precipitate consisting of zinc sulphide and strontium sulphate in equivalent proportions is thus obtained; this is collected, washed, dried, and calcined. The calcined mass is finished by grinding, levigating with water, and drying.

This white has similar properties to the two whites described above.

Besides the processes described above for making zinc sulphide whites, the following may be mentioned as having been made the subject of patents:-Parnell dissolves zinc in alkaline solutions and throws down the sulphide by means of an alkaline sulphide, the pigment being finished by calcining as above. William's process does not differ much from that of Orr; he takes a solution of zinc chloride and throws down the sulphide by means of a solution of calcium sulphide; after separating the solution of calcium chloride which is formed, a solution of barium chloride is added and then one of sodium sulphate; the combined precipitate of zinc sulphide and barium sulphate is then finished by calcining; sometimes a little magnesia is added to enable the pigment to mix with oil more perfectly. Claus dissolves zinc carbonate (either the natural variety calamine, or the artificial product) in ammonia; to this is added a solution of barium sulphide which throws down a precipitate of zinc sulphide and barium carbonate. This precipitate is collected, dried, and calcined out of contact with air; while still red hot the calcined mass is thrown into cold water, then ground, levigated with water, and dried. The use of the waste liquors of galvanisers is mentioned as a source of the zinc solution by Claus in his patent specification.

One of the many difficulties met with in the preparation of the zinc whites just noted is that they are very liable to turn yellow