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ORR'S WHITE ENAMEL OR CHARLTON
This zinc pigment was first patented in 1874, and is the parent of this group of zinc whites. The process of manufacture consists in first preparing barium sulphide by calcining barytes with charcoal for some hours at a white heat; the calcined mass is then lixiviated with water to dissolve out the barium sulphide which is formed. The solution so obtained is divided into two equal portions. To one of these is added a solution of zinc chloride, whereby a precipitate of zinc sulphide is obtained according to the equation
The precipitate is not separated, but to the mass is now added the rest of the barium sulphide and sufficient solution of zinc sulphate, when a combined precipitate of zinc sulphide and barium sulphate will be obtained. This precipitate is washed with water, filter-pressed, dried, and then calcined at a red heat in a suitable furnace. While still hot the mass is thrown into cold water, which causes it to become rather denser than it otherwise would be, and thus have more body; the product is finally ground as fine as possible and dried, when it is ready for use. The calcination causes some of the zinc sulphide to become converted into oxide, and thus the final product will consist of a mixture, more or less intimately united, of zinc sulphide, zinc oxide, and barium sulphate. It is quite possible that the zinc compounds are in a state of more intimate union than would be the case if they were simply prepared separately and then mixed together; that, in fact, an oxysulphide of zinc is formed. The exact composition of the pigment will depend upon several factors, the proportion between the barium and zinc salts used may be varied, while the duration of the calcination will influence the result by increasing the proportion of oxide in the pigment.
As made by this process Orr's white has the composition
Barium sulphate, Ba S04
The ordinary pigment is made to contain one equivalent of Zn S to one of Ba S 04, 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
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 1 to 2 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 de 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
Fig. 10.-- Zinc sulphide white plant. 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
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 : Water, hygroscopic,
1:362 per cent. Water, combined,
3.712 Zinc oxide, Zn 0,
9.182 Zinc sulphide, Zn S,
23.041 Barium sulphate, Ba S 04,
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