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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
during the process of calcining, which yellow tint is preserved in the finished product. This yellow tint is obviously objectionable and is probably due to over-oxidation. Several remedies have been proposed to avoid this defect. One of these is to calcine without exposure to the air so that oxidation cannot take place; another remedy is described in the notice of Griffith's white; still another is not to wash completely, the precipitate, but to leave a little of the soluble salts in it which tend to preserve the purity of the colour.
The defect is not so prominent when the pigment is made as proposed by Knight, by mixing 1 part of zinc sulphide with 2 to 3 parts of zinc oxide and 4 or 5 parts of barytes with a little magnesia; this mixture is calcined out of contact with the air, so that there is little or no discolouration; still this method does not give as good results as the other processes which have been described, due to the difference in the properties of natural barytes and precipitated barium sulphate.
White lead is liable to change; zinc (oxide) white, although permanent in colour, is expensive, and has comparatively little body. To obtain a moderately-priced white possessing the good qualities of white lead and zinc white is the object sought for in the preparation of the zinc sulphide whites; the measure of the success may be gauged by the fact that the production at the present time is 100 tons per week, and is steadily increasing
Orr's process is the only one now worked in this country.
As the method of manufacture involves both a wet or dry process it is by no means easy to produce a perfectly satisfactory article. The precipitate produced by the barium sulphide and the zinc solution has to be carefully dehydrated before it becomes a pigment, and, unless precautionary means are taken, the product has a peculiar tendency to darken in sunlight, passing through gradations of grey often to a deep slate colour, the white colour being restored after long exposure.
During the calcining or furnacing operation, besides the yellowing effect noted above, there is a tendency to the production of grit which is troublesome during the subsequent washing and grinding.
Notwithstanding all the difficulties in the way of its production a very fine white pigment is found on the market, fast in colour, fine in condition, and of good covering power and staining properties.
ASSAY AND ANALYSIS OF ZINC SULPHIDE
As these are of such variable composition it is necessary to assay them for colour and covering power in the usual way.
An analysis of these whites is rarely required. When such is the case, the analysis of Orr's or of Griffith's white given above, and that of the lithophone which is given below, will show what to look for in these whites.
They are distinguished from other white pigments by being partially soluble in hydrochloric acid, sulphuretted hydrogen being given off during the solution of the pigment in the acid.
The solution in hydrochloric acid will show the characteristic tests for zinc given under zinc white, and will contain any magnesia which may have been added to the white, as also any calcium which may be present. If the white has been made with barium or strontium carbonates, the solution will also contain barium or strontium ; these may be tested for by adding sulphuric acid, when, if present, a white precipitate of the sulphates will be obtained, and can be differentiated by applying the distinguishing tests.
The residue which is left undissolved, on treatment with hydrochloric acid, will consist of either sulphate of barium or sulphate of strontium ; these may be distinguished by using the flame tests.
A sample of lithophone, one of the zinc sulphide whites examined by the author, had the following composition :
Barium sulphate, Ba S 04,
67.96 per cent. Zinc oxide, Zn 0,
7:32 Zinc sulphide, Zn S,
24.85 This pigment possessed a good body and worked well in oil ; it had a slight yellow tint.
Barytes is one of the most important white pigments at the disposal of the painter; probably, in this respect, ranking next to white lead as to the extent of its use, although its qualities as a pigment are much inferior.
In composition it is the sulphate of the metal barium and is obtained from both natural and artificial sources. The composition is indicated by the formula Ba S04 In nature it occurs in large quantities very widely distributed, forming the mineral barytes, heavy spar, or, as it is called by the lead miners, cawk, or lead bloom. Artificially, it is obtained whenever sulphuric acid or solution of a sulphate is added to a solution of a barium salt.
In this country barytes is found most abundantly in Derbyshire; while in Cornwall, Devonshire, Cumberland, several places in Wales and Ireland, it is also found and worked. It occurs in the form of large crystalline masses, generally opaque, but sometimes transparent pieces are found; in some cases the crystalline character is not very strongly developed and then the barytes appears to be more or less amorphous. The normal colour of the barytes is white, but often it has a faint yellow tint, due to the presence of small quantities of oxide of iron; it often has small streaks of lead ore (galena) distributed throughout its mass.
The following analyses of two samples of Derbyshire barytes will show the average composition of this mineral:
5.02 Barium sulphate, Ba S 04,
98.796 98.566 Ferric oxide, Fe, 03,
.830 Calcium sulphate, Ca S 04,
•504 Magnesia, Mg 0,
017 As a rule the transparent variety is heavier than the opaque variety, as is seen from the above analyses. Of two samples of Cumberland barytes, one, a transparent variety, had a specific gravity of 4.5, while the other, an opaque sample, had a specific gravity of 4.3.
PREPARATION OF BARYTES. In principle the preparation of barytes as a pigment is very simple. It merely requires grinding to a fine powder, when it is ready for use. In practice, however, to ensure the best results a little more than this is required. The crude barytes comes to the grinder in large lumps more or less contaminated with foreign minerals; the crude material is first hand-picked to separate out all pieces that are not barytes and in some cases to pick out the small pieces of lead ore with which the mineral may be mixed. The subsequent treatment varies a little at different barytes works, according to the character of the machinery which is available, but the following may be taken as a description of the general method in use for preparing barytes :- After being picked, the pieces or lumps of barytes are ground to a coarse powder under edge-runner mills; from these it is transferred to flatstone mills, in which it is further ground, usually under water, a constant stream of which is caused to pass through the mills during the operation; from these mills the ground barytes is passed to settling tanks, which are usually constructed of stone; in them the barytes settles in a few hours, the super-. natant water is run away, while the top portion of the mass of barytes, being the finest, is taken and stored ready for the next operation; while the bottom portion, which is usually much coarser, is sent back into the grinding mills again to be reground. The grinding cannot be too well done as on it depends the quality of the resulting pigment.
After the grinding the barytes is next subjected to a bleaching process to free it from any yellow tint it may possess. This tint is due, in almost all cases, to oxide of iron, from which barytes is rarely free. The process is carried out in stone cisterns, the size of which will vary in different works according to the quantity of barytes it is desired to treat at one time; a steam pipe is fitted to the cistern, so that the contents may be heated, an operation which facilitates the bleaching. The wet barytes from the settling tanks is run into these stone cisterns, and heated up to near the boiling point; then a quantity of hydrochloric acid is run into the cistern, the amount used varying considerably. As a matter of fact it should be proportioned to the amount of oxide of iron in the crude barytes, but, as a rule, it is added in a rule of thumb sort of way at most works; on an average about 1 cwt. of acid to 1 ton of crude barytes is used.
The acid extracts the oxide of iron, leaving the barytes quite