Other salts of zinc, such as the nitrate (Zn 2 N O3), the sulphate (Zn SO4), the sulphide (Zn S), are also altered when heated and yield a residue of oxide; but they require rather a higher temperature to effect their decomposition than do the first two compounds named.

Neither of these two compounds occurs naturally in a sufficiently pure form to be used as a source for the preparation of zinc white by an ignition method, therefore they have to be prepared artificially; the carbonate, by precipitating, by means of a solution of sodium carbonate, any solution of zinc, when it is obtained as a white powder insoluble in water. The hydroxide is similarly obtained by adding a solution of caustic soda to a solution of zinc. In this case care has to be taken not to add the precipitant in excess or otherwise some of the hydroxide first thrown down will be redissolved again, zinc hydroxide being soluble in caustic soda solution. The precipitates in either case are well washed with water by decantation, and then drained on a filter. It is not necessary to dry them. When as much water has drained from them as possible, the mass is thrown into a crucible and heated to a white heat for some time, when a bright yellow residue of zinc oxide is left in the hot crucible; this, however, as it cools turns white. When cold the oxide is ground up for use as a pigment. The white obtained by this process is a good one, has good body or covering power and is of good colour; but the process of manufacture is more costly than that by combustion, consequently, it is rarely, if ever, used. Attempts have been made to utilise by this method many waste solutions of zinc which are obtained in various industries, but hitherto without much success. The principal difficulties are the trouble of collecting these liquors and the influence which any impurities they contain may have on the colour of the resulting pigment, these impurities being difficult, if not impossible, to eliminate by any practicable process.

PROPERTIES OF ZINC WHITE.

Zinc white is a rather bulky fine white powder, having a specific gravity of 5.6. In hue it is rather bluish. It is quite insoluble in water, oil, alcohol, and turpentine. It dissolves in dilute sulphuric acid, hydrochloric acid, acetic acid, and many other acids without effervescence, giving colourless solutions. It is also soluble in ammonia and alkaline solutions. It is unchanged by exposure to air and light, and sulphur

or sulphur gases have no visible action on it, as the sulphide formed is white like the oxide; as a pigment, therefore, it is quite permanent. It mixes well with all vehicles used in preparing paints. For preparing the stiff paste, which is sometimes sold, it requires about 22 per cent. of linseed oil, a much larger proportion than is required by any other pigment. It mixes well with water, and in this form it is largely used by watercolour artists, under the name of Chinese white. It can be mixed with all other pigments without undergoing any change or changing the other pigment. The chief fault of zinc white is its want of body, which is not equal to that of white lead. Its covering power is rather better. This is, no doubt, due to its bulkiness, which is inseparable from the mode of preparation. Many attempts have been made to produce a dense zinc white. The plan most commonly in use is to grind the pigment for some time under edge runners, so as to break down, so to speak, the voluminous character of the pigment, and make it more powdery, in which form it has greater covering power. Another proposed plan is to heat the zinc white to a red heat, and then drop it into cold water; the sudden cooling effects the breaking up of the particles of zinc white, but this method is not so effective as the grinding process, and is rather more troublesome to carry out. Its cost, want of body, and want of freedom in working under the brush, have limited the use of zinc white much below that justified by its permanent qualities.

ASSAY AND ANALYSIS OF ZINC WHITE.

Zinc white can be assayed for covering power and colour in the usual way.

Zinc white, being a rather costly pigment, is very liable to adulteration with other white pigments, such as china clay, barytes, whiting, terra alba, &c.

Zine white should be completely soluble in dilute sulphuric acid without effervescence; a property possessed by no other white pigment. Effervescence indicates an addition of white lead, whiting, or magnesite. Most other adulterants are left as an insoluble residue. The solution in acids is quite colourless. On adding ammonia, a white precipitate is first obtained; but, on adding an excess, this is redissolved; any lead, if present, is left as an insoluble residue. On adding ammonium sulphide to the ammoniacal solution, a white precipitate of zinc sulphide will be obtained; this precipitate should be quite white, or, at most, have a faint yellow tint, due to its containing traces of the yellow ammonium sulphide; any other colour would indicate

impurities. This precipitate should be filtered off, and to the filtrate a solution of ammonium oxalate be added; no precipitate should be obtained; if there is any it indicates the addition either of calcium carbonate in the form of whiting (which is also indicated by effervescence with acids), or of calcium sulphate. If to the solution or filtrate from the ammonium oxalate sodium phosphate be added, the formation of a white precipitate indicates the addition of magnesium. If a preliminary test with dilute sulphuric acid shows that adulterants are present, it will be best to treat the pigment with hydrochloric acid, and boil it, filtering off any insoluble residue, and then allowing the solution to cool; if crystals form, then lead is present, and has been added either in the form of white lead or lead sulphate, the character of the results obtained will indicate which. In the solution zinc, calcium, and magnesium may be looked for as described above. If an insoluble residue is left it may contain barytes, china clay, or gypsum, which substances may be tested for in the way described under their respective heads.

The quantity of zinc oxide in a pigment can be thus ascertained-Weigh out about 1 gramme of the pigment, dissolve it in hydrochloric acid, and to the solution add first ammonia in excess, then ammonium sulphide in slight excess; the mixture is placed on one side for a few hours, and then filtered; the precipitate of zinc sulphide is well washed, dried, placed in a porcelain crucible, and ignited at a bright red heat, taking care that plenty of air reaches the material in the crucible; by this means the sulphide is converted into oxide. When the reaction is considered to be complete, the crucible and its contents are allowed to cool, and, when cold, are weighed; the weight of zinc oxide found gives at once the weight of zinc white in the pigment.

SULPHIDE OF ZINC WHITES.

When ammonium sulphide is added to alkaline solutions of zinc salts, or when sulphuretted hydrogen is passed through similar solutions, a white precipitate of the sulphide of zinc is obtained. This is a body composed of sulphur and zinc in the following proportions—

The first mention of the use of this compound as a pigment

occurs in a patent granted in 1852, the process for its preparation being to add a solution of sulphide of potash to one of zinc sulphate. The patentee did not prepare it with a view of using it as a painters' pigment, but for the purpose of colouring india-rubber.

During the last twenty years very considerable attention has been paid to zinc sulphide as the possible base of a white pigment. The consequence has been that it is now manufactured on a large scale by several colour manufacturers and sold as a pigment under a variety of names, as Orr's Charlton white, Orr's enamel white, Griffith's patent zinc white, patent zinc white, lithophone, &c. None of these bodies consist only of sulphide of zinc, but contain oxide of zinc, barytes, magnesia, &c.

When alkaline sulphides are added to solutions of zinc salts, a white precipitate of the sulphide is obtained, according to the equations

which may be considered as typical of the operations carried out on a large scale.

Zinc sulphide is prepared on a moderate scale and used by indiarubber manufacturers for colouring white rubber goods, it having been found that zinc white (zinc oxide) or zinc sulphide are perhaps the best pigments to use for this purpose. The zinc sulphide employed must be of good colour, free from any impurities, and should contain from 97 to 98 per cent. of dry zinc sulphide, the rest being water.

Zinc sulphide possesses what to the painter is of great importance, "body;" the colour is apt not to be good. It is extremely difficult to remedy this defect with pure zinc sulphide, but by introducing other bodies and preparing the sulphide in a special way, a pigment possessing both good colour and good body may be obtained.

5

ORR'S WHITE ENAMEL OR CHARLTON

WHITE.

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