volatility, but it is obvious that in zinc-white making it is not possible to separate it out from the finished product. It is chiefly on account of the presence of these impurities in the zinc ores, and the discolouring action they have on the white, that they are not now used for preparing it.

Occasionally it is found that the white collected in the first chambers is not so good as that collected in the following ones; this is due to the presence of small traces of impurities in the zinc which is used and which it is difficult to entirely eliminate. These impurities are found to collect mostly in the first chamber, and the white in this is, when found to be of a poor colour, sold as an inferior quality of zinc white.

The general methods of working is to raise the retorts to a white heat, and then to throw in the ingots of zinc; all apertures are now closed up excepting those leading from the air to the combustion chamber and into the collecting chambers. The zinc soon begins to volatilise, and on issuing from the mouth of the retort burns. It is essential to prevent the combustion taking place within the retort because it would then soon become choked up by a deposit of infusible oxide. This can only be done by making the mouth of the retort narrow, and scraping the crust formed on the mouth of the retort by means of scrapers. The zinc white formed by the combustion of the vapours is of two kinds, light and heavy; the former passes into the chambers and is there collected, the latter drops down the combustion chamber, being rather heavy, into a barrel placed for its reception, and being usually of a poor colour, may be returned to the retorts with the next batch, along with a small quantity of carbon in some form or other.

The zinc white made by this process, while being of a good colour, is very light and, in consequence, is rather deficient in body, still it gives a more uniform product than any other process hitherto devised, and is, therefore, the chief, if not the only, process now in use for making zinc white.

2. Ignition Methods.-When carbonate of zinc, Zn CO3, is heated it loses its carbonic acid and leaves a residue of zinc oxide, Zn O, according to the equation

Similarly the hydroxide, Zn H, O2, loses water and leaves oxide behind

Other salts of zinc, such as the nitrate (Zn 2 N O2), 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 or covering power. 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. This want of body, coupled with the fact that it is rather costly, has 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.

Zinc 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 cruicible; 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

Zinc,
Sulphur,

its formula being Zn S.

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 sulphite 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.

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