A full analysis of these lead sulphate pigments is rarely required; but they should be assayed for colour, hue, covering power, &c., by the methods described in another chapter.

SULPHITE OF LEAD WHITE.

Sulphite of lead is a white insoluble powder containing lead oxide and sulphur dioxide, and having the formula PbSO. It was first proposed to be used as a pigment in 1850 by Dr. John Scoffern, who took out a patent for its production and use for painting. The process he describes for its preparation is to prepare a solution of basic acetate of lead, and to precipitate the sulphite from this by passing a current of sulphur dioxide gas through it. A solution of normal acetate is left, which may be used for preparing fresh basic acetate; while the pigment which is precipitated is collected, washed with water, and dried. This sulphite white is said to be of good colour and body, to mix better with oil than the sulphate, and to keep its colour better than the carbonate white.

In 1881 Andrew French patented a process of manufacturing a white pigment, which consisted of a mixture of about 80 per cent. of sulphite of lead, with 20 per cent. of oxide of zinc, although these proportions were liable to vary from time to time. This pigment was made by mixing ores containing both lead and zinc with carbon in such forms as coal, charcoal, coke, &c., and treating the mixture in a low cupola furnace fitted with a hot blast. By this treatment the ore becomes partly reduced to the metallic state; while some is oxidised and carried by the blast of air in the form of fume into flues and collecting chambers, where it settles out. The amount of air supplied in the blast is so arranged that the oxidation is limited, and the sulphites of the metals are produced and not the sulphates (see Sublimed and Hannay's white leads). The pigment so made is described as being superior to ordinary white lead, inasmuch as it retains its colour longer and is superior to sulphate whites in having more covering power or body, and mixing with oil better.

The author has not seen any of these sulphite whites, and they are not now made.

ZINC WHITES.

Several white pigments containing zinc as their base are now used as pigments on a fairly large scale. These usually contain as their essential constituent one or other or both of two compounds of zinc--viz., the oxide, Zn O, and the sulphide, ZnS;

the former has been used as a pigment for a long time, the latter is of comparatively recent introduction. The oxide is generally sold under the name of zinc white, and is always the body or compound which is understood to be designated by that term. The sulphide is rarely sold pure, but is more or less mixed with oxide of zinc and other white pigments, and sold under a variety of names, which will be given below.

ZINC WHITE.

This pigment, also known as Chinese white, consists entirely of the oxide of the metal zinc, which has the composition

It can be prepared in two ways

1. By Combustion of Metallic Zinc.-Zinc is a volatile metal, and when heated to a white heat it is readily converted into vapour; if this comes into contact with air or oxygen combustion can take place, and the oxide is formed by the union of the metal with oxygen, as shown in the equation—

Zn +0=Zn O.

The usual method of manufacturing zinc white is based on this reaction.

2. By the Action of Heat on certain Zinc Compounds.—When the carbonate or hydroxide of zinc are heated strongly they lose part of their constituents, leaving a residue of oxide behind. This method of preparing zinc white has been used and made the subject of many patents.

1. Combustion Method. This is the principal method for preparing zinc white. The process is carried out in a special form of plant, which is shown in Figs. 8 and 9. This plant consists essentially of two portions, one in which the zinc is produced in the form of vapour, and the other of chambers in which the white formed by the combustion of the zinc vapour is collected. The vapourising plant consists of a furnace, A A, in which are placed a number of fireclay retorts, BBB. These furnaces are made double, back to back, so as to accommodate two sets of retorts, as shown in the drawing; they are of the reverberatory type. The retorts are made of fireclay; the usual form is shown in the drawings, although it varies a little at different works. The mouths of these retorts open into a kind of com

bustion chamber or flue made of sheet-iron; one end of this flue is made funnel-shaped, and is open to the air, so that a current of air can pass through it, the other end of the flue opens into the first collecting chamber. The white hot vapour of zinc,

when it comes from the retort into this flue, burns, and fumes of zinc oxide are emitted in large volumes; these are carried by the draught into the collecting chambers. The collecting chambers, CCC, are large chambers varying in number at different works,

but usually six set in two series are employed. They are constructed of wood, and each chamber is complete in itself; it communicates with its neighbours on either side by means of small

apertures so placed alternately at top and bottom that the fumes and gases from the retorts must take a circuitous course through the series of chambers; in so doing the oxide of zinc which they carry with them is deposited on the sides of the chambers; finally, the gases pass through a long flue, GGG, to the chimney of the works. In this flue there are screens which serve to intercept any oxide which has escaped condensing in the chambers. The bottom of the chambers is made hopper-shaped, so that, by placing barrels underneath and opening a slide, the oxide can be taken out of the chambers with ease. Although zinc-white chambers are usually constructed of the form shown in Figs. 8 and 9, yet it is obvious that any other form of collecting chambers may be used, such as, for instance, those described under lamp-black.

The retorts measure about 28 inches long by 10 inches broad, and 6 to 8 inches high, the sides having a thickness of 1 inches. Eight of these retorts are placed in a furnace arranged in two sets of four, back to back; two retorts open into one series of three collecting chambers.

The furnace is so constructed that the flames from the fireplace first pass over the retorts, then under them in a flue, and finally pass out, or rather the products of combustion pass out, into a chimney by the flue, GGG, connected with the collecting chambers, whereby the draught necessary for drawing the zinc oxide through the chambers is created. The draught should be good, so as to ensure a plentiful supply of air to the vapour of the zinc as it passes out of the retorts, otherwise some of the zinc is sure to escape combustion, and this, passing into the chambers unchanged, causes the discolouration of the white.

Usually metallic zinc is used as the raw material for the preparation of the zinc white, but it has been proposed to use zinc ores, such as calamine (carbonate of zinc), and zinc blende, or black Jack (sulphide of zinc), and any products containing zinc, which, by being heated in a retort with reducing agents like coal or charcoal, can be reduced to the metal, which, being volatilised in the operation, can be burnt and collected in the manner described above. The great objection to this method of procedure is that the zinc ores are rarely pure, being accompanied by varying quantities of other metals-cadmium, antimony, &c. Some of these are volatile like zinc, and on burning form oxides which pass into the collecting chambers and contaminate the zinc white, spoiling the colour very considerably. This is especially the case with cadmium, which is a very common constituent of zinc ores. This metal forms a brown oxide, which, if present, always spoils the zinc white. In the ordinary process of zinc smelting it is eliminated by taking advantage of its greater

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-

Zn CO3 = Zn O +

2

CO2.

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