When the settling tank is sufficiently filled with white-lead, the solution is passed into other vessels, and the white-lead is washed in washing tanks provided with wooden horizontal stirrers having a rotary motion. The settled pigment is covered with twice its volume of pure water, and stirred; 3 washings take place, and at each the material is allowed to deposit, and the water above is decanted. The white-lead is conducted into large basins built of porous stone, which absorbs part of its dampness. After a few days, the material is divided into blocks, pounded by wooden vertical stamps, put into boxes holding about 8 cwt., and carried to the drying-room.
A modification of this process is practised in England. The lead is smelted in a cast-iron kettle with a spout, which delivers it upon the bed of a large reverberatory furnace, into which air is constantly injected by a ventilator. The lead becomes divided, offers a large surface to the air, and runs into a channel whose lateral sides are perforated with small holes. The lead is oxidized, and the litharge escapes through small apertures. The silver, if any, remains at the bottom of the channel. This mode of preparing litharge is very easy and rapid. The litharge is then finely divided, and after being moistened with 1 per cent, of lead acetate dissolved in water, is put into horizontal troughs, closed on top and inter-communicating. A stream of impure carbonic acid, produced by the combustion of coke in a reverberatory furnace, with air projected by centrifugal ventilators, passes meanwhile through the layers of oxide. The pressure exerted by the ventilators is sufficient to overcome the resistance of the layers of litharge. The gases are cooled in pipes immersed in water. To bring all the particles of oxide into contact with the carbonic acid, and aid the combination, a system of rakes, moved by machinery, keeps the mass constantly stirred.
The white-lead thus obtained is good for painting, perfectly white, covers well, and is preferred in England to that prepared in the wet way, which contains crystalline particles.
The method in use in Germany is a modification of the Dutch, plates of lead being simultaneously exposed to the action of the air, aqueous vapour, carbonic acid, and the vapour of acetic acid; the product covers as well as the best substance made by the Dutch process, and better than that made by the French, being denser and of a finer grain; it is never crystalline, and hence requires less oil for the preparation of paint. The best approximates in composition to the formula 2CO2 PbO + PbO . H20; some specimens contain less lead hydrate than this, but when the proportion of this constituent becomes too low, the product is hard and useless; as a rule, the more hydrate is present the better the white-lead. Excess of carbonic acid in the atmosphere of the white-lead chamber has a tendency to produce a crystalline structure: when the composition approaches that of neutral carbonate, it is extremely crystalline; the residues of knottings left on the sieves approximate to the same composition, and are useless as pigment.
2. Good ceruse; second quality.
3. Usable ceruse; third quality.
4. Bad; residues.
5. Abnormal chamber products; useless.
No. 1. 86.80
The richer in carbonic acid the more grey is the tint. When the acetic acid is too small in quantity, anhydrous lead oxide is sometimes formed; this is yellow or red, and its presence considerably damages the lead; a specimen had the following composition: -
Carbonic acid .... 5.30 Water...... 0.90
Lead carbonate . . 32*22 Lead hydrate .... 12.05 Anhydrous lead oxide . 55.64
This process requires lead oxides and acetic acid, or lead acetates and carbonic acid. Among the oxides, litharge and massicot are the best; red-lead does not suit. The acetic acid should be free from substances which would discolour the white-lead and impair its value. When acetate is used, the neutral acetate (sugar) of lead, and the basic solutions called extract of Saturn and Goulard's water, are best.
Following is the manner of manufacturing: - If the oxide is in big lumps, it is ground to powder, which need not be very fine. Litharge seldom requires this operation. The oxide is mixed with acetic acid or lead acetate, and sufficient water is added to make a consistent paste. This is spread in thin layers over trays covered with sheet-lead, disposed one on another in a room for the purpose, into which the carbonic acid enters, and, being absorbed, combines with the oxide to make lead carbonate or white-lead.
The absorption is hastened by stirring with rakes, and thus presenting fresh surfaces to the action of the carbonic acid. If the gas is not sufficiently damp, water is added to the mixture, to render it more ready to absorb the carbonic acid. As the operation progresses, the oxide gradually becomes white; and when the mixture is free from colour, the treatment is finished, since all the oxide has been transformed into carbonate. The length of the operation varies with the proportion of acetic acid or acetate employed, the rapidity of production of carbonic acid, and the attention given in stirring and maintaining the dampness. With the proportions given further on, a sufficiently rapid production of carbonic acid, and proper care, the carbonation requires 3 to 6 days.
It is economical to mix at once part of the oxide with the whole of the acetic acid or acetate, and when this oxide is very nearly transformed, to add a new proportion without more acetic acid or acetate. This new mixture being exposed to the action of the carbonic acid, the free oxide is very rapidly converted into carbonate. A new proportion of oxide is again added, and the operation is continued as before, always with a proper amount of moisture. These successive additions of oxide are repeated until the acetic acid or acetate is reduced to 1/4, or even less, of that which was in the original mixture. When the carbonation is finished, the mixture is spread in a stove-room, allowed to dry, ground in a mill with water in the ordinary manner; the ground and floated product is dried again, and is white-lead for painting and all other purposes. The carbonated mixture may be ground immediately after removal from the trays, without drying first; but the latter operation improves the quality. For 100 lb. of lead oxide, is used the same weight of a solution of acetic acid which contains 18 pints of No. 24 or proof vinegar.