(5) The soft paste is poured into conical earthenware pots, which are dried in a stove-room. The greater part of the water is expelled, and the blocks, becoming contracted, are easily removed from the pots. Their thorough drying is finished in another or the same stove-room. The sides of the pots are coated with white-lead, which is scraped off with an iron tool, or by washing in water.
Some white-lead is sold, after drying, in the shape of conical blocks, wrapped in paper, and put into barrels without breaking them. Some is not put into pots, but the soft paste is poured upon a cloth, which is then folded so as to form a square flat bag. Several such bags, separated by square wooden trays, are afterwards squeezed in a hydraulic press, which expels the greater part of the water. After unfolding the cloth, the block of white lead is cut into prisms or bricks, having sufficient consistency to be carried immediately into the drying-room.
(6) Most of the white-lead in lumps requires to be ground and sifted again before it is ready for sale. This second grinding is done with an edge-runner mill. The ground stuff is shovelled into the hopper of a cylindrical silken sieve, enclosed in a wooden box, where the fine white-lead falls* That whrch does not pass the sieve is collected in another box, and re-ground. The sifted white-lead is removed from its box after the dust has subsided, and packed in barrels by shaking or ramming.
(7) The prisms of white - lead are ground in a kind of coffee-mill, and the powder is put into a horizontal cylinder with oil, and mixed by iron paddles fixed to the shaft running through the cylinder. Thence the paste passes between cast-iron rollers, and becomes fine and homogeneous by more oil being added if necessary. The finished paste is kept under water in large tubs. When white-lead is ground in oil, it is not necessary to reduce it to fine powder, and this avoids one of the most unwholesome operations. It is therefore advantageous that all white-lead (by far the greater part is always ground in oil) should be mixed with oil in the works themselves. English factories deliver the greater part of their product in the shape of a paste holding 8 to 9 per cent, of oil. (Riffault.)
Pelouze observes that the vinegar used is made from inferior beer, and contains but a small proportion of acetic acid, less than l 1/2 per cent, of the weight of lead employed; and in good operations nearly the whole of the metal is transformed into white lead. No white-lead is obtained when drafts are not established between the different parts of the beds. The theory of the process is therefore simple. The air produces oxidization, and the vinegar, volatilized by the heat of the fermenting manure, unites with the lead oxide, being then displaced by the carbonic acid disengaged by the manure. A considerable portion of acetic acid is found in unwashed white-lead made by the Dutch process.
The chemical reactions on which it is based are as follows; - If a solution of basic lead acetate be treated with carbonic acid, part of the oxide is converted into lead carbonate, and the remainder becomes neutral acetate (sugar of lead). By adding a new portion of litharge (lead oxide) to the solution of neutral acetate, this becomes basic again by the solution of the oxide. These reactions permit the manufacture of white-lead by a continuous and economical production of basic acetate.
In practice, a solution of basic lead acetate, marking 16° to 18° B., is made by boiling a solution of neutral acetate with very finely powdered litharge. When the litharge has dissolved, and the basic solution is well saturated, the liquor is decanted from the impurities into a closed vessel. Carbonic acid is then introduced, being previously well washed. When all the basic excess of oxide is transformed into carbonate, the liquors are allowed to settle. The carbonate falls to the bottom, and the supernatant solution of neutral acetate is decanted, to be boiled again with oxide, and become basic acetate. Each operation entails a loss of neutral acetate, which must be replaced. The settled carbonate is first washed with a little water, added to the decanted solution of acetate. The washing is continued with larger quantities of water. The paste of white-lead is put into pots, and dried in the stove-room. This white lead is in impalpable powder, and as white as snow, but has less density and body than Dutch.
In the manufacture as conducted at Portillon, near Tours, the lead oxide is moistened with water, and spread over a wooden floor above 2 saturating pans, copper - lined, supplied with stirrers composed of a wooden frame with bronze projections, which reach to about 1/2 in. from the bottom. One pan is raised above the other, so that the excess of liquid in the upper runs by a spout into the lower. The latter, at the middle of its height, is connected with a duplex bronze pump. The two pans are filled with water rendered acid by about 1/40, of pure pyroligneous acid marking 30°. While the stirrers are in motion, damp lead oxide is put in, and becomes partially dissolved. The pump then forces the solution into 3 large copper-lined tanks, connected with each other, and having stirrers kept in motion during the operation. These 3 apparatus are provided with pipes and inverted gutters, perforated with numerous small holes, through which a continuous stream of carbonic acid escapes. The sp. gr. of the solution is 5° B. By this operation the pump takes the solution of basic acetate from the saturating pans, and carries it into the precipitating tanks, where it is brought into contact with the carbonic acid.
White lead is immediately formed, and the liquid, which must still retain a certain proportion of basic acetate, passes into the settling tanks, where the white-lead deposits. The liquor then goes back to the saturating tanks, and the operation is repeated.