There are several processes for making this pigment.
This is a modification of the Dutch method. The purest metallic lead is used. Originally it was subjected to the chemical operation in the form of loose rolls of sheet-lead. The American method is to cast the lead into circular gratings, looking very much like shoe-buckles. In whatever shape, the lead is put into earthen jars, with a little vinegar at the bottom, the lead being supported by earthen ledges from coming into contact with the vinegar. Sometimes the pots have openings in the sides to permit a free circulation of the vapours set free in the process. An immense collection of the jars, tens of thousands in number, is then packed in alternate layers with layers of some fermenting material which will give out carbonic acid gas. Originally stable manure was employed; now tan bark is preferred. The layers of jars and bark are carried up sometimes 20 ft. high, the bark being kept out of the jars by sheets of lead and boards. The fermentation sets free much carbonic acid. Basic acetate is first formed on the surface of the lead in the pots, which is decomposed by the carbonic acid gas, forming carbonate and free acetic acid. The latter acts again on the lead.
Very little vinegar is required; and the process goes on continuously, assisted by the heat of the fermentation, until, at the end of 10 or 12 weeks, fermentation stops. The process is then at an end. The stack is taken to pieces, and the lead is found in its original form, though increased in bulk and weight, and converted into a very white and soft carbonate. If the conversion has not been thoroughly done, a core of metallic or blue lead will be found in the interior of some of the pieces. The pieces of lead are thrown into large tanks filled with water, in which they rest upon copper shelves full of holes. They are beaten to separate and pulverize the carbonate, the water preventing the fine dust from poisoning the air and injuring the workmen. Grinding and washing in water then follow, until the carbonate is reduced to an impalpable powder. It is then dried in steam pans or upon tile tables, and put up for the market. The carbonate obtained in this way is superior; but a fair article is made by boiling solutions of nitrate or acetate with litharge, and precipitating the solution with carbonic acid.
White lead is not alone employed as the best white paint; but it constitutes the body of almost all other paints, it being coloured by intermixture with other pigments. (Scient. Amer.)
The Dutch process comprises the following operations: - (1) Fusion and casting the lead in sheets or grates (buckler). (2) Alternate layers made of lead and stable manure, or spent tan. The lead is put into pots holding weak acetic acid, and remains in the beds 35 to 40 days when stable manure is employed, and 70 to 90 when spent tan is used. (3) Successive uncovering of the layers of lead, most of which has become carbonate. Separation of the white-lead from the non-corroded metal. First grinding and separation of the blue lead. (4) Grinding the white-lead with water under stones. (5) Moulding and drying the floated white-lead. (6) Grinding and sifting the dry white-lead, and packing in barrels that which is to be sold powdered. (7) The white-lead which is to be made into paste with oil is not sifted, but mixed with 7 to 10 per cent, of its weight of oil, in a closed stirrer, and passed between cast-iron rollers. When the paste has become fine and homogeneous, it is received in a tank filled with water, from which it is taken and packed for sale.
(1) Fusion is effected in cast-iron kettles, and no dangerous fumes are emitted except from old lead or the residues of previous operations, still covered with carbonate. The kettle is placed under a hood receiving its draft from the chimney of the furnace. The top edge of the furnace is connected with the hood by a metallic cylinder, having doors which open for charging the lead, or for casting the fused metal. These precautions seem sufficient.
(2) The buckles or thin sheets of lead, rolled into spirals, are put into earthenware pots, and there supported upon 2 or 3 projections, the vinegar being at the bottom of the pot.
(3) In separating the white-lead from the non-corroded metal, the workman picks up by hand the large and slightly adhering scales of white-lead, and separates the remainder by twisting and bending the non-corroded lead. This is generally done in the bed itself, or in a special room, where the corroded metal is carried as it comes from the pots. The buckles or sheets, with some still adherent white-lead, are put one by one upon an endless cloth, which carries them to an inclined hopper, from which they pass between two pairs of grooved rollers, and thence through an inclined cylindrical sieve. What passes through the holes of the sieve is re-ceived into a hopper, which delivers it into a trough on wheels. The metallic lead falls from the lower opening of the sieve into another trough. The whole of the machinery is enclosed in tight wooden partitions, the only free opening of which is that for the passage of the endless cloth. The trough, filled with white-lead, is removed when the dust has subsided, and its contents are mixed with the scales picked up by hand.
The next dry grinding is effected In an edge-runner mill The ground lead is shovelled into a cylindrical metallic sieve with fine holes, and enclosed ill a wooden box. The powdered white-lead is collected at the bottom of the box, and the small flattened particles of metallic lead fall from the lower end of the sieve into a special receiver. The sifted white-lead is mixed with water, and thoroughly ground under millstones.
Danger ceases to exist when the separation of the scales, their grinding and sifting, are effected under water, or subjected to sprays of water immediately after leaving the grinding apparatus.