The manufacture of mineral and lake pigments

by Josef Bersch

The stacks, built up of pots and manure, are of different sizes; it is not advisable to make them too small, or the loss of heat would be considerable. A stack 4 to 5 metres long, 3½ metres wide and 6 to 7 metres high will contain 6,000 to 8,000 pots and 9,000 to 11,000 kilogrammes of lead.

The stack consists of a rectangular pit walled on three sides; the fourth side is open, with the earth dug out in the form of an inclined plane, in order to permit the introduction of the pots and the manure. The construction of the stack is commenced by placing the pots at the bottom in rows, avoiding interspaces as much as possible. In a stack of the size mentioned a layer contains 1,000 to 1,200 pots. Between the pots containing lead and acetic acid are arranged a number of larger ones containing acid only; the object of these is to furnish acetic acid vapour. When the pots are in place, 3 or 4 lead plates are placed on each spiral, the top plate forming the cover; immediately over the pots strong wooden planks are laid, and upon them a layer of boards, which must fit so tightly that nothing can fall through. On the boards is spread out a layer of fresh stable manure, with which the space between the outside row of pots and the wall is also filled. The layer of manure is 30 to 40 centimetres thick.

Upon the lowest layer of pots, a second, third and so on are built up exactly in the same manner, so that the whole stack is filled with alternating layers of pots and manure. In order to prevent the cooling of the uppermost layer of pots, it is covered with a thicker layer of manure 60 to 70 centimetres thick. When lead plates and the taller pots are used, a stack will generally contain 15 layers; but when gratings and the smaller pots are employed, 18 layers can be packed into the same space. In arranging the layers of pots, care should be taken to leave spaces at tolerably equal distances, so that the air necessary for the oxidation of the lead may enter. To prevent the cooling of the stack at the front, where it is not protected by masonry, when full it is walled up with boards; a board roof also protects the erection from rain.

In place of manure, spent tanners’ bark can be used, which in the same manner ferments, producing heat and carbonic acid. In places in the neighbourhood of large tanneries this spent bark is generally obtainable at lower prices than stable manure, which is more valuable for agricultural purposes; the former has also a very considerable advantage, white lead made by means of spent tanners’ bark being generally of a purer white than that made with manure. The reason for this is that in the decomposition of animal excrement small quantities of sulphuretted hydrogen are produced, a gas which produces black lead sulphide when it comes into contact with lead compounds.

According to the results of practical experience, pigs’ dung cannot be used in the manufacture of white lead; so much sulphuretted hydrogen is evolved from it that the white lead is not white, but has a greyish tinge.

When bark is used in place of manure, a discolouration of white lead by sulphuretted hydrogen is not to be feared, but there is the drawback that a longer time is necessary for the corrosion of the lead, because the bark decomposes more slowly than the manure, and accordingly gives out less heat and carbonic acid.

According to the climate of the country, the stacks may be differently erected. In colder countries it is necessary to sink them in the earth and surround them with masonry, as directed above; but in warmer climates such effectual protection against cooling is unnecessary, yet in all cases it is better to sink the stack in the earth on account of the regularity of temperature so obtained.

Instead of sinking the stacks in the earth, they may be built in the open when there is a plentiful supply of manure or bark; but they must then be surrounded by a very thick layer of manure to prevent cooling. It is a desirable alteration in the construction of the stacks to provide the pots with lids, and so avoid the use of the layer of boards separating each two layers of pots. The object of the cover is simply to prevent dirt from falling into the pot. It should not fit tightly on the edge, or the entry of carbonic acid into the interior of the pot would be made difficult. The lids are therefore rounded and fit loosely on the pot. When pots with lids are used, the lowest layer is covered with manure in the ordinary way; upon this again comes a layer of pots, and so on.

The transformation of the lead may be regarded as complete in four to six weeks when manure is used, but with bark the time extends to ten weeks. The quantity of white lead obtained varies in different cases; for example, from a stack 5 metres long, 4 metres wide, and 6 metres high, into which 12,000 kilogrammes of lead were put, 10,000 kilogrammes of white lead were obtained, and 4,000 kilogrammes of lead remained unaltered. In another case, for a stack of 8 layers 280 kilogrammes of vinegar and 9,600 to 12,000 kilogrammes of lead were used, and there was a residue of 10 to 15 per cent. of lead.

3. Removal and Grinding of the White Lead.—The stack is pulled down after the lapse of the necessary time; the lead plates and rolls are collected in wooden boxes and brought into the room where the white lead is separated from the metallic lead. Formerly the white lead was removed from the sheets exclusively by manual labour, an operation extremely dangerous to the workman. It is quite impossible to prevent the formation of white lead dust, so that the men were continuously in an atmosphere charged with the poisonous material, and, as a consequence, suffered from the various forms of lead poisoning.