Manufacturer and Builder 4, 1872
In former years the only white-lead known was that produced after time method invented in Holland, from which country the manner of its production has spread in all parts of Europe and America. Half of the white-lead used in the world is still made in this way. The method is based on time following peculiar behavior of lead under the circumstances stated:
If a sheet of lead of four to six inches in width, and of the thickness of an old copper cent, is rolled up in such a way that it does not touch itself at any point, then, if this coil be placed in a glazed pot, with vinegar on its bottom, in such a meaner that, by means of pieces of wood, it is kept above the vinegar, without touching it, and this pot be placed iu a warm locality — for instance, behind the stove in winter — keeping it loosely covered, it trill be found that, in it few months, the lead will be changed through its whole mass into a white substance. This body, when rubbed up into a powder, produces the finest white-lead, which is a compound of oxide of lead with carbonic acid, made chiefly at the expense of the vinegar, which is decomposed and produces carbonic acid and water, while the air also furnishes a certain amount of carbonic acid, and a larger amouet of oxygen. That the substances mentioned are derived from both sources is proved by the fact that white-lead can not be made by placing water in place of vinegar on the bottom of the jet; neither can the operation succeed when the air is excluded by closing the vessel hermetically.
The operation here described is an exact representation of the Holland method, on a small scale. When operating on a large scale, special pots are used of nearly a foot high; the lead plates are cast in a peculiar way, after the same method as is used in China to cast the leaden lining of the tea-chests, namely, by spreading the melted lead, by a dexterous movement over a cool, flat surface. Then a cheap way of warming is used, namely, burying the pots in stable manure, which, at the same time, develops more of the necessary carbonic acid than is found in the atmosphere.
One of the largest establishments where white-lead is manufactured in this way is situated in Philadelphia, near the Schuylkill River. The manure used is that of horses, and the quality of the same is of much more importance than at first sight would. be supposed. The decomposition of the same is, in fact, a slow combustion under development of carbonic acid, and both these are wanted in the process — a gentle heat, without raising the temperature to ignition, and the pure carbonic acid gas. If the manure becomes old and dry, the heating process stops, and it most be replaced by a fresh portion, while that which has served is good for the farm. If the horse manure accidentally become mixed with that of pigs, or any other carnivorous animal, during its combustion sulphureted hydrogen is also developed, which gives rise to the formation of sulphide of lead, which is black, and would ruin the whole of the white-lead, of which pure whiteness is the principal merit.
In Holland, a mixture of horse manure and refuse tan bark is used, in Germany only manure, in England oak bark, and in the northern lands of Europe and Russia, willow bark. But, whatever the material, the arrangement of the rooms is the same. They are built on the principle of ice-houses, so as to guard against the exterior temperature, and to keep a uniform heat inside, which is of great importance in northern climes, while otherwise the operations would be totally interrupted in winter. In some of these buildings artificial heat, by actual fire, is even added to the heat of the manure.
Two kinds of pots are used, with and without covers. They are piled up in layers, and, in case pots without covers are used, a floor of boards is to be placed on the top of each layer, to prevent the entrance of dirt. A layer of manure of one and a half to two feet thick is first placed on the floor, and trodden down level, then moistened, and the pots put in place. In order that the weight of the upper layer shall not all come on the lower one, proper supports are placed between the rows of pots; these details differ in different establishments, according to the custom and judgment of those superintending the same. In any case the pots should not be closed hermetically, as is clear from what has been said in regard to the theory of the operation.
It seldom takes more than a few days to have the fermentation of the manure in full action, which is seen by the hot watery vapor which proceeds frOill all cracks and sometimes reaches as high as 170° Fahr. As soon as this is noticed, nothing more needs to be done for from four to six weeks, when the vapors disappear and the whole has become dry and sometimes cold. it is evident that here the heat and the carbonic acid is obtained in as economical a way as is possible.
The pots are then removed, and, by opening them, it is found that all, or nearly all, the lead is changed into white-lead, and that time vinegar on the bottom has disappeared. The contents are placed in a box to separate any remaining lead, and the pots at once filled with a fresh charge and replaced in the fermenting-rooms. The old way of separating the white-lead from the metallic core is to break it off, in which operation the laborers cover up their mouths and noses, in order not to inhale the dust too freely. As this operation is so injurious to the workmen, it has, in all well-regulated modern establishments, been superseded by machinery, in which the whole is passed between grooved iron rollers, by which operation the same separation of the metallic lead is accomplished, with the production of much less dust. The remaining pieces of lead are, of course, incited up with the metal for the next castings.
The amount of white-lead thus obtained should be, theoretically, 132 parts for every 100 of lead consumed, but, practically, it is only about 125-parts. When the operation goes on prosperously, 90 per cent of the lead is corroded, but sometimes only 70, so that nearly one third of the lead remains.
The next operation is to grind the white lead obtained as fine as possible; this is done, after mixing it with water, in proper mills, which differ in different localities, while the resulting paste is dried, and then either pulverized or ground up again in oil.
The last operation performed is the adulteration with chalk or barytes, in order to obtain paint of different prices, and a proper degree of cheapness, which, unfortunately, is one of the most essential conditions of an article which is produced for the purpose to make it sell well.
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