Pattinson's white-lead is distinguished by its composition, which is a basic chloride and an oxychloride, instead of a combination of oxide with carbonic acid. He prepares his white-lead from crude galena (lead sulphide), which often contains silver. This latter metal is collected, and the sulphur is also employed. The finely-powdered galena is heated in closed leaden vessels with concentrated hydrochloric acid, which is produced in large quantities in soda works, and is very cheap. By this treatment the sulphur is transformed into sulphuretted hydrogen, which is burned in the furnaces of sulphuric acid works, and assists in the production of sulphuric acid. The lead is transformed into chloride, and as this salt is but slightly soluble, large volumes of boiling water are employed to separate the silver sulphide contained in the galena. The boiling solution' of lead chloride, to pass to the basic state, needs to be mixed with lime-water very rapidly, in order to obtain the basic chloride in the shape of an exceedingly fine powder which covers well. The proportion of lime also should be exactly calculated for neutralizing half the chlorine of the lead chloride, and the precipitated basic salt should contain equal atoms of chloride and oxide of lead.
The clear solution of lime is in one tank, the hot one of lend chloride in another, and they are mixed by regulating their running into a third tank. An inconvenience is, that the chloride of lead, being but slightly soluble even in boiling water, very large vessels are needed, and the consumption of fuel to heat the water is considerable.
Pattinson's white-lead has a slight brownish shade, which is scarcely sensible when a small proportion of black or blue is added. On the other hand, it covers particularly well. It is very bulky, possesses great body, and absorbs a large proportion of oil.
The use of petroleum to prevent the lead-poisoning of workmen is useful. Fastre verified that most cases of lead-poisoning resulted from absorption of white-lead through the pores of the skin. He remarked that men at work with the scrapers, or the horizontal grinding-stones, are affected, as well as those who pound the white-lead; and yet the scraping and water-grinding produce no dust. In such work it is through absorption by the skin the poisoning is effected. Fastre found in petroleum an energetic antidote. Before beginning work, at midday, and in the evening, the workmen are obliged to wash their hands with petroleum. The cases of lead-poisoning have decreased 90 per cent. Benzole, existing in the petroleum, scours the skin, and takes the white-lead completely away; the fat substance in the oil prevents the absorption of lead-salts during work. This simple process, which is said to give such good results, may supply useful applications in many industries where the workman has to handle salts of copper, mercury, and other such products. (Revue Indust)
As the result of examination of hundreds of samples, Wigner and Harland express a decided opinion that white-lead consists of a mixture of a neutral carbonate with a hydrate, and that its value as a pigment depends almost entirely upon the relative proportions of these ingredients. In general terms, if lead is converted into a hydrate, it will combine with oil, and form a kind of paint or varnish; but this, although it will spread over the surface of the material to be covered, will not really cover it in such a way and with such a degree of opacity as to hide the natural colour of the substance over which it is spread, but, on the contrary, it will appear like a muddy film of varnish or lacquer spread over it. Or, taking the other extreme, if the compound consists entirely of lead carbonate, it will form an emulsion with the oil, resembling, to some extent, the emulsion which chalk will form with water or syrup, and although it will possess a certain degree of opacity, it will not cover the material in such a way as to render it suitable for paint.
They come to the conclusion that the combination of the two compounds is necessary to secure a good paint - that is, the hydrate must be present to enable the mixture to form a paint instead of an emulsion, and the carbonate must be present to give covering power.
The results of analyses of the best brands of commercial white-lead show that the percentage composition corresponds in most cases with admixtures between certain limits. Muter appears to have practically hit upon the true proportion, which he puts down as 3 equivalents of lead carbonate and 1 of hydrate.
The facts brought forward seem to give evidence of the reasons why zinc white, carbonate of magnesia, oxide, and other metallic carbonates and similar substances, have not been used as paints with success. In the case of the white-lead, a positive chemical compound has been formed, and the 75 per cent., or thereabouts, of lead carbonate present has been dissolved in the chemical compound, and so a paint has been formed which possesses a covering power in excess of any other. Until some means can be devised by which some other substance can be dissolved in the same way in a chemical compound, so as to form a paint possessing characters somewhat different from those of a mere emulsion, it seems useless to argue that, is regards durability or covering power, they can equal a good well-manufactured sample of white-lead; and while inventors attempt, in order to increase the yield of paint from a ton of lead, to precipitate the whole of it in the form of carbonate, it is useless for them to think that such paint can possess a covering power to be compared with that of a genuine article. (Analyst.)