Scientific American 23, 2.12.1868
For the Scientific American.
The adulteration of white lead with sulphate of baryta has become so common that it is one of the regular steps in its preparation in all factories. The pure white lead of the most finely ground quality is called "Silver White;" when mixed with equal parts finely ground sulphate of baryta it is called, on the European continent, "Venice White." When adulterated with double its weight of sulphate of baryta it is known as "Hamburg White;" and even three parts of the baryta and more to one of lead are frequently used. This adulteration is not entirely a deterioration, and many of these adulterated qualities are preferred for certain purposes to the pure article.
There exists another kind of white lead, called "Kremner White," which owes its pure white color to the original purity of the lead employed (which is free from silver and iron), and the carefulness in the method of manufacture, clearing it from all powdered metallic lead or sulphuret, which, especially the last, even in the smallest quantities, injure many other qualities of white lead.
The method described on page 298 is usually called the Dutch process, and being very injurious to the workmen has in certain localities been supersedes by the so-called French process, of which Thenard first established the principle. It consists in making a solution of a soluble salt of lead, and by passing carbonic acid gas through it the lead is precipitated as a carbonate. This process may be executed on a very small as well as on a large scale, and requires the following steps: first, a saturated solution of acetate of lead (lead sugar) is made, either by dissolving this salt in water, or by heating metallic lead with pure vinegar; this solution is boiled with oxide of lead (litharge) till it cannot dissolve any more of it; one part of pure strong wood vinegar (pyroligneous acid) will dissolve a little less than one part of litharge (oxide of lead) and form a neutral acetate, when dissolving twice that quantity of litharge in it (correctly 60 parts of acetic acid to 112 of litharge, one atom of each) we obtain a socalled subacetate, a basic solution, which colors litmus paper blue, and when dissolving three times this amount of litharge the solution is saturated, and the excess of lead above the netral solution will be readily precipitated as carbonate of lead by passing carbonic acid gas through the solution, till the solution becomes neutral again, or even acid.
This carbonic acid gas may be obtained by the action of sulphuric acid and water on charl or marble, as is done in the preparation of the so-called soda water, or it may be obtained from the combustion of charcoal, but in this case it must be purified, chiefly from sulphur vapors, as these color lead black, and consequently make the precipitate very dirty looking. The best way is to pass the gas resulting from combustion first through a separate solution of lead, before passing it into the receptacle from which the white lead is to be precipitated. As soon as this precipitation is completed the liquid is left to settle, the supernatant neutral acetate of lead solution is decanted off, and boiled with another dose of litharge; thus a limited amount of acetate could be used for an indefinite period, if there were not unacoidable losses during the process, which have to be supplied from time to time with fresh acetic acid. It is clear that during this method of operation, the white lead being obtained from the first in a wet condition, the workmen are not exposed to the poisonous dust, as is the case in the old process described on page 298.
Several modifications of this French process have been proposed; for instance, button and Dyer make a solution of litharge in nitric acid, and precipitate with carbonic acid obtained from the combustion of coke. Richardson uses sulphuric acid to precipitate the solution of acetate of lead, and thus forms not a carbonate but a sulphate of lead; and Leigh precipitates a carbonate from a solution of the chloride of the metal by means of carbonate of ammonia, which is only a more expensive way of operating without compensating benefit. Pattinson has a similar method, but precipitates the white lead by means of a solution of carbonate of magnesia in carbonic acid water, which solution he obtains from the mineral hydrate of magnesia, or from magnedia limestone; the solution he uses contains chloride of lead, and he treats the precipitate with caustic potash or soda, and he asserts that in this way his white lead becomes equal to the best known.
A method was recently patented in England and the United States to simply use an impure ore of lead of such a kind as is soluble in acetic acid, boil it with the acid, decant and filter the solution till clear, and then precipitate with carbonic acid. A common lead ore of this class is a mineral carbonate of lead of a reddish brown or gray color. It is abundantly found in England, but when introducing this method in the United States a great drawback was found to consist in the fact that not such a lead ore had been found here. Fortunately railroad cuttings in Missouri quite recently brought to light large deposits of this mineral, which are now being used for the manufacture of lead, white lead, and other lead compounds.
Dr. Vander Weyde, of New York, recently patented an apparatus by which the wood vinegar necessary for the solution of this ore, could be distilled from the wood at the mine, and the residue of the distillation, the charcoal, while hot in the still, was converted into carbonic acid gas, by simply blowing a current of air through the still, as soon as the volatile products were drifven off by the distillation; this carbonic acid gas, after passing through cooling and washing tubs, is used for the precipitation of the carbonate of lead, the whole process thus being accomplished in one apparatus and one operation.
Ny this process of using the lead ore, the labor of reduction to the metallic state is entirely saved, a labor required when following either the old or so-called Dutch method, or when using the lead sugar, or when dissolving in acetic acid the litharge which is manufactured from the metallic lead.
Generally the white lead obtained after the French method by precipitation, has not the body, or else does not cover so well as that prepared after the old Dutch method; the cause is recealed by the microscope; the precipitated white lead consists of little semi-translucent crystals - the Dutch white lead - out of opaque white grains, but later improvements in the French method have overcome that difficulty to a great degree; they consist in precenting the formation of these small crystals by the use of nitric, sulphuric, and hydrochloric acids, and thus form a compound which consists not only chiefly of a carbonate, but also of a sulphate and chloride, which last two, by themselves, are inferior to the carbonate, but when combined in the formation of the precipitate, appear to improve the pure carbonate in a manner not yet precisely explained.
Chemical analysis has proved that the pure white lead manufactured after the Durch process, is a compound of two atoms of carbonate of lead and one atom of hydrated oxide of lead, therefore is it probable that when the carbonate of lead obtained by precipitation after the French process was boiled with a sufficient quantity of a pure solution of subacetate of lead, it would take from this solution some hydrated oxide of lead, and become also a compound of carbonate and hydrated oxide of lead, and be as opaque, and dense of body as the Kremner white. A hint worth trying.
Of course the white lead manufactured after the French method is also adulterated with sulphate of baryta in different proportions, and this will be the case till a method is found of making pure white lead directly from the ore, and as cheap as the baryta, in which case the adulteration would not pay any more and come to an end.
V.
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