French Method of Making White Lead.

Manufacturer and Builder 9, 1872

(See page 257, VOL. III., and page 8, VOL. iv.)

Even as the oldest way of making white lead, invented in the old Republic of Holland, is still called the Dutch method, so the way later invented in France is universally known as the French method. It is founded on the fact that neutral acetate of lead or lead-sugar when in solution can dissolve still much oxide of lead, and this oxide may then, by means of carbonic acid gas, be precipitated from the solution as carbonate of lead or pure white lead. The whole process may be executed perfectly on the smallest scale. The product of this manufacturing method is the best looking white lead which can be made. Notwithstanding this, it does not enjoy the universal favor; the reason of this we will see later. The beauty of this method of manufacture is, (as already remarked, page 257, Vol. III.,) that the purity of the lead, or of the litharge used, is of no influence on the result; the color is always a very pure white anyhow, as the carbonic acid only precipitates the lead and leaves the impurities in the solution.

The manufacture is divided into two main operations, which are separately executed, firstly, the solution of lead oxide in acetate of lead, or the preparation of the basic acetate of lead solution; secondly, the precipitation of the oxide of lead by means of carbonic acid, and the preparation of the latter.

For the first operation, two methods are followed, according to the desire to use metallic lead, or litharge, which is its oxide, and this is only a question of econnomy regulated by the prices of either.

For the solution vinegar is used, which must be pure from foreign organic matters, or one may use at once crystallized lead-sugar, which consists of lead oxide and acetic acid. This is dissolved in water, in large wooden troughs, in which steam can be blown for heating; the lead-sugar solution or the acetic acid is then made to act on finely powdered litharge; it dissolves quickly when the solution is kept boiling hot, and the hydrometer indicates by raising an increasing specific gravity of the liquid. This goes to a certain point, when at once a basic salt is separated, and the solution becomes lighter and poorer in lead again. This point is to be avoided by adding more acetic acid or lead-sugar; and when the safe maximum saturation is reached, the liquid is left in rest to settle and clarify, when it is drawn off for the second operation, while the former vessel is at once again filled with acetic acid or acetate and litharge.

In case metallic lead is to be used, it is either loosely rolled up in thin plates, as in the Dutch method, or granulated by casting it through a sieve into water. This lead is then loosely spread on the bottom of several wooden tanks, and on this poured so much acetic acid till the lead is covered. When any lead-sugar solution is on hand, this may also be used. After half an hour, the liquid is drawn off; it serves only to moisten the lead; this moistened lead becomes then soon so hot by oxidation that watery vapors arise. When the heating has become considerable, the liquid which was drawn off is pumped up again, and now it will dissolve the coat of oxidized lead in a few hours, when it is again drawn off; then the lead becomes again hot by renewed oxidation. The liquid is put on and off till saturated, and when the metallic lead has nearly disappeared, it is coated with a black dirt; this is washed off with water, and the black washings utilized, as they often contain silver, for which reason it is sold to silver-refiners; its principal impurities are carbon and sulphuret of lead. which make it black. The washed lead is piit in the troughs again with fresh lead.

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The second main operation, the furnishing of the required large quantities of carbonic acid gas, is the most difficult. There are mineral springs which produce tolerably pure carbonic acid in large quantities, and such localities would therefore be excellent for such factories as we describe here. We know thus far, however, of only one who was taken advantage of such a cheap supply of the needed ingredient; it is one on the shores of the German Rhine. If the gas is so disposable, that it may be stored in a gas-holder, and given a pressure equal to a water-column of two to three feet high, it may be used at once by passing it through the lead solutions, but it is advisable to purify it first from sulphuretted hydrogen, by passing it through an emulsion of oxide of iron in water, in order not to obtain sulphuret of lead, which being black, would spoil the white lead.

There are no doubt many places on this continent where such naturally escaping gases could be utilized, to great profit for the manufacturer, but where this is not the case, the cheapest way to obtain it is from a coal fire. The air is blown or drawn through the same by a suitable blower, passed in a gas-holder over water, cooled, and this then blown in the solution under certain pressure.

In the latter case a precaution is required, namely, to give free escape to the non-absorbed gases, which are nitrogen and carbonic oxide; the latter being especially poisonous must be removed carefully by proper ventilation, in order not to injure the workmen. Even when using carbonic acid prepared in any other manner, it is advisable to have well-ventilated rooms. The wooden vats in the same are 6 to 7 feet wide and 2 to 2½ feet deep; they are provided exteriorly with iron straps, and on the inside painted with white lead and oil. They are provided with a well-fitting cover, while a copper tube of two inches diameter enters, and is bent in a circle along the bottom of the vat. This circular tube is full of small holes not larger than pinholes, out of which the carbonic acid gas flows and ascends through tire liquid. At the lower side of the vat is a cock to draw off the liquid, and also an opening at the upper part, connected with a flue which leads the the escaping gases out of a window or the roof.

The vats are filled to a height of 1 to 1½ feet with the saturated lead solution; the carbonic acid under the necessary pressure is then admitted by opening a cock in the copper tube, and ascends through the solution in very minute bubbles, by which the carbonate of lead is formed, and floats in the solution as long as there is any excess of oxide of lead present.

As soon as this is exhausted any further supply of carbonic acid is useless, and in order to see if this point is reached, a sample of the milky fluid is drawn and tested with blue litmus paper; as soon as this begins to become red, which usually happens after a few hours, the gas is turned off, and the liquid drawn out in another vat narrower and taller, in which it is left for twelve hours to settle, when the clear solution of acid acetate of lead is drawn from this and used again, to dissolve a fresh portion of litharge, which when saturated and alkaline is again placed in the shallow vat, to be treated with the carbonic acid; and so the operation is repeated.

It is evident that this method of manufacture possesses a regularity without any difficulties, which makes it very economical in its operations. By means of well-arranged pumps, the liquids may be transmitted by any kind of power, while a single man may then operate a tolerably large establishment; the heat obtained for producing the carbonic acid by combustion may be utilized for drying nearly all the white lead produced.

The latter must first be cleared from the adherent lead solution. It is important to do this perfectly, because in this case the waste of acetic acid is greatly reduced, which otherwise reaches a considerable amount in the course of a year.

In order to do this, the wet white lead is placed on filters, consisting of square pieces of linen, loosely stretched on frames, where the adhering solution slowly drips out; when this dripping has ceased, the thick mass is mixed with its own volume of water, and again filtered; this may be repeated when the drippings become very dilute; or the mass is placed in a press and the liquid pressed out.

Much trouble has been taken to give this French white lead the appearance of Kremser-white, namely its hardness, but in vain. It is found in the trade also in square pieces, in similar packages as the Kremser-white, but it may be at once distinguished from it by its lesser weight, lesser compactness, and greater friability, while the Kremser-white is very hard and compact. Most of the French white lead is therefore sold as powder packed in kegs.

Considering the method of its manufacture, it is probable nothing but pure neutral carbonate of lead, without any hydrated oxide of lead, which is contained in the other kinds of white lead, and this is the cause of its different properties. Probably, if boiled with a sufficient quantity of basic acetate of lead, it would take from this oxide of lead enough to become identical with the Kremser-white.

French white lead is, as well as all other kinds, adulterated with heavy spar (sulphate of baryta.) This is done in the factory by mixing very finely pul verized heavy spar with the milky emulsion of the white lead before it is dried. Of course, the white leads of whatever quality as sold in this country, ground in oil, are all more or less adulterated in this way, sometimes twice, first in the factory, and for the second time by those who prepare them as paints by grinding them in oil. No wonder that they do not have the body of the old-fashioned white leads.