Pigments, Paint and Painting: A practical book for practical men

by George Terry

The washing out of the acetate is never perfect, and it involves a large amount of labour.

Opinions differ as to the effect of this acetate if allowed to remain in the product. White lead makers on the “stack” principle aver that it should and must be washed out, lest it should damage the qualities of the paint. This is questionable, and not one can produce practical evidence of its being the cause of any damage if still contained in white lead. Facts seem to deny its harmfulness in this respect, inasmuch as the best prepared samples, those washed and dried from the most careful makers, will be found upon analysis to contain more or less of acetate of lead.

A large proportion of this salt in white lead may not be beneficial for many reasons, but a small percentage can do no harm; nay, for many purposes it may be good.

There is no substance used for driers for white lead that is more esteemed than this acetate of lead, commonly known as “sugar of lead.”

A small amount of this salt present in white lead would communicate drying properties, and this alone is what it could do.

Granting that we can discover a method of producing white lead of amorphous character, of good density, free from all discoloration, free from all particles of metallic lead, and free from all but a small percentage of acetate of lead, then washing will not be needed.

Stoving and drying become unnecessary. The work of women, their deadly occupation, so burdensome to the operatives and to all with whom they are concerned, is done away with.

Condy’s Process.—An improvement in the manufacture of white lead was patented by Condy, of Battersea, in 1881, which, though giving perfectly satisfactory results when carefully conducted, necessitated special precautions, and led to his substituting in practice the following additions and modifications, which are of great consequence in rendering the process more certain in the quality of its product, and more valuable as a commercial manufacture on the practical scale, by virtue of its offering greatly increased facility and economy in production.

The results of numerous and repeated experiments on the larger scale induced Condy to qualify the recommendation contained in his first patent, viz. that of employing a solution of tribasic acetate of lead and bicarbonate of soda in proper proportion to precipitate nearly the whole of the lead, and further stating that he preferred to employ “a slight excess of tribasic lead salt rather than find carbonate of soda in excess.” Though, when carefully conducted, if the greatest nicety is observed, a satisfactory result is obtained; in practice, the least variation from the exact composition of the two substances is attended with the drawback that the white lead is liable to a slight uncertainty of tint after it is ground in oil, whereas by the process hereinafter described a positive and reliable result can be obtained, as the white lead produced will be of a uniform white colour, and not liable to turn when ground in oil. Though the earlier process was in itself complete for the manufacture of white lead from oxide of lead, it afterwards occurred to Condy that a great object would be attained in rendering the process more valuable and more practical, if a method were devised, worked out, and described for the manufacture of tribasic acetate of lead to be made entirely from metallic lead by the action of acetic acid or neutral acetate of lead on metallic lead, and not to be dependent on the employment in any way of previously manufactured oxide of lead.

This portion of Condy’s invention relating to the manufacture of tribasic acetate of lead may be described as follows: he melts, and, after skimming carefully, feathers the metallic lead by dropping it into water; he places this granulated lead in wooden vessels or vats previously fitted with perforated false bottoms under which are fixed taps for drawing the liquor off into other vats or tanks placed on a lower level. Having filled with granulated lead the vessels fitted with the false bottoms, he fills up the interstices with a dilute acetic acid composed of one part, by weight, of acid (specific gravity 1·045 at 60° F.) and 12½ parts of water, and after allowing the dilute acid to stand for two hours, draws it off through the taps into the lower tanks. This allows access of atmospheric air to the lead, which has the effect of heating the lead so that oxidation takes place.