The theoretical constituents of normal Dutch process white lead are claimed to be two molecules of lead carbonate (PbCO3) and one molecule of lead hydrate (PbH2O2), which would mean about 70 per cent lead carbonate and 30 per cent lead hydrate. The author, however, found in a series of extended tests, made in a practical way in conjunction with those made in the laboratory, that all other features being normal, i. e., fineness, purity and dryness of the material from a number of batches, the average producing best results after grinding in oil proved to be 72 per cent lead carbonate and 28 per cent lead hydrate. Batches showing 68 per cent lead carbonate or under proved to be deficient in opacity and without tooth under the brush, while when 73 per cent lead carbonate was exceeded oil absorption and spreading quality was lacking. Tests made with quick process lead on a similar plan and extended over a long period showed a decided difference, probably due to a difference in the structure of the particles of the pigment. Here it was ascertained that the extreme limit within which the constituents gave good results was not less than 75 per cent nor more than 78 per cent lead carbonate, and the best average 77 per cent. lead carbonate and 23 per cent lead hydrate. The pigment referred to here as quick process white lead, however, is not to be confounded with the many later day processes of manufacture, such as, for instance, the precipitation or electrolytic methods. It is undoubtedly the best process for quick corrosion, differing only in one respect from the old Dutch process or stack method, that instead of the metal being cast in buckles and placed in earthen pots that are imbedded in tan_ bark, the pig lead is melted, and while in this state is "blown" into fine particles by jets of high pressure superheaded steam, and the powder thus obtained placed in large wooden revolving cylinders into which a supply of acetic acid is introduced and the manipulation kept up by spraying the mass with water. In the meantime carbonic acid gas that is generated in a furnace by burning coke and washed and freed from sulphur is also passed into the cylinders through flues. The average time of corrosion by this process is twelve days, as against sixty days by the stack method, and the product shows a lesser percentage of uncorroded lead. We do not intend to dwell upon the superiority or deficiencies of the various methods of white lead manufacture, having gone into this description of corrosion merely to illustrate how closely this quicker method resembles the other and yet how results differ. In the stack method, it may be said, that nature is given its sway to a great extent, while in the cylinder corrosion method artificial means are employed to hasten the result. In the former process very little attention is required after the pots have been set ,while in the latter constant vigilance is required in order to obtain a good and uniform product. It is claimed for the latter method that the process of corrosion can be inspected at any stage and at all times, day or night, and that this fact favors the output of a superior product.
While this should be true to a great extent, the fact is that white lead corroded by this method, no matter how uniform it may be turned out, is somewhat deficient in density and hiding power when compared with a sample of normal white lead corroded by the stack method. Nearly all, if not all, of the white lead made by any of the quick processes exhibited a tendency to lighter specific gravity than that made by the stack method, requiring a greater percentage of oil for grinding into the paste form, in many instances as high as 25 and even 50 per cent. more, and until this defect was overcome it was difficult to market the product, because the consumers found a sad deficiency of body (hiding power) in the product. These brands of white lead lacked what the painter calls "tooth" or "life," because when the paste lead was thinned to a consistency of what the painter considered just right for application with the brush, it either flowed out like grease over the surface or was too "short."
The argument that the lead has the spreading capacity will not hold good with the painter, because it is the labor that counts for far more than the cost of the material, and if he can cover up a surface well enough with two coats of white lead paint he will not use three, and therefore will purchase the brand that gives him the result he looks for.