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

by George Terry

When a stack is charged, the chamber containing it is enclosed. The tan or dung within soon commences heating, and the heat soon causes the acetic acid in the pots, and the water in the tan or dung, to rise in vapour and penetrate the stack. Air is admitted to the stack through openings left for that purpose, and carbonic acid is evolved from the heated decomposing tan or dung, and this gas also penetrates the stack, and the process of converting blue lead into the white lead gradually proceeds, and the blue metal becomes corroded and incrusted with a white crust or covering.

As to the exact chemical changes and combinations proceeding in the working of a stack, differences of opinion exist, but we may fairly conclude that the process resolves itself into this—first, the formation of sub-acetate of lead, which, decomposed by the agency of the carbonic acid gas, becomes reduced to the condition of normal acetate by loss of a portion of its basic oxide of lead. The reduced sub-acetate then again takes up an additional molecule of oxide of lead, and is re-converted into its original subsalt state, to be again attacked and reduced by the carbonic acid gas, and so on continually during the working of the stack. It will be evident that the “nascent” state of the various substances disengaged during the chemical changes which are proceeding in the stack is an important factor in this process, and must be taken into account in considering the philosophy of the operation.

It will also be evident that the mode of proceeding in white lead making by the stack process is most crude and clumsy, and a most uncertain method, one governed by rule of thumb, and, by no element of certainty or science. White lead makers, as a rule, know nothing of the chemistry of their subject. This absence of chemical knowledge of the subject, by those who are engaged in this manufacture, may explain the curious circumstance that for hundreds of years this industry has been pursued in the same old-fashioned and uncertain way, and the stack, or Dutch process, still holds its ground and displays little or no advance in knowledge or improvements in its method of proceeding, even in the present age of precision in almost every branch of manufacture. The uncertainty of the stack process is shown most clearly in this: That stacks may work and some do not work. In the latter case all the time and labour spent in forming the stacks, and all the acid they contained, is lost.

No amount of foresight will avail to determine beforehand which stack shall accomplish the conversion of its contained metallic lead, and which will not.

The stacks are generally allowed to remain in operation, after they are charged, three or four months; in this time it is presumed all profitable action in the stack has ceased. The temperature of the stack, which had risen gradually from the normal temperature to 100° or 150° F., will have gradually fallen, and this falling temperature is the indication that the corrosion of the lead in the stack has terminated.

After the three months’ action of the stacks, they are stripped and pulled to pieces. Some will be found to be done better than others, and one part of the same stack will be done more perfectly than another. The coating on the lead will also differ; some will be smooth, regular, and equal in formation, some will be rough and blistered, and far from uniform. The rough blistered casting is rejected as unfit for white lead making: the workmen call it “dross.” The smooth laminated coating is the one preserved for the after manufacture.

It is a curious fact connected with the consideration of the total want of educated guidance in these matters that prevails, that in all factories of this description some chambers are noted as always working well, and others are equally well known to always do their work the reverse of well. No one knows why! No one stays to seek the reason. The factory way goes on filling and emptying these white lead chambers whether the stacks be working well or no.

The incrustation that is most esteemed by the manufacturers of white lead in this old-fashioned style is a hard, china-like material, formed of thin deposits, layer upon layer, in a slow, continuous, regular way. It is at once conceivable that in the rough-and-ready manner of stack manufacture most irregular action must proceed.

It would be almost impossible for the contents of the pile or stack to be submitted to the same action of the gases throughout. Some parts of the stack and its contents will be under more favourable conditions than others, hence the reason why, in practice, it is invariably found that some stacks, and some parts of a stack, work better than others. Under the microscope, this good crust of white lead, the proper incrustation from which to prepare white lead, will be readily seen to consist of very thin coatings or layers of white lead, which have been slowly formed on the metallic lead and piled one upon another to the thickness of an eighth or a quarter of an inch. This formation constitutes the hard, china-like substance, which alone possesses the chemical constitution and the properties to form good white lead paint.

White lead makers, recognising this peculiar incrustation as the only one capable of fulfilling their desired purpose of making good white lead paint, do not even recognise any other as of any service for that purpose, be it good or bad. Such a material, if obtained, however good, would be outside their experience and beyond their philosophy. After the stacks have been stripped, the gratings or buckles with their adhering coating of white lead are moistened with water and are passed through crushing rollers to separate the unconverted lead; then the crust which has been detached is ground under heavy edge runners with water.