This detached crust of white lead will vary much in colour: it will be white in some parts, yellowish or greyish in others. These discolorations arise from various causes, but they are principally caused by the contact of the moist wood and tan. The white lead is now a rough crushed material, very hard, and requiring to be ground to the finest powder. It contains, also, small fragments of blue lead which have passed the crushing rolls, and a quantity of acetate of lead. The presence of acetate of lead is always found in larger or smaller quantities, which vary with every operation, and which invariably accompany white lead produced in the stack.
To remove discolorations—to separate the fragments of metal and to dissolve out the acetate salt—much water and washing are employed. The material is ground with water under the heavy edge runner stones, it then proceeds to a series of horizontal mills, each succeeding mill set closer than its fellow, and is further and further ground to fineness with water. From these mills it runs as a milky liquid to a series of settling tanks, where it is allowed to subside, and the clear fluid is run off to waste, or into tanks to be used over again. This waste water will now contain the colouring matter removed from the incrustation, and the principal portion of the acetate of lead which the incrustation previously contained, and any other soluble matters removed from the washed and ground material. The small fragments of lead which passed the crushing roll and edge runner mills will have been previously removed by subsidence in water.
The white lead deposited in the tanks is in some factories ladled out into skips and agitated by a “dolly,” which further enables the heavy powder to get free from the water in which it is entangled. The moist powder is next placed on trays or dishes, and is conveyed to the stove or drying chamber. Women always perform this work.
The drying or stoving room is a large enclosed space heated by a “cockle” arrangement; rough scaffoldings are erected within this chamber, on which women mount to stow the trays on shelves fitted for the purpose. The trays and their contents remain in the heated atmosphere of this chamber for two or three weeks, by which time they become dry and ready for removal, to be packed in lumps for certain markets, or ground to dry powder and packed in barrels for others.
Women are employed to fill and also to empty the drying or stoving chamber, and during this work they are fully exposed to its contaminating atmosphere. Hot and dry, and charged with fine dusty particles of white lead, it becomes a dangerous trap, and contaminates the blood of those engaged with its deadly poison. It is in this part of the manufacture that the principal damage to health occurs. This is the most laborious work; heat makes it very fatiguing, the atmosphere within this chamber being always much above the exterior air.
Recent Government regulations have sought to curtail these and other evils in this manufacture. Women engaged in these stoves are ordered to wear overclothing, headdress and respirators. The general experience of their practice, notwithstanding Government regulations, is this; that they cannot work in them with ease and convenience, and more often wear the respirator around their necks than in front of nose and mouth. The excessive mortality in women who work in these stacks and stoving houses scarcely requires assertion. Few, even of those who employ them, know the extent of the deadly operation. Recently, medical men have made public that cases are within their knowledge of children born already contaminated with lead poison. Woman labour should surely be restricted by Government enactments in all such deadly occupations.
We may sum up the whole matter as regards white lead making by the stack or Dutch method in a few brief words: It is a most tedious and uncertain operation; it is a most dangerous occupation for all concerned; it is founded upon no true principles of any kind; and of science its whole course is ignorant. White lead making is ruled by a “happy-go-lucky” philosophy. The representatives of this manufacture are completely ignorant of the scientific details relating to it, and hence we may not be surprised to find amongst them an enormous amount of ignorance and prejudice.
Good white lead will not differ materially in its composition by whatever process it may be made, but it may differ seriously in its physical character, and in its fitness to produce a substance adapted to the uses to which white lead paint is applied. Good white lead is a compound which contains hydrate and carbonate of the metal, in the proportions either of one molecule of hydrate of lead combined with two of carbonate, or is made up of one molecule of hydrate with three of carbonate of lead.
If we consider the first compound roughly
PbH2O2,2Pb CO3