Owing to these difficulties in purifying carbonic acid, Prof. Gardner prefers to prepare a practically pure carbonic acid in the first place, and this he does by allowing petroleum, benzine, paraffin, or other hydrocarbon or carbonaceous liquid to gradually fall into a retort containing chalk or other suitable carbonate at a high temperature, whereby relatively pure carbonic acid gas is generated; and while still in a highly heated state it encounters a stream of air or oxygen, ensuring its complete combustion before entering the converting chamber.
Another method of procuring fairly pure carbonic acid is by the combustion and oxidation of any of the liquid hydrocarbons mentioned above, in suitable lamps; and when carbonic acid gas of exceptional purity is required, it may be obtained by heating bicarbonates to a sufficient degree to drive off one molecule of carbonic acid, reducing the bicarbonate to carbonate, from which it can be reproduced by treatment with carbonic acid gas obtained from cheaper sources.
The modus operandi adopted by Prof. Gardner is as follows: The lead is granulated and prepared for conversion in one operation, thus—an iron or a slate slab about 2-3 inches thick is placed in a tank containing acetic or nitric acid solutions rising about 3 inches above the upper surface of the slab. The lead is melted at low red heat and poured from a height of 4 to 6 feet into the acid solution, through which it falls till it encounters the slab, and thereupon passes away into the surrounding solution, being thus converted into a spongy condition. In this condition the lead is spread on frames or trays, which are then lifted bodily into their places in the converting chambers. The latter is closed and heated to 120° F. for 3-4 hours, or until the whole contents have assumed a uniform temperature. Thereupon the acid vapours and air are admitted and distributed, taking care that the temperature is not thereby reduced below 110° F. nor increased above 125° F., steam being temporarily shut off if necessary. This treatment is continued for 24 hours.
The admission of aqueous vapour should be so regulated that while a dry atmosphere is avoided, yet there is no appreciable condensation of moisture in the chamber. While ensuring this condition, the temperature may reach as high as 130° F. for a second period of 24 hours, but must not overstep the limits of 120° F. minimum and 135° F. maximum.
After 48 hours’ treatment, the supply of duly warmed carbonic acid gas is admitted for a period of two hours, without discontinuing the introduction of acid vapours, air and steam; and this addition of carbonating gas is repeated for two hours at a time with intervals of four hours during which it is cut off. When, after four or five days, efflorescence or exfoliation appears on the lead, the supply of carbonic gas is increased to two hours in every four, or four in every six; and the admission of acid vapours, air and steam may also be augmented so long as the temperature is not allowed to exceed 130°-135°F. The whole operation is completed in seven to fourteen days.
Of this process, Mr. Carter Bell, in his paper before referred to, speaks in the highest terms. No washing or drying is necessary. No women are engaged in the manufacture, and but few men. The white lead thus produced by the aid of electricity is deposited in a peculiar state of disintegration, it is perfectly amorphous and non-crystalline, of the purest quality; its density is 5·8. When ground in oil and made into paint, it possesses great body and a covering power inferior to no other paint, if not superior to them all.
Painters who have used the paint, practical men, and amongst these we may observe coach painters, have pronounced its excellence and superiority to the best ordinary white lead paint.
By this electrical process of manufacture, not only is the time consumed in the making and in the preparation of this material greatly shortened, but the cost of preparation is reduced, and added to this is the important fact, the vital factor in our consideration. The labour of women is unnecessary. No lives are sacrificed to its working requirements.
Prof. E. V. Gardner, who has been for some years occupying his attention with the subject of white lead making, with a view especially to remedying its existing evils, has invented his electrical chamber process of manufacture, and an entirely new course of after treatment. He has for the last seven or eight years been more or less occupied perfecting his conception, and accommodating it to practical and commercial claims.
Chamber processes are not new, there have been several patents enrolled for making white lead in closed chambers, but none has proved commercially convenient or practically successful in its adaptation, and none has survived to the present time.