The gaseous mixture, consisting of volatilised acetic acid, carbonic acid, and aqueous vapour, is admitted from the boiler and furnace into the chamber in quantity and proportions best adapted to each stage of the work. The chemical reactions resulting are analogous to those which take place in the Dutch process, and the Brumlen and Dahn process. Each operation lasts six weeks, and gives a product of about 20 tons of white lead, with a consumption of 7 per cent. of acetic acid, and 9 tons of coke. The residuum of lead is about 10 per cent. of the quantity placed in the chamber. At the end of each operation the white lead taken from the chamber is washed and purified in large tubs, some of which are furnished with filters. Finally, it is packed in earthen vessels, and dried, when it is ready for the market, and is sold either in cakes or powder.
The establishment at Cogoleto, above referred to, is able to produce annually about 2000 tons of white lead, of which 1200 tons are produced by the Rhenish process, and 800 tons by the Brumlen and Dahn process. There is also in a manufactory in Milan, the process of revolving heaters. The process of precipitation has not yet been used in Italy. The Rhenish process, as above described, furnishes the greater part of Italian white lead. The smaller manufacturers still use the Dutch process, but the Brumlen and Dahn process is generally regarded as unsatisfactory.
It is believed that the production of white lead in Italy during the first ten years of this century was about 300 tons per annum. The total annual production at present is, in round numbers, about 3500 tons, of which 2800 are produced in the Ligurian manufactories, and about 300 in those of Naples. Milan also produces about 300 tons.
The product finds a market almost exclusively in Italy. The annual amount exported is about 300 tons, and goes chiefly to Constantinople. Of the quantity of 2000 tons per annum produced at Cogoleto, about 300 tons is sold mixed with oil. No exact statistics are attainable as to the total amount of Italian white lead which is annually sold other than in the dry state, but it is believed to be little, if any, larger than 300 tons.
When the price of white lead was high, sulphate of baryta was mixed with carbonate of lead to produce white lead of inferior quality, but in consequence of the present price of white lead the use of sulphate of baryta has almost entirely ceased. No other methods of adulteration are known to be in use. In Italy, white lead is universally known as “biacca” when it is sold in cakes. When sold in powder it is known as “carbonato di piombo” (carbonate of lead). The lead from which all the white lead made in Italy is manufactured comes from the lead mines of the island of Sardinia, with the exception of a very small quantity of argentiferous lead coming from Spain. The acetic acid used in the process of manufacture comes from France. The market price of white lead in Italy is now about 45 lire the quintal (equivalent to 18s. per cwt.).
Lewis’s Process.—Many attempts have been made to substitute for carbonate of lead—the ordinary poisonous white lead, that slowly but surely induces paralysis in those who come in contact with it for any considerable period—some less deleterious pigment. Zinc white has often been put forward as a substitute, and is indeed largely employed; but it is open to the objection of not possessing sufficient body or opacity. Sulphate of lead is not poisonous; but, when prepared in the ordinary way by precipitation, is of a crystalline nature, and, therefore, wanting in both these qualities.
A sulphate of lead has, however, been produced by John T. Lewis, of Philadelphia, by sublimation, which, when treated by the Freeman process, is stated to possess a body and colour superior to the best white lead made by the ordinary process, with the additional merit of being cheaper, while it is also non-poisonous.
In the smelting of lead ore into pig lead, 15 per cent. goes off in fumes, and 10 per cent. are all that it has hitherto been found possible to recover; by this process, however, the whole 15 per cent. are recovered. The sulphate may also be produced from a low quality of galena, or lead ore, which is not suited for smelting into metallic lead, and from the slag formed in the process of smelting.
The plant consists of simple subliming furnaces, iron cooling pipes, suction fans, and a series of flannel or calico bags, arranged vertically in a building, well ventilated, so as to allow the filtered gases to escape, leaving behind the sublimed white lead, which is then merely shaken down from the bags into barrels placed beneath them. The manufacture is carried on at Joplin, Missouri, with four subliming furnaces, about 500 feet of cooling pipes and towers, with suction fans, which drive the fumes into 300 bags, 20 inches in diameter, and 38 feet long, arranged vertically. About 50 tons weekly of white lead are now being produced from waste fumes, slag, and poor ore, by this establishment alone, at a cost not exceeding that given in the following figures:—
| £ | s. | d. | |
| 20 cwt. of galena (with 82 per cent, of ore) | 8 | 0 | 0 |
| Cost of subliming and catching, including repairs, labour and all expenses | 1 | 10 | 0 |
| Casks | 0 | 6 | 0 |
| 9 | 16 | 0 |