CHAPTER VII.
THE MANUFACTURE OF WHITE LEAD.
The processes by which white lead is or was manufactured may be divided, according to the principal operations, in the following manner:—
1. Processes in which metallic lead is subjected to the action of acetic acid vapour, whilst the vessel in which this operation is conducted is exposed to a higher temperature. In the oldest so-called Dutch method this increase of temperature is effected by the decomposition of manure, by which the vessels containing the lead and acetic acid are surrounded. In consequence of the heat produced by this fermentation, acetic acid and water are volatilised, and oxygen also being present, lead acetate is formed.
Also, as a consequence of the oxidation of the lead, heat is produced, which accelerates the process, and lead oxide is formed in large quantity, which unites with the neutral acetate already formed to produce a basic compound. The vessels in which this process is taking place are in an atmosphere containing much carbonic acid produced by the fermentation of the surrounding organic matter; this carbonic acid converts the basic lead acetate into white lead.
The German or Austrian method is to be regarded as an improvement on this rough process. The heat necessary for the normal course of the chemical reactions is produced from fuel; the carbonic acid formed by the combustion of the fuel is used to convert the basic lead acetate into white lead.
2. In the above methods the manufacture of white lead commences with the production of white lead from metallic lead and acetic acid. In the so-called French method a solution of basic lead acetate is decomposed by carbonic acid into white lead and neutral lead acetate, which latter is again converted into basic acetate.
3. The English method. The principle of this process consists in moistening litharge with a solution of lead acetate and exposing it to the action of carbonic acid, whereby white lead is formed.
Methods for manufacturing white lead, which are often advanced as entirely new processes, may be always traced to one of the above, from which they deviate but little in principle, and the deviations cannot always be regarded as improvements. In the following detailed account of the manufacture of white lead we shall adhere to the classification just given, according to which there are three principal methods: (1) Manufacture of white lead from metallic lead, acetic acid and carbonic acid (Dutch and German processes); (2) manufacture of white lead from basic lead acetate (French process); and (3) from litharge moistened with lead acetate solution (English process).
We should remark that every year “new” processes for the manufacture of white lead are patented. The majority of these will not be mentioned; to those acquainted with the principles of chemistry they at once appear impracticable.