White Lead

This substance was known to Theophrastus, Pliny, and Dioscorides. Theophrastus describes its preparation by the action of vinegar on lead, and Pliny mentions the same process. The Latin Geber also gave an account of its manufacture by the action of the vapour of vinegar on lead; and until Bergmann, in 1774, showed that white lead is a compound of lead calx and fixed air, this substance was supposed to be a compound of the calx and vinegar.

White lead was known as ψιμυθιον by Theophrastus, cerussa by Pliny, and luft-saurer blei-kalk by Bergmann.

Various processes have been employed for the manufacture of white lead.

1. Dutch Stack Process. - This is the oldest commercial process for the manufacture of white lead, and it is based upon the reaction which was known to the Ancients. Lead sheets, rolled into spirals, are lodged vertically upon three projections fixed at one-third of the way up conical, glazed earthenware pots, and are covered with lead plates, after the pots have been filled to one-fourth of their depth with vinegar. These pots are placed upon horse-dung in a shed, and by the use of boards are piled tier upon tier. Within four or five weeks the greater portion of the lead has been converted into white lead by chemical changes, the course of which is as follows: The fermentative oxidation of the dung liberates heat which vaporises the acetic acid; and this vapour, in conjunction with air, converts the lead into basic acetate. Meanwhile carbon dioxide has been generated by the fermentation of the dung, and this together with water-vapour acts upon the basic acetate, converting it into white lead or basic carbonate, and normal acetate. This latter by further action of carbon dioxide is changed into white lead with liberation of acetic acid; and the liberated acid then attacks fresh lead so that the process is repeated. The white lead is finally removed from any remaining " blue " or metallic lead, ground with water into a paste, washed so as to remove the soluble acetate, and then dried. The process of drying the lead pulp is attended by risk of poisoning to the workmen; and since the product is usually to be made into a paint, it is mixed with oil while wet, according to Ismay's process, and agitated in a " pug mill," which separates most of the water, the remainder being eliminated by surrounding the mill with a heating jacket whilst a partial vacuum is maintained.
2. Electrolytic MethodsEnglish Stack Process. - This method differs from the Dutch method in the substitution of spent tannery bark for stable manure and dilute pyroligneous acid for vinegar. The lead also, instead of being in sheets, rolled into spirals, is cast into straps which are laid on the pots. At the end of the process, which is rather more lengthy than the Dutch process, the white lead, which preserves the shape of the strap, though the bulk has increased, presents a white, porcelainlike appearance; and when the mass is broken a thin core of lead is found within it. The product is now crushed and sieved, whereby the lead is flattened out and separated from the carbonate. After washing and drying the pigment is ready for the market. If the white lead shows a yellowish tint owing to staining from the bark, this is counteracted by the addition of a minute quantity of Prussian blue or indigo.
3. Electrolytic MethodsChamber Process. - The chemistry of this process is similar to that of the former processes, but the use of dung or tan is dispensed with. A usual continental practice is to hang thin straps of lead over a mixture of vinegar and wine-lees in boxes heated to 30° C., and then to introduce carbon dioxide. The process is quicker than the Dutch process, and the product consequently suffers somewhat in quality.
   
   According to a patent process, the fermentation gases escaping in the manufacture of spirits are employed for the manufacture of white lead. These gases contain water-vapour, carbon dioxide, and a little alcohol. They are driven by means of compressed air into a special chamber, where the alcohol is oxidised to acetic acid, and thence into the chambers containing the lead.
   
   In Cookson's Chamber Process straps of lead are suspended in a series of chambers, and all the necessary gases - carbon dioxide, oxygen, water-vapour, and acetic acid vapour - are generated separately and introduced into the chambers by means of pipes in the floors. Perfect control of both temperature and gases is obtained, and the product is very white, being entirely free from contamination with tan, etc., which so frequently occurs in the old stack process.
4. Electrolytic MethodsFrench Method. - According to this method, which was originated by Thenard, litharge is dissolved in pyroligneous acid of 1.056 density, basic lead acetate being produced in solution. Carbon dioxide is then forced through the solution by means of a fan; and this precipitates white lead, leaving neutral acetate in solution, which can afterwards be made to dissolve more litharge so that the process of carbonation may be repeated. This process has undergone several modifications.
5. Electrolytic MethodsAmerican Method. - Metallic lead in a molten condition is broken into fine spray which is brought together with acetic acid into rotating barrels. Filtered combustion gases are blown with steam through the barrels for seven days, and the white lead thus produced is washed by means of water from the unchanged metal. The chemistry of the process is essentially the same as that of the preceding processes.
6. Electrolytic MethodsBronner's Method. - Freshly precipitated lead sulphate is warmed with caustic soda to 70° C., so that a basic sulphate is formed thus:
   
   3PbSO₄ + 2NaOH = 2PbSO₄.Pb(OH)₂ + Na₂SO₄;
   
   and when this product is warmed with sodium carbonate solution white lead is produced, thus:
   
   2PbSO₄.Pb(OH)₂ + 2NaCO₃ = 2PbCO₃.Pb(OH)₂ + 2Na₂SO₄.
7. Electrolytic MethodsMilner's Method. - The process of Dale and Milner consisted in grinding litharge or any insoluble lead salt between millstones with water and sodium bicarbonate; but the Milner process introduced as an improvement consists in grinding together for three or four hours 4 parts of litharge, 1 part of common salt, and 16 parts of water. The mass gradually swells up and becomes white and pasty, there being produced a mixture of basic lead chloride and caustic soda. Carbon dioxide is then passed through the paste with constant stirring until its reaction to litmus is neutral, the basic chloride thus being converted into basic carbonate. Too much carbon dioxide would spoil the product. A somewhat similar process is that of Hof, in which lead oxychloride, together with magnesium oxychloride, is produced by dissolving lead oxide in magnesium chloride solution. The former is then decomposed by carbon dioxide with the formation of white lead.
8. Electrolytic MethodsBischofs Method. - This process, which is carried out at the works of Messrs. Mond at Brimsdown, Middlesex, consists in heating litharge to 250°-300° C. in a stream of water-gas, whereby it is reduced to a black suboxide which is converted into a yellow hydroxide with water. Carbon dioxide then changes this hydroxide into white lead.
9. Electrolytic Methods. - The principle of these methods consists in producing lead hydroxide by electrolytic oxidation of metallic lead, which is then carbonated by the passage of carbon dioxide through the electrolytic cell. In a German process a 1.5 per cent, aqueous solution of 80 parts of sodium chlorate, and 20 parts of sodium carbonate, is electrolysed with an anode of soft lead, and a cathode of hard lead; the electrolyte is kept slightly alkaline, and carbon dioxide is passed through the solution.

In a French process a 1 per cent, solution of sodium chloride is electrolysed with lead electrodes, whilst a current of carbon dioxide is passed through the solution to precipitate white lead from the alkali plumbite formed.

White lead is an earthy, heavy, amorphous powder, which is seen under the microscope to consist of rounded or oval grains of diameter varying between 0.0001 and 0.00004 inch. This pigment is valued on account of its covering power, in which it much exceeds zinc white or baryta white; it is, however, rapidly blackened by hydrogen sulphide, and the product has the composition 4PbCO₃.PbS.Pb(OH)₂.

The covering power depends upon the density; the greater the density, the smaller the amount of oil absorbed and the more opaque the pigment. The presence of lead hydroxide in the pigment has been found by experience to be essential, and the nearer the composition of the white lead approximates to that of the basic carbonate 2PbCO₃. Pb(OH)₂ the better is its quality. When the composition approximates to that of the normal carbonate the pigment is useless.

White lead is frequently mixed with the much cheaper heavy spar, or with gypsum. Certain mixtures of white lead and heavy spar are of technical importance. Thus Venetian white is a mixture of equal parts of white lead and barium sulphate, Hamburg white of 1 part of white lead and 2 of barium sulphate, whilst Dutch white consists of 1 part of white lead to 3 of barium sulphate.

Owing to the poisonous properties of white lead attempts are continually being made to utilise other lead salts for pigmentary purposes, but hitherto with imperfect success, for the products do not possess quite the same "body" and smooth-working properties as genuine white lead. Thus Pattinsorts white lead was the basic chloride PbCl₂.Pb(OH)₂, the pigment galenite is a basic sulphate, and Hannaifs white lead is the sulphate made from galena, which is said to possess considerable covering power.