White Lead.
This pigment, which in addition to other good properties has remarkable covering power, was amongst the earliest known artificial pigments. Already in the fourth century before Christ, Dioscorides described the preparation of white lead, which was obtained by exposing lead to the action of the vapours of vinegar, removing the white layer and treating it with water. The Roman writers describe a similar method; they use the name cerussa, under which white lead is known to-day in commerce.
Although white lead has been so long known, it was left to Bergmann, in 1774, to show that it contained carbonic acid; before that it was believed to be lead acetate. The development of analytical chemistry was followed by a knowledge of its constitution and the use of more rational methods of manufacture. Whilst in the middle ages the manufacture of white lead was almost exclusively in the hands of the Dutch and Venetians, in later years it gradually spread, and now many works are concerned in the manufacture of this pigment. That adulterations of white lead were not rare in former times appears from the writings of Basil Valentine, an alchemist of the fifteenth century.
Commercially white lead is known under most varied titles, of which the following are the principal: White lead, Venetian white, Dutch white, Krems white, Kremnitz white, flake white, etc.
According to its chemical composition, white lead is a compound of lead carbonate and lead hydroxide, that is, a basic lead carbonate. Commercial white lead, apart from intentional admixtures of other white substances, contains lead carbonate and lead hydroxide in varying proportions, as is shown by the following analyses by Mulder, who found that all the samples examined by him were composed according to one or other of the following formulæ:—
| 2 PbCO₃.Pb(OH)₂, | containing | 86·27 | per cent. of | PbO. |
| 5 PbCO₃.2Pb(OH)₂ | ” | 85·86 | ” | ” |
| 3 PbCO₃.Pb(OH)₂ | ” | 85·45 | ” | ” |
| 4 PbCO₃.Pb(OH)₂ | ” | 85·00 | ” | ” |
According to Hochstetter, the manufacture of white lead must be directed to obtaining the compound 2 PbCO₃.(OH)₂, which possesses the following percentage composition:—
| PbO | 86·32 |
| CO₂ | 11·36 |
| H₂O | 2·32 |
The compound of this composition is distinguished by being completely amorphous, and so possesses the greatest covering power. Commercial white leads, as is seen from the formulæ of Mulder, may differ appreciably from this composition, when they will have a smaller covering power, since they will contain some quantity of neutral lead carbonate, PbCO₃ (containing 83·46 per cent. of PbO), which is crystalline.
White lead is prepared according to very different methods, the principle of which consists in subjecting a solution of tribasic lead acetate to the action of carbonic acid, by which the basic carbonate is produced and the neutral acetate formed, the latter being then reconverted into the basic acetate, which again serves to produce white lead and so on.