White Lead is a carbonate of the metal. The best is produced by the Dutch process, which consists in placing gratings of pure lead in tan, and exposing them to the fumes of acetic acid; by these they are corroded, and covered with a crust of carbonate, which is removed and ground to a fine powder.
Clichy White is produced in this way by the action of carbonic acid gas upon acetate of lead.1
Pure white lead is a heavy powder, white when first made; if exposed to the air it soon becomes grey by the action of sulphuretted hydrogen.
It is insoluble in water, effervesces with dilute hydrochloric acid, dissolving when heated, and is easily soluble in dilute nitric acid.
White lead may be purchased either pure or mixed with various substances - such as sulphate of baryta, sulphate of lead, sulphate of lime,2 whiting (see p. 254), chalk, zinc white, etc. These substances do not combine with oil so well as does white lead, nor do they so well protect any surface to which they are applied.
Sulphate of baryta, the most common adulterant, is a dense, heavy, white substance, very like white lead in appearance. It absorbs very little oil, and may frequently be detected by the gritty feeling it produces when the paint is rubbed between the finger and thumb.
When sulphate of baryta has been added, its presence is in most cases avowed; the mixture is called by a particular name, which indicates to the initiated the proportion of sulphate of baryta that it contains. Thus - Genuine Dry White Lead, Newcastle White, Nottingham White, Roman White, London White, are all names for pure white lead.
Kremnitz, or Krems White, known also as Vienna White, imported from Austria in small cubes; French White, or Silver White, in drops, from Paris; and Flake White, made in England in small scales, should also all be pure white lead, but they differ considerably in density.
2 Barium sulphate, lead sulphate, calcium sulphate.
Venice Wliite contains 1 part white lead to 1 part sulphate of baryta.
Hamburg White „ 1 „ „ 2 ,, „
Dutch White, or
Holland White " 1 " " 3 " ,,
"When the sulphate of baryta is very white, like that of the Tyrol, these mixtures are considered preferable for certain kinds of painting, as the barytes communicates opacity to the colour, and protects the lead from being speedily darkened by sulphurous smoke or vapours." 1