White lead

White lead
Other names	basic lead carbonate
Identifiers
CAS number	1319-46-6
Properties
Molecular formula	(PbCO3)2·Pb(OH)2
Molar mass	775.633 g/mol
Appearance	white powder
Hazards
Main hazards	lead poisoning
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

White lead is the chemical compound (PbCO₃)₂·Pb(OH)₂. It was formerly used as an ingredient for lead paint and a cosmetic called Venetian Ceruse, because its opaque quality made it a good pigment. However, it tended to cause lead poisoning, and its use has been banned in most countries.

White lead has been the principal white of classical European oil painting. There have been claims that it is partly responsible for darkening of old paintings over time, reacting with trace amounts of hydrogen sulfide in the air to produce black lead sulfide. Other authorities dispute this; the most traditional view is that impermanent pigments and dirty varnish (which is often cleanable) are more likely responsible for darkening.

Paintings and the role of varnish, which might protect the white lead yet itself darken, aside-- according to Michelle Facini, a paper conservator at the National Gallery of Art, lead carbonate to lead sulfide is indeed what happens to some lead chalks/paints in drawings and watercolors and other works done on paper and unvarnished. Varnish is meant to be removable from an oil painting, to strip off when it dirties or cracks; but on paper it soaks through and becomes inseparable from the paper fibers, ruinous as it ages. This is why works on paper are never, or should never, be varnished. Thus far more frequently for paper works than for paintings, lead white was exposed directly to sulphur in the air (particularly from unfiltered coal say in 19th ce industrial London, for example) to turn to the black sulfide.

In any event, white lead has been mostly supplanted in artistic use by titanium white, which is structurally weaker than white lead. Winsor & Newton, the English paint company, was recently restricted from selling its flake white in tubes and now must sell exclusively in 150mL tins; even as in the US dutch-method flake white becomes increasingly popular as traditionalists, fascinated with the pigments of old, seek them out. Critics argue that many of these substitutes are much less permanent. White lead is less used by today's painters, not because of its toxicity directly; but simply because its toxicity in other contexts has led to trade restrictions that make lead white difficult for artists to obtain in sufficient quantities.

Historically, white lead was produced by the Dutch process. This involved casting metallic lead as thin buckles. These were corroded with acid in the presence of carbon dioxide. Next they were placed in pots with a little vinegar (containing acetic acid). These were stacked up and left for six to fourteen weeks, by which time the blue-grey lead had corroded to white lead. The pots were then taken to a separating table where scraping and pounding removed the white lead from the buckles. The powder was then dried and packed for shipment.

In the eighteenth century, white lead paints were routinely used to repaint the hulls and floors of Royal Navy vessels, to waterpoof the timbers and limit infestation by teredo navalis worms.

White lead occurs naturally as a mineral, in which context it is known as hydrocerussite.

Other synonyms (as an art pigment)

Venetian Ceruse, flake white, silver white, slate white, Berlin white, Cremnitz / Kremnitz white, Crems / Krems white, Nottingham white, Vienna white

References

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