|Other names||basic lead carbonate|
|Molar mass||775.633 g/mol|
|Main hazards||lead poisoning|
| (what is this?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.
Other synonyms (as an art pigment)Editar
- ↑ 1,0 1,1 Inorganic Chemistry,Egon Wiberg, Arnold Frederick Holleman Elsevier 2001 ISBN 0123526515
- ↑ 2,0 2,1 Stack Process White Lead (Old Dutch Method). Commercial supplier, offering original pigments to the conservation and reproduction markets
- ↑ Zinc White Problems in Oil Paint. The Smithsonian's Museum Conservation Institute exposes long-term problems with zinc white
- ↑ Lead411.org based on Warren, Christian. 'Toxic Purity: The progressive era origins of America’s lead paint poisoning epidemic'. Business History Review. Winter 1999, Vol. 73(4)
- ↑ Hough, Richard (1994). Captain James Cook. Hodder and Stoughton. p. 56. ISBN 9780340825563.
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