Thursday, 29 March 2018

Bright orange

Now in the past five years, I've had rather a lot to say about orange and also written some essays about arsenic. Up until now, however, my Venn diagram of language has had no intersect between these two concepts. Then my restless eye flitted across to the BBC and picked up a piece about orpiment [see chunk L; etymology: a contraction of aurum = or + pigment]. It seems that orpiment has been dug out of the sides of volcanoes for hundreds of years because, it makes a fabulously orange pigment and has been used by a succession of artists that we (you and me both) have heard of: Fragonard; Gaugin; Munch; van Gogh.

But it turns out that orpiment As2S3 is seriously toxic but not to be confused with realgar As4S4 although both are arsenic sulphides. They have quite different colours for starters. Wikipedia claims that orpiment was used in the manufacture of oil-cloth: I guess the bright yellow variety used for sou-westers and other nautical rain-gear. I guess it's marginal toxicity, even if absorbed through the skin through the salt-water boils that sea-farers got where the skin chaffed at the edge of their oil-skins, was better than hypothermia or pneumonia. Boils are usually caused by Staphylococcus aureus [bloboprev], a commensal bacterium that is normally kept on the outside by the physical barrier if the skin. Although I doubt if any 19thC sailor was presented with an informed consent risk assessment. Orpiment, mixed with Calcium carbonate and clay, is used as a depilatory in many Asian countries and so is readily available for accidental or deliberate consumption. It featured in a list of 9 toxic minerals in Forbes.

If orpiment is so toxic, how else do we get yellow into paintings and products?
  • If you like your yellow dull, then there is yellow ochre which is basically ground clay mostly ferric oxide with other earthy crud. It is accordingly quite variable in tone and quality. For completeness, there is an Australian yellow ochre butterfly Trapezites lutea.
  • Another brighter pigment with an ancient pedigree is Naples yellow chemically Pb2Sb2O7 that was known in Olde Bablylon.
  • Gamboge is derived from the resin of a number of different far-Eastern trees of the genus Garcinia. If you think of Buddhist robes, they are probably dyed with gamboge. Can't give you a chemical formula: it's organic and therefore complex.
  • Indian yellow, is also organic but a reasonably pure xanthanoid; formally known as (2S,3S,4S,5R,6S)-3,4,5-Trihydroxy-6-(8-hydroxy-9-oxo-9H-xanthen-2-yloxy)-tetrahydro-pyran-2-carboxylic acid or 2S3S to its friends. You may be skeptical about the producers' guff that it was [expensive because it] could only be obtained by feeding mango leaves to emaciated cows.  That is no longer the standard method of production, if it ever was.
  • There's a satirical book by Aldous Huxley called Crome Yellow. Chrome-with-a-h yellow is PbCrO4 = lead II chromate, as a pigment is was favoured by van Gogh and Seurat, but that's super toxic too. This is the colour of US school buses.
  • Cadmium yellow is CdS cadmium sulphide, although cadmium appeared often in my lectures about heavy metal poisoning and remediation of cadmium-contaminated industrial sites, CdS is considerably less toxic than orpiment and chrome yellow and has largely replaced the latter because it is brighter and spreads better.
  • Painters nowadays prefer a synthetic aniline-based dye called Hansa yellow because it is far less toxic than all the yellows above which feature metal ions Pb, As, Sb, Cd, Cr etc.

No comments:

Post a Comment