A simple little observation today – last week I used my iron ladle to melt some lead for a trial splindl ewhorl casting. Then I used it for melting modern lead free pewter. Once liquid, instead of the usual purple oxide layer, it formed a yellow one instead:
Looks like brass to me.
Only it’s actually tin, with a little bit of lead, and what I think might be happening is that the residual lead has gone into solution in the tin,which means that there is some lead on the surface, and what you get is a thin bit of lead oxide, which is yellow.
Of course I can’t prove it one way or another without some nice equipment which I don’t have access to. Even better, it suggests why some alchemical recipes manage to produce yellow colouration.
Or I can read up on it. This book (linked to at the bottom of this entry) in google books, about tin/ lead solder in the semiconductor industry, mentions that lead oxide is yellow, and that a certain tin oxide, SnOx.H2O, is yellow. On the other hand it also says that the oxide layer has interference colours of purple/ blue/ yellow. I’ve only ever seen purple/ violet/ blue on this lead free pewter before though, and so I reckon I am correct in ascribing this to the presence of the lead, but whether it is having an effect on the oxide layer thickness I don’t know.
It is so frustrating being, effectively, stuck right back in the 19th century, without any of the equipment to see exactly what is going on. I remember reading an old paper, from the early 20th century, about the surface colouration of metals through their oxides or similar, and even then they weren’t sure about what was going on and had different theories.
That just makes me feel even more amazed that they ever managed to work out what was going on, through the work of lots of individuals and their communication within the scientific community over many years.