Gents
For my sins I was once an aspiring Geologist. It was a long time ago but one thing about minerals sticks clearly in my mind. They differ in composition and colour a lot. If the relative amounts of arsenic to trace elements is important for the etch to work there is no saying that this will always be the same colour. Often other trace elements will alter the colour, sometimes a lot.
The result of this is that a good "etching " realgar may be red in one mine but may be yellow in another.
Regards
Roy
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