Experienced traders can make a quick estimate of the purity by rubbing it against a touchstone, which has been used since ancient times. And by treating the rub

Experienced traders can make a quick estimate of the purity by rubbing it against a touchstone, which has been used since ancient times. And by treating the rubbings with mineral acids you can make even more accurate determinations, although I'm not sure if this was done in the 1620s. https://en.wikipedia.org/wiki/Touchstone_(assaying_tool) https://en.wikipedia.org/wiki/Touchstone_(assaying_tool)