Assaying, Microscopy, Mineralogy & XRF/XRD

A cupel has the property of absorbing oxide of lead and other oxides, such as those of copper, bismuth, zinc, etc., which are soluble in oxide of lead; while metals which do not oxidize in the atmosphere, such as gold, silver, platinum, etc., are not absorbed.

If there be more than three or four per cent, of platinum in the gold, its presence is readily perceived, from various situations:

- Evaporating the lead a higher temperature is necessary than is commonly the case, to make the metal flow and acquire a round form

- The surface of the button is crystalline or rough, and when large, flat and quite irregular, besides looking dull and having a more or less grayish color

- The nitric acid is frequently discolored

- The little roll of gold is not of a pure gold yellow, but rather inclining towards steel gray.