Hoke talks about a purple stain when testing gold. My experience indicates that to be true. Other elements may discolor your test, but if you use a spot plate, test, then empty the cavity, If gold is present, there will be tell-tale signs of purple in the cavity. It is for this reason that the spot plate should be kept scrupulously clean, so previous tests don't lend confusion to current tests. The purple stain will generally wipe out with a towel, but don't hesitate to clean with a drop of AR if necessary.
You can also get a little creative and test with a crystal or two of ferrous sulfate when you have questions about the presence of gold and the reaction with stannous chloride is difficult to define. The addition of the crystal(s) and close observation will generally disclose a film of gold, plus the solution tends to shift color with the precipitation of any gold that is present.
A third trick, one I rarely used, is to place a drop of the unknown solution on a piece of paper, and allow to spread. Test the drop and watch for colors to develop. Not nearly as reliable as the spot plate test, but can serve in a pinch.
Harold
