Excellent posts Gentlemen: I made my own cupels out of cow bones, sometimes mixed with 5 - 10 % cement, but bone alone is preferred for it's selective porosity / Pb absorption.
What is being posted is a form of Scorification. In the case of the potato, it is being converted to carbon which why it works as a form of cupel.
One futher point do not wear any synthetic cloth if you are smelting more than a tiny bit. The heat of a normal smelt will promply melt a synthetic and believe me, it sticks to you exactly the same was hot glue does, and burns nicely. I have a nice scar on my tummy to show how I learned this.
AS has been posted, NO WATER anywhere near your work, and everything has to be moisture free. The explosive reaction of water scatters your Gold everywhere including upon your pore suffering skin.
Believe me, it hurts like hell both financially and physically.
Don Jose de La Mancha