Heavy metal.

Vorto cups.

These cups are another favourite of mine. Been making them for years now. The glaze is totally food safe too. Something that bothered me for years. To get a nice solid chocolate or black you generally load a clear glaze with heavy metals like manganese dioxide, cobalt carb., copper carb., black/red iron oxide etc. Choose three of these and away you go. I'd prefer not to use any of them, as in various quantities they are deadly. The iron oxide (basically rust) is what I do use though; still toxic in quantity, but you’d need to eat a fair bit.