to see what happened when they were fired. Some worked, some didn’t. It’s all about the colorants used. As long as the materials you’re using don’t generate seriously toxic materials in the kiln . . . test! ;- )
Also, when you fire, think about the other ingredients, and when they’ll either burn off or evaporate just in case it will affect the final result. I look ingredients up for their flash point, etc. and try to have a hold low enough below that to prevent the flashing — or boiling in the case of water.
.
.