They don't use any clays with chemicals, not to mention, I try to save my questions in Yixing for more important things. I would guess that the cobalt is added when the raw material is mixed up, so I'm assuming not by the potters. But regardless, my company's factory doesn't use any chemical additives for any clay. This is just me being curious.
But if you really want me to ask, MarshalN, I will.