I've been casting stoneware with factory made slip and it's been going well. When I pour it, it's grey and of course glossy when wet, and then dulls down as it dries and becomes white like plaster in a day or two. For one of my mould experiments, it was simply too viscous/sluggish to work well, so I added some water to the slip to make it flow easier.
However, the next day the surface of the result was dark and greasy/oily!
I have not tried to fire that result yet, as I want to glaze it and everyone says that glaze won't stick if the piece even has too much oil from your fingertips, so I come here to ask for tips on how to deal with this. Has anyone encountered anything like what I'm describing? If so, what did you do?