Hi. I have been using blue black for my vessels that I slump through a mould. I also use a pattern of dense white and French vanilla next to the blue black, butting against it and sometimes overlaying slightly. The firing process is usually fine but sometimes creep away from each other. After slumping I have quite often (but not always) have trouble taking the rims off. The glass is taken off as Amanda Simmons does with a cutter and creating a run around the rim. The glass develops runs down into the vessel and I loose it. This has happened a few times which I'm getting very concerned about as it's getting expensive to loose the glass and the firing. I was wondering if there is a viscosity problem, what should I be aware of please? The blue black is perfect for the work I'm currently engaged with. As I said it only happens sometimes but enough to be getting expensive. Many thanks.