The black clay I used was a porcelain with black mason stain. These were a single fire, but I have used it on bisque ware and there are still bubbles if it is thick.
I have a slow warm up at the beginning but after that its just
300 degF/hr to 1980
110 degF/hr to 2180 or whatever bends cone 6
No slow cool or hold.
Here is one more of tile one through a magnifying glass, shows ever-smaller bubbles. Now I have to go fire up the Electron microscope, where did I put that thing?