I have been having a strange issue with my Cone 6 Black Stoneware Clay from Standard (266). It gets these big cracking bubbles upon glaze firing. I bought this clay to marble with Porcelain (Standard 365) because it had a similar shrink rate. The marbled pieces came out SUPER bubbly, really horrible. I thought it was just poor clay preparation on my part because of the mixing of the two. However it has been happening with pots I make solely out of the Black Stoneware. I prepare this clay the same way I do all my other clay bodies and never really have this issue otherwise. Has anyone else had this problem with this clay? Any advice?