http://northstarglass.com/product/ns-119-super-unobtainium/

So I've been trying to use this for hollow work this weekend - which the site says is OK but it cracked on me every time.

First I tried layering it over some china white 25x4 tubing and prepped a couple of blanks. They cracked overnight, hours after the kiln.

Fuck it, I blamed the white and layered some more over 25x4 clear tubing and it seemed to behave a bit better. I went straight from prep to making a minitube.... The glass behaved fine while working it but I put it back in the kiln and the fucker cracked on me while cooling down last night.

NS says this can be used for hollow work but I can't figure out what I'm doing wrong. Any ideas?