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underglaze causing cracks?

updated sun 10 jan 99


Penny Hosler on sat 9 jan 99

Perhaps someone can help with this. I glaze fired a bunch of 9x9 tiles
(low-fire slip cast)to =5E06. One test tile had been underglazed in the =
firing, one underglazed =26 glazed after bisquing, and the rest just glazed =
bisque with no underglaze. I was trying to see if the underglaze recipe =
work on bisque as well as greenware. All used the same clear glaze. Both =
the tiles with underglaze cracked nearly in half. The UG recipe is :
25 - EPK
25- OM4
15- Frit 3124
5 - Talc
20- Flint
+ various 2-15=25's of Mason stains

Is there something inherently wrong with my UG recipe that could be causing
this? Also, the colors on the bisqued tile came out somewhat less intense =
when used on greenware. Normal?
Penny in WA