Expert: Leisha Hiester - 4/15/2010

Question
Hi,
I recently purchased molds, slip and bisque from a lady who decided to retire.  Upon examination of the slip I notice it is a 05 cone slip.  Would it be safe to fire it to 04 cone?  I tried to contact the slip manufacturer, but they have gone out of business.  
Her bisque had been fired to 05, but I have been advised to refire everything to 04 so it would be compatible with underglazes and glazes.  Does this sound right to you?
Any help you have on this I would appreciate,
Thanks,
Tammy

Answer
Tammy,
It is fine to fire at 04. The difference between the two is only about 40-60 degrees. Lots of ceramicists fire at either 04 or 05 for bisque. If I'm ever unsure about anything I always, test, test, test first. That's my mantra.
Hope this helps. Leisha