Expert: Sam Kelly - 7/14/2008

Question
I have bisque fired some small rectangles of clay to 1040 oC.  I now want to glaze them.  I think i've bought the wrong glaze.  It says the firing range is 1240 - 1280.  Can I do this??? I'm under time pressure and I've spent a fortune buying the glaze - it's a matt white.

Answer
The glaze you have purchased is a midfire glaze to stoneware glaze. If the clay you have is for cone 7 to cone 9/10 then everything is ok.
If you have a cone6 or below(earthenware 1100dC) clay then the glaze you have purchased is not correct.
If you are firing to cone6 I would test the glaze if you can't return it to the store. Cone6 is only 20deg less than your minimum firing range and a soak for half an hour or so at the end of the firing may work.
Your bisque temp. is a little high though not unusual. Mostly a bisq tenmp. of Cone06(1000degC)is ample. Going higher will help to fit a crazing glaze but it also makes it harder fer the glaze to stick and dry on the pot.
Here is a link to a cone/temp chart, the middle list is probably the normal, http://www.evenheat-kiln.com/technical/pyrochart/pyrochart.htm

Sam