Expert: Sam Kelly - 2/15/2009

Question
My Questions are:  I was told that when Bisque Firing Clay most clay bodies are fired with 04 Cone for 2 hours low, 2 hours medium, and two hours on high.  I have been leaving the top hold open.  Sometimes if my pieces have dried for less time then I prop the lid open and fire on low for about two hours the day before - or just an extra hour on low during the same day firing.  Is this correct?  Am I supposed to close the hole after a while in a BISQUE firing?

Now for glaze firings - I was then told for low-fire clay bodies - glaze fire on low for an hour, medium for an hour and high for an hour.  I did this the other day (05 cone) and just let the kiln shut off on it's own.  My glazes really didn't turn out how I had expected them to.  I used purple haze and wanted it to turn black and purple, instead it was a light purple with little speckles.  I wanted it to run more.  It also crackled.  I think that I may have accidentally Bisque Fired at 05 cone - which would explain the crackling.  

I guess what I really want to know is - Did I glaze fire correctly - and if I want to achieve a darker color with glazes DO I need to fire on low for two hours and then graduallyl move up.  I also read that people turn up the bottom dial for an hour - then the top for another hour - etc.  I was told a glaze firing should really only last about four hours - Doing it that way - it would be about 6 hours.

ha - long long question.

Please write back whenever you can.  Thank you!!
Elaine Zobro


Answer
Hi Elaine,

When bisque firing you can close the hole at 600degC, at this temp all chemical water has been removed from the clay, do not open the kiln above 200degC.

Put cones throughout the kiln on the next firing and see what the temps. you are getting throughout the firing. YOU SHOULD BE USING CONES, DON'T RELY ON THE KILN FOR TEMP. SATURATION. Colour is usually associated with thickness of aplication and the amount of stain used to color it. Speckles indicates that the glaze was not mixed enough, I use a paint stirrer on a drill and then sieve through 120 mesh strainer. You can intensify the blue by adding cobalt carbonate at 1%.

Cracking claze is called crazing and is usually associated with glaze to body fit, your chosen glaze may not fit the body You are using, additions of silica(flint) can rectify this.

My bisque and glaze firings usually take 6 to 8 hours, if going to stoneware 10 hours.

Fire to the cones.

Sam