What is the best kiln firing schedule to avoid devitrification?

by Leslie
(Lansdale)

I am so frustrated and I didn't have this issue with the same kiln firing schedule in the cooler weather - now that the weather is hotter outside, I'm getting a white, rough scum on the bottom of my fused, 1 layer glass cabs!! Is there a firing schedule that will avoid this. I don't understand why this is happening. All my pieces were coming out the kiln fine in the winter/spring and now that the summer has arrived I'm now getting yucky pieces!! Can you please help with a kiln firing schedule for me? Thank you, Leslie

Milly's reply:

Firstly, are you using the same kiln wash / separator or ceramic fibre paper as before? Thinfire paper can cause dullness on the bottom of fused glass, particularly if you try and reuse it. Has the kiln or kiln shelf got damp - maybe with humid heat?! Do you get that in Lansdale?!
The first step in avoiding devitrification is cleanliness - I can't stress how important this is. If all your glass is spotless, you could try a devitrification spray before fusing glass. You can buy them ready mixed, spray it on thinly and wait for it to dry thoroughly before firing.
Devitrification occurs when you hold the glass at the fusing temperature too long. You don't say what type of glass you're using, but a full fuse temperature is around 1450F/788C to 1500F/816C. Hold for 10mins, and cool as fast as possible. You could crash cool at this stage by opening the glass kiln door, but wear goggles, fire resistant gloves and any protective clothing you can get your hands on!
I hope this helps a bit - it's really annoying when something that has worked suddenly stops working, and you haven't changed anything... Grr.