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Stone Spiral

Disappointing Glaze Firing - Help?

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I just completed my first glaze load. There were several things that I need some help with - and would love to hear your insight. It was a slow ramp to Cone 6 with a 4 minute hold.

1 - Several pieces cracked.

Three plates, a bowl, and a cup. Why might this happen? How can I avoid this happening in the future?

2 - Some of my glazes did not turn out correctly - at ALL

One was Spectrum Crimson, the other a home made turquoise. The commercial crimson turned out glossy white - with the occasional purple-ish streak. The turquoise turned out a matte white, with an almost chalky quality. The glazes were well mixed and sieved; I'm wondering - is it possible for colour to burn out? For example, the commercial glaze says it's good for Cone 4-6, and I fired to Cone 6. Could this particular glaze possibly need to be fired to Cone 5 only? Other ideas?

3 - Bubbling

A few pieces came out with bubbly glaze - very badly on one piece in particular. It wasn't the same glaze (one was cobalt blue, the other a glossy black). I was told that soaking the load for longer would help avoid this... I also heard that one shouldn't soak too long, as it can cause over firing. What do you think - and how can this be avoided?



4 - Guide Cones
The cone 5 guide cone was completed melted, not just bent over/slumped. The cone 6 guide cone hardly had a wobble in it. Why would cone 5 look SO over-done, while it looks like cone 6 was barely reached?


Thank you so much! I was pretty bummed when I opened the kiln... however, I still consider it a success as I am learning so much. There were several interesting pieces, nothing exploded, nothing stuck to the kiln shelf... (I like to look on the bright side).


Thanks again for your insight!

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1. Uneven walls is the number one cause for this most of the time. Or a combination of this and firing to fast.


4. Cone 5 and Cone 6 are pretty far apart. I contacted Orton about this and they actually have started making a Cone 5.5 in order to help solve this problem of 5 being down and 6 being stiff. Maybe get some 5.5 Cones. I use them now. 


Also don't be to upset about a bad load it happens. Figure out what you want to change in your firing process and make more pots and fire again. Remember only change one thing at a time.

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It sounds like your kiln milght have fired unevenly ... next time put a set of guide/witness cones on each layer of shelves.

Some electric kilns fire almost a Cone cooler on the bottom or top shelf.


I highly recommend the book "Clay, a studio handbook" by Vince Pitelka.

It addresses most pottery issues and has a hundred pages on glazing and firing.

It is worth every penny.

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1. They probably cracked in the bisq firing (most do) and the glaze firing finished them off.

2.I second Christ Campbell: sounds like very uneven heat in the kiln.

3. Any chance you used two different clays?  porcelain and stoneware?   stoneware is more susceptible to bubbles and pinholes. Although cobalt can boil at cone 6 pending flux levels.

4. see answer 2.


Extended holds will help with pin holing and bubbling. If your glazes are rated to cone 6, they should survive just fine. Did you run up to 2190F with a hold, or to 2230F with a hold?  This also makes a slight difference.



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2 & 3 make me suspect that the firing was actually too hot, at least for the commercial glaze.  Through my trial and error with Spectrum glazes, I've found that they don't like a lot of heat work.  Any time I've gone above ^5 with them, they've gotten bubbly/blistery even though the container says they can go to ^6.

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