[Natania Hume]

I used to have this problem with the brown clay I use from standard clay co. (266), but then I tried bisqueing more slowly thinking that organic gasses needed time to escape the body and it seemed to work. However these unsightly things seem to be back and I am wondering if it is from firing too hot? The clay (still standard 266) is supposed to go to cone 6, but I guess I'll try cone 5 and see what happens...any advice?

[bciskepottery]

Standard 266 (and many other black clay bodies from other manufacturers) contains manganese dioxide which can bloat if fired too hot. My kiln fires to a hot cone 6 and I get some bloating; in kilns at a local studio, I've fired it to cone 6 without problems. Dropping down to cone 5 will help.

[Natania Hume]

bciskepottery, on 08 February 2013 - 09:26 PM, said:

Standard 266 (and many other black clay bodies from other manufacturers) contains manganese dioxide which can bloat if fired too hot. My kiln fires to a hot cone 6 and I get some bloating; in kilns at a local studio, I've fired it to cone 6 without problems. Dropping down to cone 5 will help.

I will try cone 5. Thanks!

[neilestrick]

266 cannot go to cone 6. Keep it at 5 and you'll be fine.

[Natania Hume]

Update: the blisters are all but gone at cone 5, but to the discerning eye they are still present in some of the pieces. I can see/feel little bumps that most people would probably miss in the glaze surface. Perhaps my kiln fires hot and I should try cone 4 (no, there were not witness cones in the firing, but I suppose I should do that too). Any ideas?

[bciskepottery]

Those bumps are still an outgassing problem and defect. At some point, they could open and pose a serious sharp edge. One possibility is the glaze is healing over before the clay body is through outgassing. Is this happending with one particular glaze, or all glazes? Are you holding your peak glaze temperature or allowing the kiln to start cooling after hitting the peak? Consider firing in your next glaze load a piece that is unglazed and see if the blisters or bumps present themselves.

You might want to consider bisqueing higher, or holding your peak bisque temperature to give more time to burn out the problem ingredients. At cone 4 you are not going to get vitrification . . . which is important for functional ware. If you continue to get the outgassing bumps, then you might have to move on to another clay body.

[Natania Hume]

bciskepottery, on 10 February 2013 - 08:39 PM, said:

Those bumps are still an outgassing problem and defect. At some point, they could open and pose a serious sharp edge. One possibility is the glaze is healing over before the clay body is through outgassing. Is this happending with one particular glaze, or all glazes? Are you holding your peak glaze temperature or allowing the kiln to start cooling after hitting the peak? Consider firing in your next glaze load a piece that is unglazed and see if the blisters or bumps present themselves.

You might want to consider bisqueing higher, or holding your peak bisque temperature to give more time to burn out the problem ingredients. At cone 4 you are not going to get vitrification . . . which is important for functional ware. If you continue to get the outgassing bumps, then you might have to move on to another clay body.

It only happens with the one glaze I've been using, but I am getting ready to try another one. I bisque to 04 and go quite slowly but haven't been soaking at that temp. So I'll try that. The blisters appear on the bottoms of the pieces which are unglazed. Not sure exactly how to soak at the peak bisque temp. Would I turn the knobs down to "med." instead of letting the kiln turn off?

[bciskepottery]

Yes, dial down to medium after hitting peak at bisque and hold for 10 minutes before turning off.

Not sure about blisters on the unglazed bottom . . . you might see them there, but they are probably also under the glaze. You might need to slow down the last part of your glaze firing to allow the clay body and glaze more time to mature and outgas any nasties, combined with a short hold at peak to even things out. The cause may be firing too fast at the end, then stopping and starting to cool before the gases are fully released.

[timbo_heff]

Natania Hume, on 08 February 2013 - 08:58 PM, said:

I used to have this problem with the brown clay I use from standard clay co. (266), but then I tried bisqueing more slowly thinking that organic gasses needed time to escape the body and it seemed to work. However these unsightly things seem to be back and I am wondering if it is from firing too hot? The clay (still standard 266) is supposed to go to cone 6, but I guess I'll try cone 5 and see what happens...any advice?

You might should try to hold for an hour or two or so just below sintering temp: say 1580F : this gives all the organic stuff a chance to burn out of the clay before the surface sinters and traps outgassing .
Do this during bisque fire and I'll bet all blebs be gone !

[Natania Hume]

I will try these suggestions to soak the kiln at the sintering temp., and also at peak temp. And see what happens.
Thanks for the suggestions everyone!