[pam sinclair]

I am a potter of some years and have been working with a Grolleg cone 6 body for the last year or so. I am continually frustrated by bloating. The bisque has been totally fine tuned. The problem is with larger pieces - thrown from 8 to 14 lbs. Is this air trapped in the walls? out gasing? I am firing the schedule from the Cone 6 Glaze guys . . . I am making extra everything for orders to cover the losses. HELP!!

[neilestrick]

If it's truly bloating, you'll generally get a lot of little bubbles. If you're getting just a couple of larger bubbles, then it's probably air bubbles. They tend to show up in porcelain. If it's bloating, you might try a different firing schedule or check the calibration of your kiln.

[OffCenter]

I don't think air bubbles cause bloating but they can look like bloating but those that look like bloating are already there before the pot is fired, just maybe not as noticeable. Bloated is often caused by over firing. For some clays even a slight over firing will cause bloating. If you're mixing up the clay, be sure it is well mixed because if it is not that can cause bloating. Organic matter can cause bloating but that's probably not a problem with porcelain. Carbon trapped in the clay can cause it and that's not only a problem in reduction (which I guess you're not doing) but in electric kilns with thick pieces tightly packed.

Wedge the clay more to make sure it is well-mixed. Test to see if you're over firing. If that doesn't solve the problem then try not packing the kiln as tight as usual and slow down around 700 degrees to let carbon escape. If that doesn't work, you need a new porcelain.

Jim

[Natania Hume]

pam sinclair, on 09 January 2013 - 01:03 PM, said:

I am a potter of some years and have been working with a Grolleg cone 6 body for the last year or so. I am continually frustrated by bloating. The bisque has been totally fine tuned. The problem is with larger pieces - thrown from 8 to 14 lbs. Is this air trapped in the walls? out gasing? I am firing the schedule from the Cone 6 Glaze guys . . . I am making extra everything for orders to cover the losses. HELP!!

I've had this problem with a cone 6 brown body. Someone told me that the cause was bisqueing too fast so carbon didn't have time to escape. I changed to a very slow bisque and that seems to have done the trick, although I noticed a few bumps/bubbles on a recent load where I glaze fired very quickly so perhaps firing slowly all the time is the key. Good luck!

[Denice]

I have been working with Cone 6 clays for 40 years the only time I have bloating is when I over fire. I'm a handbuilder mostly so I tend to fire slowly, I have noticed that some clays will bloat at Cone 7 so it doesn't take much. I learned this when I was learning how to use my dual pyrometer set to down fire and hold the temp in my kilns. Denice

[Mark C.]

I'm guessing this is bloating not air bubbles-That comes from overfiring the clay body-one can also over reduce which can lead to bloating. If this is not the case and the clay is rated to cone 6 try bisquing a cone or two hotter. I see no connection to form size and bloating(small no bloat large bloat) unless the big pots are way thicker?
Mark

[mregecko]

I recently had a similar problem with a ^6 stoneware... Went through the theories of carbon coring, outgassing, air bubbles in clay, etc etc etc.

Come to find out my clay was just overspecced, and was in fact a ^5 clay. (Straight from the manufacturer's mouth)

I'd say try firing some pieces at a cone lower and see if you have the same problems.

Alternatively, you can bisque fire higher (04-02) and at a long cycle. Personally, I find that's A.) Annoying, and B.) Hard to glaze after.

Good luck, and let us know how things go!

[Mark C.]

Many clay manufactures make cone 5 clay-but most seem to fire it to cone 6 it seems. One would think a cone 5 porcelain would take cone 6?
Mark

[OffCenter]

Mark C., on 10 January 2013 - 10:34 PM, said:

Many clay manufactures make cone 5 clay-but most seem to fire it to cone 6 it seems. One would think a cone 5 porcelain would take cone 6?
Mark

I've tested a lot of them and never ran across a commercial cone 5 porcelain that wouldn't take cone 6. Pam hasn't replied yet so we don't even know that it is a commercial porcelain. If she mixes it herself, the problem may be in the mixing.

Jim

[neilestrick]

OffCenter, on 11 January 2013 - 10:12 AM, said:

I've tested a lot of them and never ran across a commercial cone 5 porcelain that wouldn't take cone 6.

Jim

I agree. Plus they usually have slumping problems before bloating shows up.

[Wendey]

Why do manufacturers make clays that fire to Cone 5 when most potters who make their own glazes fire to cone 6!????