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Pieter Mostert

Will Re-Bisqueing Stop Bloating?

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I've had trouble with bloating using Standard 266. I know I'm not the first person to have this issue, but my options for preventing it are limited. I fire my pots in a community studio, so can't change the firing schedule (we bisque to cone 06 and glaze fire to cone 6, both in an electric kiln). Is it reasonable to assume that putting a piece through the bisque again will burn out the remaining organic matter, and therefore prevent bloating? I don't know the exact bisque firing schedule, but it takes around 15-16 hours. Are there other reasons this clay bloats?

Other potters in the studio I use have used Standard 266 without bloating, but their pots have thinner walls. The other difference is that I used metallic black glaze on the pots which bloated, while the other potters didn't, if I remember correctly. I'm not sure if this makes a difference in trapping pockets of gass though, especially since I left the outside unglazed, but had bloating on both sides.

I should add that I'm not going to be using this clay anymore (mainly because I'll be leaving the US soon), so I'm just looking for a solution that will work for one specific pot, which has already been bisque fired. And yes, I now know the importance of testing a new clay body before using it for a pot you care about.

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I have never seen a bloated pot get any better only worse from refiring

that said I only see bloating in the finish high fire not in bisque ware

but if you are doing low fire then often the bisque is the same as glaze=I do not see it helping at all to refire it.The damage is already done.

Mark

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Re-bisque firing to the same temperature will not help on the bloating; you will need to fire a bit hotter -- cone 05 or 04. Or, add a hold at the top of the 06 bisque so that you create the heat work equivalent of 05 or 04.

 

I am willing to guess your bloating at glaze is the result of the combination of your "metallic black" glaze that has a fair amount of iron oxide and/or manganese dioxide in it, plus a clay body that also has a fair amount of both, and bisque that was underfired. The glaze may also be cooling and glassing over before the clay body completes outgassing . . . causing the gases to be trapped below the glaze surface. A hold at top temperature and a slower cool down might help.

 

Cone 6 is the top of the range for Standard 266. If there is an area in the kiln that fires cooler, then ask to have the wares put in that space.

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I'll have to check with the kiln tech to see if she's willing to hold at the top of the 06 bisque. However, I'd like to understand why you need to fire to a higher cone. If I understand the problem correctly (and it's quite possible I don't), bloating is caused by carbon and sulphur from organic materials in the clay which don't burn out during the bisque, and then form pockets of gass during the glaze fire. If these burn out between 1290 F and 1650 F (according to Laguna's guide to bloating), wouldn't firing twice to a temperature above 1650 F achieve a more complete burn out than firing once? It might not be as effective as what you're suggesting, but I don't see why this would have no effect.

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Burning out the organics, etc. is not just a function of temperature. From the Laguna guide: "Not only must the firing go to or beyond 1650°, but it must allow enough time for the maximum quantity of carbon to burn out. The process is not instantaneous; it takes time for all the carbons and sulfurs to combine with oxygen, and it takes time for the subsequent gases to work their way out of the body." Just reaching cone 06 temperature is probably not enough; you need time at that temperature to allow the heat to penetrate and burnout the nasties. That can be accomplished by the hold at cone 06 or by firing to a higher cone. 

 

From the time you stated for the firing, 15-16 hours, is sounds like the studio uses a slow bisque firing schedule.  So, firing too fast can be eliminated.  Two other potential causes:  too tightly packed bisque kiln that doesn't allow wares to heat completely and not achieving complete burnout of organics (remember the thermocouple measures air temperature, not actual ware temperature) or lack of sufficient oxygen to complete the burnout (is the kiln vented?). 

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Many clay bodies will bloat when over fired, including Standard 266. It won't matter if you change your bisque. Some people have no problems with it at cone 6, but others, like me, have to fire it to cone 5. Goes to show how different all our kilns are. In my studio we typically fire to cone 6, so we wait until we can fill the kiln with just 266 pots for a cone 5, or my students pay me for a half empty firing.

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bciskepottery, the kiln is vented, and as I mentioned before, other potters in the studio have fired 266 pots with no bloating, so I think the problem may just be that with a slightly thicker wall, the organic matter has further to travel to leave the body, so needs more time. I'll see if I can arrange for a longer firing or find someone who bisques to cone 05.

 

Thanks for the advice, everyone.

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It's a crap shoot as to whether or not it will bloat at 6. I've had lots of pots in my studio do fine, lots that bloated. My pots are all thinner than my students', and I got bloating. Slight variations in the kiln could also push it over the edge. Now we only fire it to 5.

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Bloating can be caused by organics not being burned out during the bisque, or by over firing.  When a body is over fired it can gas out. Either way, because the clay is no longer porous at the high end of the firing, the gas cannot escape and forms a bubble. It's able to form a bubble because the clay is soft at the peak temp. Manganese is one mineral that gasses out during over firing, although typically it's the granular form used for speckling that causes problems. From what I've read it's not due to organics but rather a breakdown of the mineral itself at high temps. In the 266, the high iron content is probably also contributing. Again, it's most likely not a bisque issue for the 266, but rather an over firing issue.

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I've had similar bloating issues with Laguna #65 and #90.

 

It's interesting to hear that the presence of manganese can be a cause for bloating. At times I'll find a bag or two of the #90 that appears speckled with manganese, even though it's not supposed to be in there. It's almost as if the mixer or pugmill wasn't thoroughly cleaned after the batch before the #90 was mixed. If I see more bloating in this clay body I'll make note of whether it's (unexpectedly) speckled.

 

At the studio where I work we often have tightly packed kilns full of thick-walled work made by youth. Surprisingly these were rarely the pieces which bloated. We'd see it in thinner, balanced, well-thrown pots... Maybe the kids' pots were so thick the bloats never made it to the surface. We'd been firing to ^07, and a bump to ^06 helped correct the issue. Our program is slow - I like to see 14:30ish - with a 4 hour hold at 200F and a 25 minute hold at peak (1800F).

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To CURE Bloating issues for clays that are prone to it: 

You must hold the temp BELOW the point of sintering during bisque fire:

Once sintering starts  the gas cannot escape:

(This will not prevent bloats from overfiring : only not overfiring prevents those)

 

Holding longer at the top of the bisque temp will not help because the gasses are already trapped under the sintered surface.

Firing bisque hotter because the gasses are already trapped under the sintered surface.

Refiring bisque will not help because the gasses are already trapped under the sintered surface..

 

Bisque to normal temp 08 to 02, doesn't matter that much,

but do a hold for an hour or two at about 1525 F

Pack pots that are prone to bloat LOOSELY in the bisque.

Vent on, peep open during bisque fire through sintering temp c011 1607F.

 

Guaranteed no bleebs.

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An update: Despite all the advice, I decided to bisque the pot a second time anyway, since there was nothing to lose. And luckily for me, the pot didn't bloat during the glaze firing (see http://community.ceramicartsdaily.org/gallery/album/803-standard-266/).

Now, I'm tempted to conclude that my initial guess was right, but I suspect it may just be luck - maybe it was in a cooler part of the kiln than the other Standard 266 pots I'd fired before, or maybe the original bisque was packed more loosely than the others. Whatever the reason, I'm pretty happy with the outcome.

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