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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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Question for Timbo regarding his info on getting rid of bloat.

My studio mate is trying Standard 266 and it bloats about half the time at cone 6 schedule we use (based on Mastering Cone 6 Glazes book).  She wants me to change to a cone 5 firing schedule (which is what Standard says will eliminate the acknowledged bloating problem with  this clay).  Since we use a lot of different cone 6 clays and many different glazes (which I have extensively tested at cone 6), I am reluctant to change everything over because it may change the glaze results and I would prefer to not do all of the testing over again.  (I could fire the 266 stuff on its own but that would mean firing almost empty kiln and more kiln sitting for me.)

So, question is:  If we do the extended bisque you indicated will the bloat-causing gases be gone so we can stay with cone 6 firing for glazes?  (Note: I am going to test but wanted to get clarification on the bisque fix since you also said "This will not prevent bloats from overfiring : only not overfiring prevents those".)

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Unfortunately, this thread is 4 years old, and timbo hasn't been around for quite some time. 

To clarify the point, there are 2 distinct causes for bloating: 

1) gasses being trapped as a result of moving through the sintering stage of the bisque too fast and trapping organics that are still burning off, or

2)  overfiring the clay, and gasses from the decomposition of metals or minerals being trapped in the heat-softened ceramic material (eg, the clay is beginning to boil because it's too hot).  

If your bloating is a result of the first set of circumstances, adjusting your bisque schedule could help (see Timbo's suggested hold). If your bloating is a result of the second set of circumstances, adjusting the bisque won't help, and might make things worse because the pieces are subjected to additional heat work.

Since Standard recommends firing 266 at cone 5 because it has known issues with bloating, that says to me the root cause for this clay is more likely over firing. It hurts nothing to test a piece, of course. It may just be that your friend has to make a full kiln load of black clay pieces for a cone 5 schedule. Which could be fun!

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Callie and timbo pretty much have it, but I wanted to add a few more thoughts.

As identified, the two main causes of bloating are

1) insufficient burnout of organic materials in the clay, (I will come back to this below), and

2) metals in the clay which will not burn out at any temperature.  These metals are mostly harmless at standard bisque temps, but at the higher temps experienced during the glaze firing these metals can melt and form gaseous pockets in the clay. 

From a clay body preparation standpoint, we would prefer these metal particles to be fairly fine grained, and to be mixed fairly homogenously with other clay body materials, so that in the overall vitrification they don't congregate in little locales.   Larger grains of metal, or poorly mixed clay bodies which leave areas of dense metal population, are the seeds of bloating trouble.  So yes, slurried, filtered clay bodies produced by reputable manufacturers with good machinery should be best in this regard.  It also helps to have a clay body which is more "open" meaning some large particle size material which promotes gaps for gases to exit.  Heavy iron bodies, or bodies with "junk" in them due to poor preparation, or overly "tight" bodies (or all three of these together!) tend to be bloat-prone.  

During the glaze firing, ideally we would like to prevent/avoid these metals from melting until relatively late in the glaze firing process, so that they do not have time to form pockets of gas in the clay that turn into bloats.  If these metals melt too early, before the rest of the clay body really starts to flux and vitrify, then the metals run around and hook up in a small area and form a pocket which get sealed.  If other clay body materials which need to offgas (say, whiting) get trapped in this little pocket as well, then they can add to the bloat.  

As others have said, bloating can be aggravated by excessive heatwork (ie, firing longer than necessary to achieve vitrification of the clay body, too long or too slowly at higher temperatures, or excessively long holds at the top temperature). 

Bloating can be further intensified by a reduction atmosphere (in a gas or wood kiln) which causes some metals - particularly iron - to flux ("melt") much earlier than usual, and to become more active than usual for the remainder of the firing.  

Another related cause/promoter of bloating may be a particular chemical reaction between iron and sulphur, which often arrive together in a clay body which has been mined where there is lignite or coal present (eg, many fireclays).  Sometimes referred to as "black-coring".  I think glazenerd had some research on this in another post a while back?  This particular chemical reaction may cause "hyper-reduction" to occur.

Finally, regarding the bisque firing.  (Plumcreative note:) The bisque is where the bloating is fixed, not the glaze firing.  The bisque firing is where the organic burnout (cause #1 above) is accomplished.  As timbo and Callie said, if you have organic matter in your clay (such as paper, small tree hair roots, animal hair, etc...), this needs to be burned out in the bisque at around 800 Celsius.  At that temperature chemically bound water has already been released, and there is enough heat that organic matter in the clay body can ignite and decompose IF sufficient oxygen is being supplied.  Note that just passing by this temperature on the way up WILL NOT DO IT.  Program a one-hour hold in to your bisque firing at 800C.  Be mindful that the burnout needs to get ALL THE WAY in to the center of your clay body, so if your pots are thicker it will take longer - so program the hold for TWO hours.  

Leave all the bungs and peeps open on your kiln until at least 900 Celsius, and yes by all means have the vent on if you have one.  If there is not enough oxygen in the kiln then burnout cannot occur, and worse you may start to get localized reduction of metals in the clay (yes, even in an electric kiln) if not enough oxygen is available, and that is what you DO NOT want, as sealed pockets can form in the clay body even at that bisque stage.  You will not know those pockets are there because at bisque temperatures the clay body does not vitrify or become flexible enough for them to manifest, but trust me there are there, and they will expand and show up during the glaze firing later when the clay body gets soft.  This is also why you do not want to stack pots which are bloat-prone - because it prevents oxygen getting in. Anyway you want the bungs wide open during the bisque to let water steaming of the clay to occur so you don't wreck your kiln from the inside out with moisture. 

DO NOT overfire your pots in the bisque.  Don't fire them above recommended temps, and don't fire them twice or more.  As per the above, this will do nothing other than make your bisqueware too "tight" (less absorbant of glaze slop - er, unless you want that), and may well promote bloating by exposing clay body constituents to excessive heatwork.   As an aside, while I mostly agree with timbo above, I think he is confusing sintering with vitrification.  The sintering that occurs during a bisque firing does not seal up the clay body (unwanted metal pockets aside) - it only welds the very tips of clay particles together, which is what makes a bisque pot stiff and resonant, but definitely not sealed up.  You know this because bisque ware is incredibly absorbant when dipped in glaze slop. 

It is too late to fix your bloating problems in the glaze firing (but you can make it worse as per the above).  Even if you were to try to replicate bisque-like firing conditions during the early part of your glaze firing, the problems is that many of the materials we use in our glazes melt early (eg Boron is fully melted by 700 C) in the glaze firing, in effect sealing up the clay body underneath, further discouraging any more off-gassing and hence promoting bloating.  

In summary, bloating has to be fixed in the bisque firing if it is due to insufficient organic burnout.  If bloating is being caused by metals in the clay, make sure your kiln is well vented (un-bunged) and avoid early, unintended reduction, and don't overfire and you may still avoid it. 

Users of paper clay, reclaim clay, or native clay (which often has lots of iron), take note.  These are likely to be issues for you.

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Wow!  Thanks all for the awesome details everyone.  I really like to learn this stuff.

Everything to do with ceramic making is overwhelming sometimes but little by little I think I may know about 5% of it all by the time I leave this earth ;>)

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Hi Curt,

I've only been running my own kiln for about 2 years so I have not experimented much with varying kiln schedules.  We use a cone 6 schedule for glaze based on schedule I found in Mastering Cone 6 Glazes.  I don't know much about changing the schedule to be 5.5.  

I asked Julie Hregdovic, technical director at Standard about changes to schedule in reference to the 266 and she said "The only thing I do differently with my glaze firing is I slow down the kiln at 1900 degrees to 175 degrees  an hour to 2198 and hold for 5 min then shut off."  Do you think that would be sufficient?

I use an L&L kiln with a Genesis Kiln Controller so it is programmed with cool down stages.  I also have a downdraft vent which runs through entire process. Can you recommend changes to our glaze schedule to bring it down to 5.5 cone that I can test with the 266?  My goal is to try to make as minimal change as possible so glazes will hopefully not change a lot.

Thanks for any additional advice you can give.  Cheers, Amy

Our cone 6

Rate

°F/hr

To °F

Hold

1

99

230

60min

2

360

1976

0

3

153

2192

15min

4

495

1832

0

5

126

1400

0

6

9999

86

0

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Hi Plum,

I can't find any chemistry for Standard 266, but I can see plenty of comments all over these forums and the internet at large over the last 5 years or so that indicate that 266 is a known bloating suspect.   Probably because of its readily visible high iron content. 

However, the reality is that you probably don't know what is causing the bloating problem (and neither do I!).

If the cause of the bloating is due to insufficient organic burnout in the clay body (and it still could be this even with high metal content), then the firing changes need to be made to your bisque firing schedule, not your glaze firing schedule.  Perhaps reread the second half of my post above, where the paragraph starts "Finally, regarding the bisque firing".   I would try this first, since it is the "easy" fix as it just means tweaking your bisque firing schedule, which should have little or no impact on any other clay bodies or glazes you are using.  You would want it to be this kind of bloating problem, because there is actually something you can do about it.

If the bloating is due to metal and/or carbon concentrations rather than organic matter, then I still would make the changes to your bisque firing programme anyway, since enabling adequate decomposition and offgassing of sulphur may still help avoid what I have just seen Laguna refers to as the "black glass" problem (just another way of describing bloating due to internal reduction in the clay body).

Finally, yes, you can try firing lower than cone 6 in the glaze firing, but as you appreciate this may create significant difficulties with other clays and glazes you are using.     I do not fire to cone 6 myself, nor do I use 266, so others are probably better placed to advise on this.

Good luck, hope you get it fixed!

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