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Large Number Of Small Air Bubbles - Anaerobic Bacteria?


bfc2

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Is it possible that particularly virulent bacteria continued to respire in the drying slipware? if yes, would that show bubbles burst at the surface visible to me before firing, or not, I wonder?  I made my own porcelain paper clay slip with porcelain powder, dispex, tissue and water. It's worked before but I left this one and it stank. following instructions from tutor I added a bit of bleach but the bleach didn't completely kill whatever it was, not totally. It was grey in colour. I poured vessels, they looked fine (but the smell and grey colour lingered until they were totally dry). Biscuit showed no signs of any problems. But stoneware temp and they've come out looking really vile, absolutely completely covered in small irregular bumps. I've broken them open and it's clear air pockets near to the surface predominantly. I can't see that this was clumped paper fibre as most of the gas holes are smooth in shape. It was a well mixed and as bubble-free as a liquid as I would usually have it. Therefore any burnt off paper could have escaped through usual channels. HOwever, if the paper gas could escape then why not this gas (if that's what it was)? Is this just a matter of something going too hot? many thanks.

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I agree it looks like bloating.  However, I strongly suspect the problem is insufficient organic burnout, not overfiring.  We have had this exact problem (even looks the same!) recently where I am at and a proper bisque firing has for the most part fixed it.

 

Introducing tissue paper into the slip means for certain you have introduced organic material into the clay that will have to be burnt at some point during firing.  The right time to do this is at the bisque firing stage, when glaze and clay are still open enough to allow the gas (mainly carbon dioxide) to escape at the point in the firing when the burnout is actually occurring.  If the bisque firing schedule does not provide for burnout (and most I have seen do not), then organic material still remains and creates problems later when the gases from burnout are trapped in the clay body by vitrified clay, or molten glaze, or both.  By definition if burnout does not happen in the bisque firing, it is going to happen at the glaze firing stage and the result is that the gases from countless small bits of organic matter burning build up but have no way to escape, and result in what you have got. 

 

Your bisque firing schedule should be adjusted to include a soak at around 800 Celsius, ideally for about an hour.  For most clay and glazes, 800 is still early enough in the firing cycle that vitrification of the clay has not really started, and before most glaze fluxes (oops, except boron) have really started to create a good glaze melt.  That means that gases CAN escape before surfaces are sealed.  If you are bisque firing in an electric kiln, make SURE the bungs are out up to and including this soak, because organic burnout requires plenty of oxygen to be complete.

 

The problem most people have with getting their heads around this is that the solution is not implemented in the same firing where the bloating actually appears!  Put another way, even when you do a good proper burnout, your bisque ware will look exactly the same as before :blink:  so you will not know at that point if the problem has been fixed.  You will only know in the subsequent glaze firing! :o  But have faith and give it a try!  And would love to hear if that fixes it. 

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Hi, thanks all. Curt, thanks. I had begun to wonder about 'burn out' of the paper, Eg, getting trapped under the glaze. As its clear that something couldn't escape, but I thought the technician knew his stuff, so I wondered whether I had presented an untoward issue. The unglazed part has only minor bump indication, so less is trapped there. I don't have control of the kiln and thought the new technician was good. The standard is 1060 for biscuit. Is it soak time that's important? I'll mention the bungs. It's a bit difficult because I don't see the technician much.... Will leave a note perhaps. I've had many of these vessels be ok, so it was a bit of a yuck shock.

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Soak time is important to make sure temperatures get high enough and burnout gets complete even in the thickest part of the ware before vitrification and glaze melt really gets going.

 

Technicians may not see this coming because commercial clays are usually clean of organic material so the problem may never appear or be missed on the odd occasion when it does. And sufficient burnout may anyway most of the time be taking place in drafty kilns with loose doors or when those firing simply forget to put the bungs in at all. But when a lot of recycled clay is being used or one is purposely blending in organic material (!) then the chances of bloating increase.

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