Will Re-Bisqueing Stop Bloating?

[Chris Throws Pots]

Hi Plum,

I'd like to add one other thought to the conversation, as I recently found myself with an entire kiln's worth of work badly bloated and had to go through the process of diagnosing the cause.

As many have already stated the two major causes of bloating are insufficient burnout of organics during bisque and overfiring. What I discovered was that I was overfiring, but  wI was oblivious to it. The calibration of my thermocouple had drifted due to corrosion on the wires that connect the TC to the controller. Essentially my kiln thought it was firing its normal ^6 medium speed program, but it was really firing a hard ^8. The thermocouple was telling the controller that the kiln was the equivalent of two cones colder than where it actually was, so all my work was getting scorched and bloated. 

Hopefully this is not your issue. Hopefully you're firing with cones either in a sitter or as a check on the controller's/TC's accuracy. But if not, include cones in your firing to make sure the ^6 you're intending to fire to is actually the heat work you're achieving. My understanding is that the Standard 266 really can't handle overfiring/refiring without bloating. So making sure your kiln isn't overfiring will be helpful in figuring out how to best proceed with clearing up your bloating issue.

[VermontSarah]

Curt, I’ve been getting boating in all my kiln loads for over a year.  I have about 60 pieces of bisque and wondering if I should rebisque with a 2 hour hold at 800c.  I can’t tell from all this discussion if there is any hope of further outgasing during a rebisque.  You said  that the clay  isn’t sealed during the bisque (“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.) which makes me wonder if I could save some of my pots.  I use Standard 365, file to cone 6.  Thanks.  

[oldlady]

sarah, you might call standard.  they have technical people on staff for questions like yours.  it is ALWAYS better to go to the source.  and NOT in an email.

[curt]

sarah, you could try rebisqueing but the downside is that you could be risking over-firing problems others have talked about above by doing this.  Normally I would not worry about this for only going up to bisque temperatures, but it seems from comments I have seen that others have also had bloating problems with Standard 365 in the past (but I have not used it).  

So not sure if a second bisque firing would bother your clay body or not.  If you are going to rebisque I would only try it with a few pieces as a test.

Rather than rebisqueing, you might try including a hold at 800C for an hour or so during your glaze firing on the way up.  That would avoid the impact of a whole second bisque firing while still providing an opportunity for some further organic burnout (if that is indeed the source of the bloating problem) during the firing process.   This idea presupposes that there are no low-melting materials in either your glaze or clay, which you will have a much better idea about than I.

[VermontSarah]

Thanks, Curt. In my last 4 kilnloads, I have only risked one new pot at a time.  I have new elements, new thermocouple, new clay and a new bisque fire schedule, but have yet to try a 2 hour hold at 800c.  I’ll let you know if it works.  Do you think extremely slow drying under plastic might be increasing the carbon matter in the clay or other factors that contribute to bloating? 

 

[curt]

I don’t think slow drying will have much of an impact. This is really a chemistry/thermal issue for the most part

[Mark C.]

Slow drying will not add or minus a thing to bloating .Apples and oranges

[neilestrick]

On 8/20/2018 at 7:41 AM, VermontSarah said:

Curt, I’ve been getting boating in all my kiln loads for over a year.  I have about 60 pieces of bisque and wondering if I should rebisque with a 2 hour hold at 800c.  I can’t tell from all this discussion if there is any hope of further outgasing during a rebisque.  You said  that the clay  isn’t sealed during the bisque (“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.) which makes me wonder if I could save some of my pots.  I use Standard 365, file to cone 6.  Thanks.  

I run a lot of Standard 365 through my studio, like several tons a year, and I have never seen it bloat. I fire it to cone 6 with a slow cool from peak temp, which gets me to cone 7. I have never heard of it bloating. Since it's a grolleg porcelain, there's not much in it that can cause outgassing problems like a dark stoneware. I regularly do a 5 hour bisque and still don't have bloating problems. What you may be seeing are air bubbles. They will become visible and bloaty looking after glaze firing. Because the clay gets so soft during glaze firing, and the layer of clay on the top of the bubble is so thin, it can inflate a little bit with the expansion of the air from heating. So make sure you're wedging well, and pop any bubbles that show up during throwing.

[VermontSarah]

Neil, that’s kind of a terrifying but interesting idea. I got a new pug mill a little over a year ago and I don’t wedge much.  I probably do have air bubbles in my clay.  One more factor to investigate—thank you!

[Eileen]

I have not had a bloating problem, but my studio partner is having a problem with this.  Is it possible if the clay is reclaimed too many times that can cause bloating?  It is a dark clay, cone 6 from Sheffield pottery.  We tried the lower temp to no avail.