Sunday, August 12, 2012

Firing and Glaze Advice Needed

Just unloaded a kiln full of pots. 
Lots of pinholing.
Believe it's a clay body problem.

Pots fired from reconstituted porcelain (same formula) in soda kiln to cone 10 next to pots fired with this particular batch of porcelain are not pinholed.
Porcelain contains:
tile 6 
custer spar

Glazes boiled/pinholed, particularly glazes that have a wider firing range, and those that flux earlier or like a lower temperature. The clay body is "lumpy", some bits are whiter than others, and some parts have shrunk away from the lumpy bits. Pots made from the newer batch that have pinholing in the glaze (amber celadon and a barium matte turquoise) are next to pots thrown from a reconstituted batch containing no scraps from this current questionable newer batch, and the reconstituted batch pots have not pinholed. Looking at older pots made last year from a completely different batch, no pinholing problems with the amber celadon in them, furthermore, even when amber celadon was thinner on those older pieces, no pinholing problems.

The question is: can I refire any of these pieces with the pinholes and have the glaze settle down? If so, how may I refire them? What would work best? electric, refire in soda?, gas no soda? no reduction? (the soda deposits seem to exacerbate the problem in areas where the glazes used are more "fluxy"). Would refiring them in soda with adding more soda make them worse?

Thanks!!! for any advice!


John Britt said...


So sorry to hear about your problem. That is too bad.

I would agree with you, based on your analysis. The problems is your recipe is so clean. (no fire clay or ball clay!) I would propose that perhaps it is the new Custer Feldspar. You might ask Highwater.

I would wonder if it is bloating in the body? If so it is definitely the body mix. Maybe clumps of feldspar unevenly mixed? Causing bloating and gas in that area??

I would first ask Highwater. Then try to refire a couple in electric and see if that heals things. I am afraid that if the problem is a bad mix it may make it worse.

But you have to try it and see.

Do you have a kiln that goes to 10?

June Perry said...

So sorry to hear you're having this problem. Did you mix the clay yourself using a new bag of one of more of the materials, or was it a purchased claybody.
If it was purchased mixed, then ask the company if anyone else has complained. You may or may not get an honest answer, so ask around if anyone else has recently purchased that body.
A porcelain body, as John pointed out doesn't have ingredients that would outgas as happens with stoneware and fireclays.
If pots with both claybodies were once fired, on the same schedule, sharing similar kiln space, and one clay is fine, then you can rule out some of the other causes of pinholing - firing too fast in the bisque range and/or too fast during the glaze melting phase, or setting pots too close together.
The fact that the clay is clumping, sounds like it is either contaminated, not properly mixed or someone made an error and opened a wrong bag. You can ask the clay maker to see copies of the material sheets of the ingredients they used in that batch of clay. Something is definitely changed with the body.

William Baker said...

Sorry for the clay troubles, excellent advice from the others.
It seems like the evidence does point to the batch, I agree to try electric refiring first. Might work to heal over the pinholes and is usually quicker and easier, assuming you have access to highfire electric kiln/oxidation. Refiring in soda might be available, but I have seen pinholes in a soda firing without glaze on the pots, so I'm not sure it's the best choice to answer the refire question.

Although it sounds like a problem in the clay and not with the soda, wondering if you can see any relation between the soda spray on the pot and the location of the pinholes?

Hope you have a better batch next time!