Potters & Sculptors - Making Rock from Mud
Is firing at a higher temperature for bisque than for glaze really that unheard of?
It's the way I was taught and have been firing. I've been working out of my home studio for the past three years, and the ceramists and potters in this area are often downright rude about how I am "doing it wrong."
I like the work I make and people buy and use it. In that sense, I'm not doing it wrong.
It seems like there should be at least some school of thought that coincides with what I was taught. Where is it hiding?
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In general- I always fell that if you like it and it works for you- then go with it. If I fire to cone X and the clay and glaze says X+1 - but it works for me then leave it be. That said. there are reasons that fellow potters say it is wrong what you do, and its pretty much based on science and instructions from ceramic suppliers and kiln companies etc.
If you fire to cone 10 - the clay is vitrified and is no longer very porous. At that stage it's hard to get a glaze to adhere nicely with most clay bodies and glazes - but if you like they way it looks- then it's fine. You might just be surprised if you Bisqued to 04 and then tried your glazes. You would find that your glazes would be more smoothly adhered to the clay body and would run off much less. Ignore the rude potters:)
I have read about it before. If your clay & glaze mature at different temps, then that is what you have to do. Especially with functional ware, where you want it to be water tight. I have also read of non-functional ware being done this way. It's all in what works. Some people want hard, durable clay, but like to use earthenware glazes. Some people don't want things leaching from the body into the glaze, so thay fire real high. It is harder to get your glaze to stick to a non-porous body, but it can be done. I re-heat all the time to re-glaze pots & it works real well. jhp
Your variation from traditional bisque terminology is covered in this definition which I pulled from about.com.
"Bisque (noun) refers to ware which has been fired once and has no chemically bonded water left in the clay. Bisque is a true ceramic material, although the clay body has not yet reached maturity.
To bisque (verb) is to fire the clay for the first time.
Bisque fire (noun) is the first firing and is usually only to between cones 08 and 06 (1720 and 1835 degrees F or 945 and 1005 degrees C). However, sometimes a clay matures at a higher temperature than the glaze that the potter wants to use on the pot. When that is the case the bisque firing may be higher in temperature, with a lower temperature glaze firing."
If you are getting enough glaze on your pots, you have a larger color palette available at cone 6. If your clay is fully vitrified at cone 10, then it is a traditional cone 10 stoneware or porcelain body, and may be fractionally more durable than the cone 6 clays that are at the low end of the stoneware range. I think I have heard of your technique being used at some commercial potteries. If I remember the rationale, it was about not wanting to glaze and fire pieces that were physically defective.
Silly rude people. They can get tiring.
Yes. Keeping clay materials-- especially gasses-- out of glazes was an important part of glazing vitrified clay. And yes, I work primarily with porcelain and make functional ware.
I guess since this is the way that I have always done it, it doesn't feel like the glaze doesn't stick well.
Thank you. I hadn't figured out a good way to search the internet in a way that would answer my question. I mostly use porcelain for thrown forms. I use a good tough high-fire stoneware for buttons, and those lie horizontal so the glaze doesn't really go anywhere.
I love the whole range of cone six glazes. I love how bright and clear they are over porcelain.
Interesting. We had always been taught to let the kiln cool for twice as long as it had fired. I wonder if that was to counteract with any issues with shivering.
Only once did I have a problem with dunting. It was a clay-heavy glaze-- closer to a slip-- that had been contaminated. The dunting was actually during heating, which made some neat but completely non-functional items. It was a learning experience.
I tried firing my green pots to maturity temp, then glazing and retiring to a lower temp but the result was not great. I've discovered tricks for getting a good layer of glaze onto a fired glaze in a refire so thought the result would be great but the glaze pulled away in places leaving the clay body showing. The richness and variations of colour didn't show up as well and it overall didn't give me a nice result.
Quartz inversion happens on the way up and again on the way down and can cause breakage but is not really a factor in the maturing of a clay body. Body maturing is to quote from pottery.about.com
"Sixth Stage: Vitrification and Maturity
The maturation of a clay body is a balance between the vitrification of the body to bring about hardness and durability, and so much vitrification that the ware begins to deform, slump, or even puddle on the kiln shelf.
Vitrification is a gradual process during which the materials that melt most easily do so, dissolving and filling in the spaces between the more refractory particles. The melted materials promote further melting, as well as compacting and strengthening the clay body.
It is also during this stage that mullite (aluminum silicate) is formed. These are long, needle-like crystals which acts as binders, knitting and strengthening the clay body even further."
So, refiring to cone 6 will expose your piece to the hazards of fast expansion and contraction, but it will not undo the vitrification that occurred in the cone 10 first firing.
I really like it that we can have differing opinions without inciting a flame war.
With my next firing, I plan to measure shrinkage at both stages. I'm curious to see the results.
in reading this discussion, my largest concern would be the fit between the glaze and the clay body. Shine, have you done the freezer/heat test on any of your pots? If they pass that kind of testing with any cracking or glaze faults, I would think whatever you're doing is working.
As george mentioned, there is a fractional difference between clay vitrified at cone 6 or 10.....probably so small it doesn't have an impact that you can see.
so then there are two things to think about:
1. you are spending more money to bisque at cone 10 (could you achieve the same results, getting the gasses out, at a lower bisque?
2. the actual fit of the glaze to the cone 10 body.
just some thoughts
Can you psot some pictures of your pots? that are fired in this fashion.