Pottery and Ceramics Clay
Choosing a Bisque Temperature
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Many people use a certain temperature (otherwise known as Cone number) for bisque firings because of habit, not because they have thought about it.
Low FireWith low fire, the bisque temperature is usually hotter than the firing temperature.
Most commercial glazes recommend bisque firing to Cone 04 and glaze firing to Cone 06, which is cooler.
The reason for this is to make sure all the carbon and other materials in the clay burn out during the bisque firing.
If you glaze fired to a hotter temperature than the bisque, it is possible that more carbon would burn out during glaze firing, causing blisters in the glaze which would not have time to heal over.
For some clays this is not a problem, because the carbon content in the clay is low or burns out early.
Because of the convenience of having bisque and glaze in the same kiln load, always bisque fire low fire pieces at Cone 06 and glaze fired them at Cone 06 also.
Usually there won't be any problems, but there always is a possibility.
Another reason that low firing bisque is done differently than high fire, is that glaze is usually brushed or sponged on, not dipped.
The amount of glaze you add is less dependent on the porosity of the clay and more on how you brush it on.
High FireThere are different strategies and by understanding them you will be able to make an better decision.
The first thing is that you want to bisque fire low so that the pottery or ceramic is still porous and can absorb glazes.
The second thing is that you want to bisque fire hot enough so that your pottery or ceramic doesn't absorb too much glaze resulting in it being too thick or actually causing the pottery or ceramic to fall apart.
This is the main reason to bisque at all.
Between these two extremes you have to decide on a bisque firing temperature.
Most potters bisque fire somewhere between Cone 010 to Cone 04, with Cone 06 to 08 as the most common.
Pros And Cons With High FireAt Cone 04, you may have problems with the pottery or ceramic not absorbing enough glaze.
This is particularly true if you do glaze layering.
However, the pottery and ceramic is much stronger when fired to Cone 04
You will have less chance of cracking during the cooling also, which can be a big problem with large or porcelain pottery and ceramics.
Besides that, you will have burned out more of the carbons, which might give better results with certain clays and glazes.
If you are bisque firing to Cone 04, you have to use thicker glazes.
This can be useful because thicker glazes don't settle out as fast.
Now, even though the pottery or ceramic is more resistant to breaking when fired at higher temperature, it is actually more sensitive to thermal shock because it is less porous and less flexible.
When doing raku you might want to fire at a lower temperature so you have more resistance to thermal shock.
If you have the right clay body this shouldn't be an issue.
Pros and Cons With Low Fire
At Cone 010 or lower, the pottery or ceramic will absorb too much glaze, besides it will crack easier when handled and more likely to crack during cooling, but will be more resistant to thermal shock.
"Copyright BigCeramicStore.com, reprinted with permission."
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