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What would happen if you made a body from 50:50 kaolin:ball clay?

It would craze glazes! This is fired at cone 6 and the crazing was like this out of the kiln. This is about as bad as I have ever seen. One might think that there is adequate quartz in this high of a percentage of ball clay to at least minimize crazing, but no so. This demonstrates the need for adequate pure silica powder in stoneware bodies to give them high enough thermal expansion to squeeze glaze on cooling to prevent crazing like this. This is also not proving to be quite as refractory as I thought, it looks like it will have about 3% porosity at cone 10.

Dilatometer curve of vitreous porcelain (red) vs. stoneware body

The 500-600C zone is the alpha-beta inversion of quartz. Notice the vitreous body experiences a bigger expansion change there. But in the 100-270C cristobalite inversion region the stoneware undergoes a much more rapid change (especially in the 100-200C zone). This information affects how ware would be refired in production to avoid cracking (slowing down in these two zones). In addition, that stoneware would not be a good choice for an ovenware body. Photo courtesy of AF

Low fire ware cracking during firing. Why?

Most low-fire bodies contain talc. It is added for the express purpose of increasing thermal expansion. The natural quartz particles present do the same. These are good for glaze fit but bad for ware like this. There are also sudden volume changes associated with cristobalite, but it forms (from quartz) at stoneware temperatures so should not be a concern in terra cotta. You could fiddle with the clay recipe or change bodies, but better to change the firing schedule. While stoneware dunting happens between 950-1150F on the way down, this could be happening anywhere. A simple fix is to slow down the entire cooling cycle. Learn to program your kiln. Use a conservative cooling rate of about 200F/hr (even slower at 1150-950F). No electronic controller? Learn a switch-setting-schedule to approximate this down-ramp (buy a pyrometer if needed).

Related Information

Links

URLs http://www.webmineral.com/data/Cristobalite.shtml
Cristobalite at webmineral.com
URLs http://www.kgs.ku.edu/Publications/Bulletins/211_4/bauleke.html
Cristobalite vs. Quartz inversion graph
URLs http://en.wikipedia.org/wiki/Cristobalite
Cristobalite at Wikipedia
URLs http://studiopotter.org/pdfs/Sohng%20pps84-89.pdf
Article about cristobalite in clay bodies
Glossary Glaze fit
In ceramics, glaze fit refers to the thermal expansion compatibility between glaze and clay body. When the fit is not good the glaze forms a crack pattern or flakes off on contours.
Glossary Thermal shock
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
Glossary Ceramic Decals
This process is done by printing a design (using ceramic inks) onto a film coated decal paper, drying it, then transferring the film to the fired ware. But beware of problems.
Glossary Cristobalite
In ceramics, cristobalite is a crystalline form of silica formed in the matrix of clay bodies as they fire in the kiln. Silica can also exist as quartz (the most common) and tridymite.
Glossary Quartz Inversion
In ceramics, this refers to the sudden volume change in crystalline quartz particles experience as they pass up and down through 573C. Fired cracks are often related to this.
Glossary Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Articles Crazing in Stoneware Glazes: Treating the Causes, Not the Symptoms
Band-aid solutions to crazing are often recommended by authors, but these do not get at the root cause of the problem, a thermal expansion mismatch between glaze and body.
Articles Firing: What Happens to Ceramic Ware in a Firing Kiln
Understanding more about changes are taking place in the ware at each stage of a firing and you can tune the curve and atmosphere to produce better ware
Minerals Quartz
Hazards Cristobalite Toxicity
Temperatures Cristobalite inversion (alpha/beta) (210C-280C)
Temperatures Quartz inversion (alpha-beta) (540C-600C)
Materials Petalite

By Tony Hansen


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