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Body Warping
Warping happens during the firing of ceramic ware when there is a high degree of vitrification and inadequate measures are taken during forming and firing to prevent it. Unexpected warping often happens with unstable shapes and over firing.
Details
The principal home of information on this issue is in the troubleshooting section in the Warping topic.
Related Information
Why the base of this bowl shape flattens on firing
The problem is a combination of the shape and the degree of vitrification this body reaches. Polar Ice porcelain has to vitrify enough to achieve translucency, that means it literally softens - not enough to fall down but enough to warp out of shape given the opportunity. A sagging kiln shelf, for example, will produce a "rocking chair bowl". A non-stable shape will do the same thing. This piece was likely made by rolling a plastic clay slab and draping it down over a bowl-form, adding a foot ring, allowing it to stiffen and then uprighting it to dry. In this case the foot ring was too small creating an extreme overhang. Had the foot ring been wider and deeper it would have enabled the rounded inside contour, provided support for the outer section and minimized the overhang. If a small foot is really needed then pieces would have to be supported by donut-shaped setters sized and positioned correctly (and the outside would have to be unglazed). Or, it would have to be bisque fired, and supported, at cone 6 and then glazed at low temperature.
Links
| Troubles |
Warping
There are multiple reasons why pottery and porcelain pieces can warp during firing, both vitreous and non-vitreous ware. Here is what to do about it. |
|---|---|
| Troubles |
Bloating
Bloating occurs when the off-gassing of decomposing particles in a body has not completed by the onset of density and impermeability associated with the vitrification process. |
| Glossary |
Maturity
A term used in the ceramics industry to signify the degree of vitrification in a fired clay. Mature clays are dense and strong, immature ones porous and weak. |
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