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.

Related Pictures

Why does this bowl shape always warp in the glaze firing?

Here is an example of how a cast porcelain piece, L3778G, warps during the glaze firing. Several factors contribute to this failure:
-It is cast and the walls are very thin.
-This porcelain is highly vitreous.
-This shape has no inherent strength to resist rim warping.
The following would help:
-Switching to machine forming (which orients particles concentrically).
-Reduce the feldspar in the recipe (to reduce fired maturity).
-Fire lower.
-Cast thicker walls.
-Change to a more flared shape.
-One other option (borrowed from bone china): Fire the piece upside down on a custom alumina setter fitted to the final rim diameter - then clear glaze it at a lower temperature.

A porcelain mug warps under the weight of its own handle

An example of a cone 10 porcelain that is over mature. It contains too much feldspar and is vitrifying so much that it is beginning to melt. The weight of the handle is pulling the lip into a oval shape, even though the hourglass shape of the piece should offer stability. For translucent porcelains one must endure this issue, using more stable shapes or firing with support.

The shapes of some pottery make them prone to fired warping

The goblet on the left is bending, not just because the clay is somewhat unstable at the temperature being fired, but because this shape is also inherently unstable. Where extreme shapes are prone to warping, ware must be made from clays that do not vitrify (that introduces issues of strength and functionality). In this case, the clay recipe is based on a terra cotta material that matures at a very low temperature. The problem was dealt with by employing a recipe of 60:40 clay:200# kyanite.

This super-vitrified clay bodies does this when fired

Although not clearly visible in this photo, the unglazed body surface on both of these MNP (Plainsman 3D + 10% feldspar) pieces has a sheen - because they are super-vitreous. And broken surfaces appear like glass (as can be seen in the broken chips on the right). Because the foot-ring of the piece on the right is too angular it is plucking all the way around. This is not happening on the piece on the left for two reasons: The angle was rounded and it was fired on silica sand. Because it is so fine-grained (wet screen to 325 mesh) and contaminant-free the body tolerates firing well past the point of vitrification - not only resisting bloating and warping but giving something else: Incredible fired strength.

Links

Glossary	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.
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.

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