A clay's 'hot plastic strength', resists the tendency to deform in the kiln. Hot stage microscopes are used for this type of assessment. However, rather than an absolute measurement, often all that is needed is a comparison between the deformation two materials. You can do this by making thin bars, supporting them at both ends and firing. The deformation from straight is observed and recorded.
A clay's tendency to warp is obviously related to its degree of maturity, however, the connection is not always what one may expect. The ability of a clay to resist deformation during firing can very volatile. Sometimes there exist temperatures at which a clay body shows good resistance to warp during burning, however at slightly higher temperatures a greater than expected tendency to warp can be noted, even though fired shrinkage and absorption curves appear to show stability. Some potters expect linear stability from a clay body, no matter how overhung the bowl, how thin the vase, flared the goblet foot or varied the firing. But a realistic appraisal of this property will reveal that much more attention needs to be paid to the cross section of ware. Mature industrial ware is made from clay bodies that would warp badly in many a potters hands.
Make these bars the same size as those used in the SHAB test, but half as thick.
Enter cone or temperature value.WARP - Warp in mm (V)
Enter deformation from straight line.
These bars were fired at cone 10, they were straight when dry. The back one is a cone 10 Grolleg body, the front one is a cone 6 Grolleg body. This simple test is valuable to determine susceptibility to warping in porcelains. If the pyro-plastic deformation is too much, for example, the weight of a handle will pull the round rim of a mug into an oval shape, for example.
When porcelains mature in the kiln they progress toward vitrification, getting softer. This simple test enables anyone to quantify the degree to which a porcelain is likely to warp. Bars of plastic clay almost never dry straight, so the measurement (in mm) to which they deviate from straight is recorded and the bar is mounted with the hump upwards. After firing the mm of firing deviation-from-straight are added to the dry value to derive a total pyro-plastic deformation measurement. This can be recorded as an absolute value for comparison with other clays or temperatures.
A porcelain mug has pulled slightly oval because of the weight of the handle. This happens in highly vitrified porcelains (e.g. translucent ones). The amount of feldspar or frit in the body determines the degree of maturity, the correct percentage is a balance between enough to get the maximum translucency and hardness but not so much that ware is deforming excessively during firing. This is Plainsman Polar Ice at cone 6, this degree of warp is acceptable and can be compensated for.
Overfired Polar Ice porcelain. This bowl fired with an oval-shaped rim and was sticking to the shelf.
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.
Tests conducted on bodies made from materials, as opposed to the materials themselves.