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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.
Tests |
Pyroplastic Deformation
How to measure the tendency of a ceramic body to warp during firing. |
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Tests |
Pyrometric Cone Equivalent
Make a pyrometric cone out of a clay or material to see what temperature it bends at |
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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