Pyroplastic Deformation - PPD
This test procedure was employed in the Foresight Ceramic Database and now is available for those having an account at Insight-Live.com. Accumulating test data using the variables defined in these procedures enables us to create tools that enable you to compare the physical properties of materials and recipes.
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.
TEMP - Temperature (V)
Enter cone or temperature value.
WARP - Warp in mm (V)
Enter deformation from straight line.
Fired deformation comparison between two porcelains
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.
Two bars ready for pyro-plastic comparison test
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.
How much porcelain flux is too much?
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.
Do not overfire translucent porcelain like Polar Ice
Overfired Polar Ice porcelain. This bowl fired with an oval-shaped rim and was sticking to the shelf.
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.
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