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<p>[QUOTE="Desertau, post: 9831219, member: 86775"]The crack is a stress crack it runs across the bottom, opposed to a spiral from the center (thrown in a circle stress follow in a circle) this suggests to me the walls were thrown without a bottom and the bottom was rolled out flat and attached (stress follow the direction the clay is stretched and the inversion pulls them back in the other direction. The clay body shrinks around 15% during the drying and firing process and around red heat there is a phase in the firing called the quartz inversion this occurs theoretically around 570c where an abrupt change in the volume of quartz crystals abruptly change in size this is and important time to carefully control the firing to slow the process and reduce thermal shock. Sometimes the crack happens because too much water is used and the clay particles are not compressed enough.</p><p><br /></p><p>the crack is also occurring while the clay body is soft from the heat when the crack happens from from cooling to quickly the crack travels through both sides in the brittle ceramics.</p><p><br /></p><p>the difference in clay body color is likely due to some small amount of residual glaze on the bottom. As glaze was applied this runny glaze had probably run down to the foot area and after glazing and before firing the excess glaze was removed with a wet sponge leaving a trace of glaze on the surface the inside only has drips. That orangish color probably is from the residual glaze.[/QUOTE]</p><p><br /></p>
[QUOTE="Desertau, post: 9831219, member: 86775"]The crack is a stress crack it runs across the bottom, opposed to a spiral from the center (thrown in a circle stress follow in a circle) this suggests to me the walls were thrown without a bottom and the bottom was rolled out flat and attached (stress follow the direction the clay is stretched and the inversion pulls them back in the other direction. The clay body shrinks around 15% during the drying and firing process and around red heat there is a phase in the firing called the quartz inversion this occurs theoretically around 570c where an abrupt change in the volume of quartz crystals abruptly change in size this is and important time to carefully control the firing to slow the process and reduce thermal shock. Sometimes the crack happens because too much water is used and the clay particles are not compressed enough. the crack is also occurring while the clay body is soft from the heat when the crack happens from from cooling to quickly the crack travels through both sides in the brittle ceramics. the difference in clay body color is likely due to some small amount of residual glaze on the bottom. As glaze was applied this runny glaze had probably run down to the foot area and after glazing and before firing the excess glaze was removed with a wet sponge leaving a trace of glaze on the surface the inside only has drips. That orangish color probably is from the residual glaze.[/QUOTE]
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