Potters & Sculptors - Making Rock from Mud
Here is a list of critical temperatures to consider when selecting a firing schedule. The same can be used to trouble shoot potential glaze and clay defects. They are given in C and F temperatures.
130-140F (55C) (Candling) used to rapid dry green ware to bone dry pieces. Most often used by production potters to meet deadlines. Can cause stress fissures on larger pieces or with certain types of clay.
200-392F (100-200C) atmospheric moisture is driven off. Common to use one hour hold at 200F to ensure complete bone dry state before ramping higher. Larger or sculptural pieces may require longer periods.
890-1112F (450-600C) chemically bond (molecular) moisture leaves clay bodies. Slight shrinkage occurs, but large increase in body porosity.
1063F (573C) Quartz Inversion. Silica changes from the alpha to beta form. Large expansion of the silica is occurring at the same time overall shrinkage is occurring from molecular moisture being driven off. Large pieces, pieces with large foot rings or surface contact, and heavy pieces are prone to stress cracks at this temperature. It is advisable to fire slowly through this period if pieces are as described.
572-1292F (300-700C) organic carbons burn off between these temperatures. Natural organic particles such as decayed leaves, twigs, or bark. Oxygen must be supplied to the kiln during this period and reduction avoided to prevent carbon build up within the clay body. Incomplete burn off can result in carbon trapping and possible bloating.
1250-1750F (667-955C) inorganic sulfides from lignite coal particles and iron disulfide burn off. Improper burn off causes early vitrification within the body by 1800F. The degree of vitrification results in blistering, bloating, or carbon coring. Dark stoneware and red bodied clays are more susceptible to these defects due to their sulfide content. Oxidation during these temperatures is essential to prevent the reduction of iron.
1800F (1000C) minimum recommended bisq temperature
1950F ( 1115C) maximum recommended bisq temperature.
1745F (950C) metakaolin begins to convert to spinel. In pottery speak, this is when vitrification begins and the glassy matrix begins to develop. Excess free silica is ejected, which can lead to cristabolite formations if feldspar levels are not sufficient.
1922-2012F (1050-1100C) spinel converted from metakaolin begins to form mullite (glass type). Feldspars begin melting at an accelerated rate. Silica begins to be absorbed in the melt.
2012F (1150C) high viscosity glass begins to form if potash spars have been used.
2044F (1118C)lower viscosity glass begins to form if soda spars have been used.
2050F (1120C) the pores of the clay body begins to close rapidly and feldspars begin to off gas rigorously. If pinhole issues are present, slow final ramp to 125F an hour to peak temperature to allow extra time for off gassing before porosity of the body closes.
2192F (1200C) both potassium and soda spars have been depleted. The porosity of the clay body is reaching its minimum levels pending type and amount of feldspars used.
2282F (1250C) any free silica that has not been absorbed into the melt converts to cristabolite.
2192F (1200C) minimum further development of the clay body after this point. Minor decreases in absorption, along with minor increases in glass content. See chart below. Typical cone six ramp hold temperature for maturity.
2192F (1200C.). Glass 62. Silica 21. Mullite 19
2372F (1300C). Glass 66. Silica 16. Mullite 21
(Ougland & Brindley)
2000-1850F. ( 1093-1010C) When most crystallization of metallic oxides occur. Zinc silicate crystals grow at optimum levels. Iron rich glazes crystallize.
1750F (955C) glaze has lost any fluid properties.
1475F (801C) used by glass artists as soft melting point. Frits begin to melt.
1350F (732C) used by glass artists to fuse pieces together.
482F (250C) Cristabolite conversion. Cristabolite that forms from 1200-1250C changes back from beta to alpha phase. When potters try to sneak a peak and the rush of cool air causes rapid cooling and "pinging" is heard. The resulting check cracks resemble crazing. Hearing pinging coming from the kiln at this temperature means the clay body recipe lacks enough feldspar to completely incorporate all the silica. There is no remedy except to use a different clay.
Sources: W.G. Lawrence- Material Science for the Potters Ougland & Brindley: " Quanitive Study of high temperature reactions"