|Monthly Tech-Tip |
Talc is employed in low-fire bodies to raise their thermal expansion (to put the squeeze on glazes to prevent crazing). These dilatometer curves make it very clear just how effective that strategy is! The talc body was fired at cone 04 and the stoneware at cone 6. The former is porous and completely non-vitreous and the latter is semi-vitreous. This demonstrates something else interesting: The impracticality of calculating the thermal expansion of clay bodies based on their oxide chemistry. Talc sources MgO and low fire bodies containing it would calculate to a low thermal expansion. But the opposite happens. Why? Because these bodies are composed of mineral particles loosely sintered together. A few melt somewhat, some change their mineral form, many remain unchanged. The body's COE is the additive sum of the proportionate populations of all the particles. Good luck calculating that!
Low-temperature white talc casting bodies have always been porous, 10-12% absorption is typical - pieces are basically just bisque-fired. People just pretend everything is fine! A stoneware potter might be horrified by this - until he/she sees the bright colors and brilliant glazes! If a dolomite body alternative is not available to you or your supplier is making a talc body that is no longer white there is an alternative: Use a middle-temperature white burning body. Why is this even possible? Their porosity at cone 05 is also 10-12% - but won't commercial glazes craze? Yes. So just make your own. This is G1916QL1 applied thickly on Polar Ice, P300, M370 (and L213 talc body), these are all plastic bodies but they can be adjusted for casting (removal of the bentonite). None crazed out of the kiln or after a BWIW test. I did this again with a thinner glaze application and at cone 03 (100°F hotter) and the results were even better. G1916QL1 can be mixed as a dipping or brushing glaze. But how can you guarantee no crazing? Fire even hotter, perhaps cone 02. Lower the thermal expansion further (as per instructions on its web page). And increase the silica in the body recipe at the expense of kaolin or ball clay (compensate with whatever bentonite is needed to get good casting properties). For a good cone 6 porcelain recipe for casting try L3778G. All of this can also be done with a plastic body, for that try L3778D.
A source of MgO for ceramic glazes, a flux or thermal expansion additive in clay bodies, also used in the manufacture of cordierite.
Co-efficient of Thermal Expansion
Ceramics are brittle and many types will crack if subjected to sudden heating or cooling. Some do not. Why? Differences in their co-efficients of thermal expansion.
Calculated Thermal Expansion
The thermal expansion of a glaze can be predicted (relatively) and adjusted using simple glaze chemistry. Body expansion cannot be calculated.
Potters know artware as pottery firing at low temperatures with brightly coloured glazes and decorated using decals, underglazes, lustres, etc.
Shivering is a ceramic glaze defect that results in tiny flakes of glaze peeling off edges of ceramic ware. It happens because the thermal expansion of the body is too much higher than the glaze.
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.