|Monthly Tech-Tip |
Talc bodies have been universal for decades. Talc has been an essential ingredient to raise the coefficient of thermal expansion (COE) of bodies to fit commercial glazes. Through time, the glaze and body manufacturers have zeroed in on a "COE target" that they can comfortably meet. In view on the situation with talc supply and litigation climate around its use, body makers are seeking a way to avoid it. Talc is a magnesium silicate and its COE reduction mechanism is thought to be as a catalyst to the growth of cristobalite during kiln cooling. The action of dolomite, one of the most common, inert and inexpensive of all minerals, appears to do the same. And in much lower percentages. This mug is made from a recipe that replaces the 50% talc with 15% dolomite and 35% nepheline syenite (the remainder being white ball clay). This body looks very promising, being better in almost every aspect.
This is a Plainsman test body, L4410K, a talc-free low fire white (L213 candidate). The glazes: Spectrum 705 Black outside and G1916QL inside (cone 04). I was able to spin the mug on a turn table to apply the black up to, and over the rim to meet the transparent liner. The process takes a few minutes but gives me the best of both worlds: The color of commercial glazes for the outside and the assurance of inside-food-safety that only a glaze I make can give. Other advantages: Glazing the inside is a quick fill-and-pour-out. I can apply the glaze across the bottom and fire on stilts, that means there is no bare clay, not even a foot ring! The range of color and brilliance is stunning at cone 04 and firing is quick and inexpensive. And, this mug is utilitarian, I use it every day for my coffee.
Replace Talc with Nepheline Syenite in low temperature white bodies
This is a development project at Plainsman Clays. It is in response to Texas talc no longer being available in North America.