|Monthly Tech-Tip |
These test tiles, which I will call A and B, were fired in the same kiln (at cone 04) and both are made from the super-white body L4410K. Both transparent glazes are 85% frit (A is recipe G3879B, B is recipe G1916QL). But A is much whiter? Why? B is suspended by 15% ball clay (Old Hickory #5), A by 9% New Zealand kaolin. That ball clay contains 0.8% Fe2O3, the kaolin 0.25%. Thus B, as-a-whole, contains 0.12% iron (as contributed by the 15% ball clay) while A has 0.02% iron (sourced by the kaolin). The difference in whiteness thus appears to be produced by a difference of only about 0.1% iron. Notice I said "appears". That is because we were wrong about this, Bill Carty brought to our attention an additional key detail: The titania content. "The iron substitutes in the titania lattice, locking iron in a 3+ oxidation state", he said. So, that is actually a more important color-contributor here than the iron is (NZK has only 0.05% TiO2, #5 Ball Clay 1.25%, recipe B as-a-whole, has 0.2% TiO2).
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.