|Monthly Tech-Tip |
Rutile blue glazes are actually titanium blues (because rutile mineral is an impure source of TiO2 and Fe2O3). The iron and titanium in the rutile react to form the floating blue effect. The GA6-C recipe has always relied on a 4% rutile addition. Its GA6-A base recipe contains significant iron (because of the 80% Alberta Slip), so could titanium oxide deliver the same floating blue effect? Yes.
These mugs are M390 clay. The top left one is the standard GA6-C (with rutile) fired using the C6DHSC firing schedule (the bottom left normal cool produces little color). But the ones on the right switch the 4% rutile for titanium dioxide (the L4655 recipe). The top right was fired using the C6DHSC slow cool schedule, the bottom right was the PLC6DS schedule.
The takeaway: Titanium is a much more consistent and reliable material than rutile. An excellent blue color is produced even without a slow cool (lower right). A lower percentage of titanium is also an option.
These two mugs have the Alberta Slip base cone 6 GA6-A glaze on the inside and GA6-C on the outside (it just adds rutile to GA6-A). The left one was cooled normally (kiln off at cone 6 after soak). For the mug on the right, the kiln was soaked for half an hour at 1800F on the way down to develop the rutile blue glaze on the outside. But during this period crystallization occurred on the inside also. This provides an insight into my this GA6-A base hosts floating blue effects but GA6-B does not: The amount of Al2O3 is much lower, that improves melt fluidity and acts as a catalyst for crystallization.
A raw TiO2-containing mineral used in ceramics to color and variegate glaze surfaces.
A super white powder used in ceramic glazes to variegate, opacify and moderate color.
Floating Blue - Substituting Gerstley Borate
A popular cone 6 glaze that employed Gerstley Borate, it was very troublesome to use and to fire. Much work has been done to create alternative recipes.
A type of ceramic glaze in which the surface variegates and crystallizes on cooling in the presence of titanium and iron (usually sourced by rutile)