The 80:20 GA6-A Alberta Slip base becomes oatmeal-like when oversaturated with rutile and/or titanium (left: 6% rutile + 3% titanium dioxide; right: 4% rutile + 2% titanium dioxide). The rough surface results from excessive crystallization of titanium-bearing phases from the melt during cooling. It is unpleasant to touch, readily shows cutlery marking and is less desirable for functional surfaces. In glazes of this type, rutile additions above about 4% can become increasingly difficult to keep smooth and consistent. Crystallization, rather than phase separation, increasingly dominates the variegation mechanism.
One way of avoiding problems is staying below the tipping point, keeping the percentage as low as possible while still getting the desired variegation (of course, that will vary depending on the melt fluidity of the glaze and kiln cooling). Surprisingly, making the glaze more fluid does not necessarily solve the problem. Greater melt fluidity increases ion mobility and can actually promote crystal growth during cooling, producing an even rougher surface.

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On this M390 red clay test tile, the titanium-variegated cone 6 calcia matte glaze G1214Z1 looks excellent (rutile has a similar effect). But that appearance depends on a combination of factors: glaze chemistry, 5% titanium dioxide, the titanium grade, application thickness, the red M390 body and the PLC6DS firing schedule. The relief of the tile especially flatters it - forcing the glaze thin over high points and allowing it to pool in recesses, displaying a whole range of effects in just a few centimetres.
The tile tells you what effects a glaze is capable of; it does not necessarily tell you what the glaze will look like on ware. These mugs demonstrate that. They were fired using the C6DHSC schedule and are made from buff-burning M340 (left) and M390. The mug on the right also has a heavier, less-even application. Surprisingly, some of the thickest runs become more transparent rather than more opaque. That is a reminder that the pale, variegated character of this glaze is not simply the result of titanium acting as an opacifier; it depends heavily on melt thickness and titanium-driven crystallization during cooling.
| Recipes |
GA6-F - Alberta Slip Cone 6 Oatmeal
Plainsman Cone 6 Alberta Slip based glaze. It can be found among others at http://albertaslip.com. |
| Glossary |
Leaching
Ceramic glazes can leach heavy metals into food and drink. This subject is not complex, there are many things anyone can do to deal with this issue |
| Glossary |
Food Safe
Be skeptical of claims of food safety from potters who cannot explain or demonstrate why. Investigate the basis of manufacturer claims and labelling and the actual use to which their products are put. |
| Glossary |
Rutile Blue Glazes
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) |
| Glossary |
Crystallization
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation. |
| Materials |
Ceramic Rutile
Rutile mineral ground to a very fine particle size (e.g. 325 mesh) contributes titanium and iron that colors and variegates ceramic glazes. |
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