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 a clay test tile, this titanium-opacified/variegated cone 6 oxidation glaze, G1214Z1, looks great. But it is important to recognize that the variegated fired appearance on the tile is a combination of multiple factors: The chemistry of the glaze, the titanium brand and grade and its high percentage (5%), a good quality laydown, the PLC6DS firing schedule, the red M390 clay body below and variations in the thickness. However, most of these factors change with the mug on the right! It is made from buff-burning M340! The C6DHSC firing schedule. Notice how it is actually going transparent where very thick. Notice the glazing drips, the uneven laydown that caused the dripping is the calcined kaolin; it contributes alumina and silica without providing the colloidal clay particles needed for suspension and thixotropy.
Look at the red clay tile again: The glaze naturally breaks thin over high points and pools thick in recesses, so one small tile displays a whole range of effects, almost guaranteeing attractive variation in a small area; smooth functional ware does not do that.
| 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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