This "Minty Matte" glaze was found wandering in a Facebook group. With high nepheline syenite coupled with very low SiO2 (thus high COE), it crazes on most clay bodies used by the potter who contacted me. Its tag traced it to Glazy 20196. I sometimes think of these as "shelter recipes", needing a "glaze rescue". The beautiful appearance of this one provides the motivation to give it a little love and rehome it! The love it needs is a simple understanding of the mechanism: It is just rutile saturation in a magnesia matte with a little cobalt to kick the blue. Recipes aren't carved in stone; the chemistry of this one can be altered to drop the COE a lot (by switching Na2O for MgO). I know that because G2934 magnesia matte also works with this mechanism (and has a far lower COE). Transplanting the rutile and cobalt into it would surely reduce crazing.
However, this recipe needs another rescue. From a misunderstanding. Although the potter contacting me was happy with the G2934 as a base for the rutile and cobalt, she had learned that durable glazes should have an R2O vs RO Ratio of around 0.3:0.7 (G2934 is 0.1:0.9). However, the R2O:RO ratio thing is misunderstood. Durability is much more about producing a well-melted homogeneous glass (but not over-fluid) with minimal phase separation and surface crystallization, good body fit and absence of large amounts of easily dissolved colorants.

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The glaze is G1214Z1 cone 6 base calcia matte on Plainsman M390 fired at cone 6 using the PLC6DS schedule. 5% titanium dioxide has been added (a high percentage). Titanium can create reactive glazes, like rutile, with no other colorants added. This effect also works well on matte surfaces, but the glaze needs good melt fluidity (that is good because functional mattes melt well). Calcia mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide, lighter colors and better blues are possible compared to rutile (iron is still needed, but it is coming from the body here). Like rutile, the effects are dependent on the cooling rate of the firing, slower cools produce more reactivity. Even application without drips is important (mixing as a thixotropic dipping glaze is best). This appearance also depends on using a dark-burning body or engobe.

This picture has its own page with more detail, click here to see it.
The body is Plainsman M390. The firing schedule is Plainsman PLC6DS. Of course, rutile could be used in place of titanium; it contains iron oxide that would enhance the development of blue. A small amount of cobalt can also be added to enhance the blue (the G2934 magnesia matte on the left, especially, will respond to that).
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Glaze Chemistry
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