The Zircon needed to opacify glazes has a side effect: Crawling. Even low percentages can induce this issue when other factors interplay (as is the case here). It often happens at sharp concaves (e.g. inside bottom corners on mugs). But here it is on surfaces disturbed during formation of the spout. This G2934 recipe has good melt fluidity, so that is not the issue. The issue appears related to adherence of the initial laydown. Here are some questions to consider:

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G2934Y, a variation of the G2934 base, is a good stain matte base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry; these make it susceptible to crawling. This base is normally fine as is, but when opacified or certain stains are added (especially at significant percentages) it can crawl. This has 10% Zircopax. Even though the glaze layer thickens at the recess of the handle join, it is still crawling. We also get this on the insides of mugs where the wall and foot meet at a sharp angle. This was initiated because the glaze cracked here during drying. Normally it would heal, but the zircon stiffens the melt, making it less mobile. The easiest solution is to adjust the specific gravity of the glaze to 1.44 and flocculate it to thixotropic; this assures that the application is not too thick. Another measure is to add a little CMC gum (by replacing some of the water with gum solution). Lastly, use a blend of tin oxide and Zircopax, as in the G3926C version of the recipe, to opacify it.

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The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

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This is M340S with G2934 matte white outside and G2926B glossy white inside (both have 10% zircopax). Consider what can go wrong. Zircon glazes love to crawl. I either add CMC gum to make it a base coat (or use a combination of tin oxide and zircopax (like G3926C). The clay has granular manganese added to produce the speck, if accidentally over-fired, even half a cone, it will bloat. And the clay body: The outer glaze is ugly on dark-burning clays. And it is drab on porcelains. It does not even look good on this same body if the speckle is not there. Another difficulty: Controlling the degree of matteness. I blend in about 20% of the glossy, otherwise it would fire too matte. And the firing schedule: PLC6DS - its drop-and-hold step is critical, without it the surface would be full of pinholes. Another problem: If the kiln is heavily loaded and cools slower than the programmed ramp-down, the surface will be too matte. Finally, glaze thickness: If it is too thin it will look washed out and ugly. Too thick it will bubble and look pasty.
| Troubles |
Crawling
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms. |
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