Crawling glaze on sanitaryware

Sanitaryware glazes are high in zircon, thus a stiff glaze melt is a part of their very nature. That means glaze crawling is a part of their nature! And, preventing it is a major effort by producers. Crawling most commonly happens inside acute contour changes (that thicken the layer), but crawl points can even occur on relatively flat surfaces. However, this time it appears on the outside of an abrupt curve. Factors that can cause crawling can compound on corners. During drying, soluble salts and binders in the clay tend to concentrate at edges and corners - these can affect glaze laydown (especially its thickness and adhesion). The slip casting process favours the concentration of the finest clay particles at the mold face, but corners see the most surface disruption during cleaning and tooling at the leather hard stage (which can expose coarser particles below the surface). Glazes containing clay must shrink somewhat during drying, a corner like this will be the first place a crack appears (and thus a crawl), especially if adhesion is not as good.

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Crawling on sanitary ware. Laydown is the first suspect.

This is glaze crawling and it underscores the need for attention to the details of all production parameters. This one small glaze defect makes this pedestal sink either a refire, a second or unsaleable. This is most common on abrupt surface contours but that is not the case here. The cause of this is likely several factors combining. The glaze is opaque white because it contains a high percentage of zircon opacifier. Zircon glazes tend to do exactly this so their successful use is doubly dependent on minimizing the percentage added and on attention to other details to compensate. This glaze has been applied thickly to ensure good coverage (thicker laydowns bring more crawling problems). The glaze is likely low in clay and thus the physical bond of the dried glaze layer depends on the binders being used, their percentages, the integrity of the way they were mixed in, and their shelf life. The ability of the glaze laydown to dry-bond with the body depends on the condition of the surface (e.g. water content, dry or bisque fired, smoothness, dustfreeness, quality of materials used in the body and integrity of body preparation, etc), the presence of surface contaminants (e.g. soluble salts) and the way in which it was applied and its thickness. The glaze melt's ability fire-bond and form an interface with the body that produces a smooth surface is dependent on its melt fluidity and ability to form an interface with the body.

There is another way to look at this problem: The process runs along crawling multiple tipping points: A viscous glaze melt, glaze application to dry rather than bisque ware, a thick glaze application, a large surface area intolerant of any defects and a glaze application technique (spraying) prone to irregularities of thickness. Rather than trying to identify the specific problem it might be better to simply make changes to move the process back from the tipping points.

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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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