Crawling on sanitary ware. Laydown is the first suspect.

This one small glaze defect makes this pedestal sink either a refire, a second or unsaleable. This is glaze crawling and it underscores the need for attention to the details of all production parameters. 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 abilitybof 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 melts 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.

Related Pictures

Two transparents having opposite melt fluidity/surface tension balances

Melt flow test demonstrates surface tension

Low-fire glazes must be able to pass the bubbles they and the underlying bodies generate (or clouds of micro-bubbles will turn them white). This cone 04 flow tester makes it evident that 3825B has a higher melt fluidity (A has not even dripped onto the tile). And its higher surface tension is demonstrated by how the flow meets the runway at a perpendicular angle (it is also full of entrained micro-bubbles). Notice that A, by contrast, meanders down the runway, a broad, flat and relatively clear river. Low-fire glazes, for example, must pass many more bubbles than their high-temperature counterparts, the low surface tension of A aids that. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). These two represent very different chemistry approaches to making a clear glaze. Which is better? Both have advantages and disadvantages - this property has implications, just not for bubble clouding, but on other issues involving glaze performance and even defects.

Surface tension differences between two glazes

Both are low fire transparents. In a melt fluidity test they flow in a similar fashion. But here, where a 10 gram ball has melted down onto the tile, differences in surface tension are clearly evident by the angle at which the edge of the glaze meets the tile.

5% tin oxide vs 9% Zircopax in a cone 6 transparent glaze

The glaze is G2926B, our standard base transparent. The Zircopax (zircon) is Ultrox Z+. We made a range of test pieces and found these two versions remarkably similar. The tin oxide version is a little bluer for some pieces, on others it was difficult to distinguish them. We even tried the two glazes on a jet-black engobe, the opacification was similar there also. The cost? $8/kg for the tin version, $5.25 for the zircon version (current retail pricing in our area). But a critical issue is crawling, high zircon whites are notorious for this. We thus now use mix of tin and zircon (3% and 5%) for tableware. Sanitaryware industry would not likely be able to justify this expense so they are motivated to investigate other options (e.g. adjust the melt surface tension by glaze chemistry or compensate by better laydown and dry adhesion).

Crawling in G2934Y Zircopax white glaze: Here are some fixes.

A matte glazed G2934Y mug where the glaze has crawled on the upper handle join

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 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.

Links

Glossary	Glaze laydown Refers to the quality of the dried ceramic glaze layer and how this affects the fired result: e.g. density, hardness, evenness, thickness, freedom from defects, etc.

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