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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).
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.
Materials |
Tin Oxide
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