This is a buff stoneware body, Plainsman M340. A L3954F black engobe was applied inside and upper outside at leather hard. The piece was fired at cone 6 using the PLC6DS schedule. The inside, totally clouded glaze is G2926B. Outside is GA6-B Alberta Slip amber transparent. Normally this inside glaze is crystal-clear on other bodies (and on this one without the black engobe). Clearly, the black stain in the engobe is generating tiny gas bubbles at the exact wrong time during the firing and the melt is unable to pass them. The outside glaze on on the same engobe, but the GA6-B glaze is demonstrating its ability to clear the micro-bubble clouding. It contains a lot of Alberta Slip, a material that is not finely ground like others. Particles across the range from 60-200 mesh are present, some of them appear to be acting as a fining agent to clear the bubbles.

While these are available as commercial products you may want to mix your own to get maximum flexibility in surface character and color intensity. The key is phosphorescent pigments added to a transparent base recipe. The pigments are often made from strontium aluminate doped with rare earth elements like europium and dysprosium. This ebay search reveals they are readily available (using the search 'strontium aluminate glow powder'). While expensive, they are much less so than materials like cobalt oxide or lithium carbonate. These pigments are known for their long-lasting glow compared to older zinc sulfide-based products. It would be best to start at low temperatures, cone 04-06. Consider trying the G1916Q glaze base recipe first (then G3879, if you can get the frit). The Q recipe is temperature, thermal expansion and gloss adjustable (using different frits and frit mixtures). A common starting point is 10-20% pigment by weight.

This is a cone 6 porcelain mug with G2934 matte glaze (with 6% black stain added). We get this satin matte effect in our test kilns using the PLC6DS schedule. Larger kilns cool slower so this glaze turns out too matte in them, we deal with that by increasing the percentage of glossy base (this is a 15:85 blend of G2926B glossy and G2934 matte). The gold decal is from Sanbao Studio. On the left, it has just been applied, other than the glossy finish revealing its location, no gold design is visible. But, after the decal firing, using the MDDCL schedule, we get the result on the right.

The G2934 base matte recipe is good for decals because it has a very low B2O3 content (unlike high boron glazes that can begin to melt very early, even in a decal firing, and alter their degree of matteness or even produce tiny pinholes or blisters). G2934 can tolerate some high-boron G2926B glossy, enough to de-matte it, and still work well with decals.

This is G1947U clear glaze with 8% Mason 6021 encapsulated red stain added. The body is P700, a Grolleg kaolin porcelain. The one on the right, having significantly reduced clouding within, has one tiny addition: 2% Zircopax. It is acting as a micro-bubble fining agent, producing a brighter color and smoother surface. But there is a possible problem: These stains are not recommended for use above 2300F. Even though the color is very good, cone 10 is just on the edge of the limit temperature, so suitability for food surfaces would require careful testing for leaching cadmium.

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.

Drying cracks are opportunistic, especially in highly plastic or fine-particled clays. They like to initiate inside sharp acute angles. The sharper the angle the greater the chance of crack. By doing this procedure before the clay gets too stiff (in the leather-hard stage) you will deny a crack a place to start. Of course, even drying is still important, the water content of a handle should now be allowed to get too far ahead of that of the main body of the mug at any time. In the pictures on the right, two tools are being used to compress and round the angle at which the handle meets the wall of the mug.

This ware is used all over the city by street-side restaurants and food vendors. And routinely used in the house. It is all lead-glazed. That glaze does not craze and thus seals the otherwise porous surface against bacterial growth. They all know to handle it with care to minimize breakage. Surfaces and edges are rough, it is poorly finished but most people value the tradition enough to not even notice. Of course factory-made ware is much stronger and more functional, and cheaper. But at meals and occasions many seek opportunities to use this at the table and show it off to their friends.

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.

This clay is exceptional in multiple ways, it clay is from Flintoft, Saskatchewan. It holds together in lumps (center picture) but when broken its sandy nature becomes clearly visible. Other sandy clays in the area are similar but when water is added this one is different: It becomes plastic, plastic enough to form well (notice the texture of the plastic material in the closeup photo on the upper left). And it dries quickly with low shrinkage (the drying test disk upper right shows perfect performance). This combination of properties is what brick makers look for. No wonder that a clay similar to this nearby was used at the Claybank brick plant for 75 years.

I ran a series of physical tests to characterize this material (data shown lower left), that brought to light other good properties:
-It is super refractory, a fireclay. The SHAB test bars (lower right from cone 10R and 10 down to 6 oxidation) correspond to the SHAB test results in the chart. Even at cone 10 this has an amazing 19% porosity. With almost no shrinkage.
-The top bar is reduction-fired yet barely darker than the one below it at the same temperature in oxidation. This indicates low iron content.
-This is low soluble salts. Even though they concentrate on the outer edge of the DFAC test disk (upper right), that part fires only slightly darker at cone 10R (inset).
-Centre-bottom: G1947U clear glaze on it fired at cone 10R. It is neither crazing or shivering. This is unusual.

All of this information is preserved in our Insight-live account. Not shown are all the picture-specific and general notes I took. I compared this with about 10 other clays, doing the same for all of them, preserving a treasure trove of data that enables comparing all of them side-by-side.

These are whiteware mugs (Plainsman M370) with tissue transfer designs that were applied at the leather hard stage. The pieces were then dried, bisque firing, clear glazed and then fired to cone 6. Tissue transfers are sold online in a wide range of designs (in full color also). They can also be made at home by silk screening the appropriate type of ink onto the tissue paper (you can make your own ink using ceramic pigments). These overglaze transparent recipes are G2934 matte (left) and G2926B glossy (right). The matte glaze softens the edges of the design. The pig is a good demonstration of how crisp the edges of lines can be (once the application techniques are mastered). This method of decorating is far less expensive than decals. And does not require an extra firing. While the ink is somewhat powdery, it can be stabilized with spray starch if pieces need to be transported for firing.

Not to be ignored are the two transparent glazes. Control of thickness is important. Too thick and they will go cloudy. Too think and they won't fire smooth. For use as a dipping glaze the slurry should be thixotropic. Using them as a brushing glaze, while taking longer to apply, does enable tighter control of thickness.