This is an example of how a glaze that contains too much plastic clay has been applied too thick. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem you need to tune a mix of raw and roasted clay. Enough raw clay is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. Perhaps you have been using a glaze having a high percentage of clay and this does not happen - the reason is likely that the clay is not highly plastic.

This is a GLFL test comparing the melt flow of the three materials at 1800F. Frit 3124 is barely out of the starting gate and the other two have crossed the finish line! With frits chemistry is a big deal, they are all about supplying oxides to the melt. Frit 3134 is low-alumina/high-boron, 3124 is medium-alumina/low-boron and 3195 is medium-alumina/high-boron. Boron is the melter. Alumina thickens the melt and hardens the glass. Just from this it appears that Frit 3195 is a better starting point for calculations to replace frit 3134.

This handle mold is for v.5 of our 3D mold-making (and discovery) project for the ball pitcher. The process to make the 3D drawing is quite simple: Cut it out of the model (top left), draw and extrude side walls (top right) and slice off and remove the pointy parts (a step-by-step video coming soon). Bottom left: A ready-to-use mold. Notice how it fits perfectly onto the side of the pitcher form (bottom right). Because of the good fit, attaching these is just a matter of using some casting slip as the glue. Casting this handle separately affords multiple benefits: It simplifies making the mold of the pitcher itself, of extracting pieces after casting and it produces a more professional-looking product (without holes inside where the handles join). And, handles can be stockpiled in a damp box, ready to use when needed.

The cracks appear to have happened on heat-up (because they have widened). Bisque firing was done around cone 04. Issue 1: The cone 10 electric firing was up-ramped at 400F/hr to 2330F (so it whizzed pass quartz inversion on the way!). Issue 2: Wall thickness variations in the pieces, they produce temperature gradients that widen as firing proceeds. Issue 3: Abrupt contour changes and sharp corners, especially when coincident with thickness variations, provide failure points that rapid temperature changes exploit. Issue 4: This new body is more plastic than the previous Grolleg porcelain used, that was likely an enabler to making these thin wall sections even thinner. But remember, practically any piece (unless it has huge in-stresses from uneven drying) can exit a kiln crack-free if firing is done evenly and slowly enough. Results of past firings are the main guide to know what to do in future ones, this is now a "past firing". So the first obvious fix here is slower heat-up, especially around quartz inversion (1000-1100F). Second: more even wall thickness.

You can decorate the underside! The one on the right is the back side of the plate. This is Plainsman Snow clay, it can have 25% porosity. But when fired at cone 06 the porous body does not absorb any of the glaze. And the plates stay flat when fired on stilts. These are done by the team of Micah & Jeremiah Wassink of Creston, BC (at Pridham Studio). They make matching mugs, but fire those at cone 6 using underglaze decoration with a clear overglaze. But these plates are decorated using a combination of heavily pigmented viscous-melt low-fire glazes and a black underglaze and then finished with a thin layer of transparent glaze.

This is G2934 cone 6 matte (left) with 10% zircon (center), 4% tin oxide (right). Although the base unopacified recipe does not cutlery mark the other two do. Although the marks clean off all of the two on the right, the zircon version (in this case Zircopax) version has the worst and is difficult to clean. Thus, a small change is all that is likely needed. One solution is to reduce the matteness of this glaze, moving to more toward a satin surface. A way to do this is to line-blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain. Another option is to switch to 400 mesh silica in the recipe, that will enable many more of the particles to go into solution in the melt, thus increasing the gloss a little (an improving the firing surface in other ways).

This is a cone 10R stoneware, Plainsman H550. Made by Donna Ratlege at Spirits of the Creek Pottery in Cranbrook, British Columbia. She titles these: "Honest! I just had it". Her new super-powered gas kiln, built by Bruno Sperling, went over temperature on the first firing. These were in different parts of the kiln, each bearing witness to the degree to which it went past cone 10. This clay body should not be fired to lower than 1.5% porosity or bloating can occur. These are a testament to Bruno's kiln-building and firing ability, these pieces have even gone beyond that - far enough to seriously warp.

These pieces were made from Plainsman Polar Ice and fired to cone 6 using variations on the PLC6DS and C6DHSC schedules. The dipping glaze is G2934Y, a recipe variant of G2934 having a finer micro-surface texture (it has the same chemistry but the MgO is sourced from a frit and talc instead of dolomite). These mugs display varying degrees of matteness depending on the cooling rate of their firings and the percentage of glossy G2926B base we blend in. As an MgO matte, this glaze is can have a surface very pleasant to the touch. It fires durable, can be quite matte without cutlery marking and it has very good slurry and application properties (as a dipping glaze). It has a very low thermal expansion (won’t craze). It works really well with stains (except purples). It melts even better than the glossy!

These porcelain mugs are sold at many tourist shops on the Alaskan cruize circuit. Made in China of course. But their quality is astounding. And they teach multiple lessons to potters - great skill in the use of decals (even inside), meeting different glazes at the rims, evenness of application, layering, the use of wax resist, etc. They likely have a glossy and matte base glaze and add stains (to get the black, blue, red, white, green). Notice they have an iron red (lower right) that is stable enough not to run and host an even more fluid melt second layer. They also have a stoney yet functional matte white (bottom left). You can make dipping glaze versions of all of these:
Black glossy: G3914A and G2926BL
Black matte: G2934BL
Iron Red: G3948A
White stoney matte: G2934Y2
Glossy colors: Add stains to G2926B
Matte colors: Add stains to G2934

This is revolutionary because it is now practical to make one-off jigger test molds in one step using a consumer 3D printer and no plaster original model. Draw, print, glue, pour plaster, peel off (or heat off using a hair drier) the printed PLA piece by piece and you are ready to jigger test mugs.

3D design of this is simple: Sketch the outside profile of the mug and the mold, join them at the rim of the mug and then rotate. While the whole thing can be printed as one piece, print-time is drastically reduced by doing them as separate pieces and gluing. It is also best to print the step section of the mold much thicker to guarantee roundness for fit into the cuphead or ring. Printing the base of the mug separately is most advantageous, that eliminates the need for generating support and enables doing multiple iterations of embossed designs or logos. Printing a small inside ring to hold it in place for glueing is a also good idea.