This custom sample board was made by Solange Roy, it is great evidence of her meticulous and thorough approach to ceramics. This picture is taken at an angle to show the surface finish and character. These are Stroke & Coat ® underglazes, they are made by Mayco and widely used and praised. Although used as such by countless potters, they are not traditional underglazes (like Mayco Fundamentals or Amaco Velvets). They are heavily pigmented and have a highly controlled viscous melt fluidity. Applied by brush they give watercolour-like effects in thin applications and increasingly opaque coverage with each added layer. These products give evidence of meticulous lab work (like this board done by Solange), technicians would have had to do melt fluidity tests of each color and fine-tune the base recipe to get the desired degree of melt fluidity (compensating for the unique effect on melting of each stain type and percentage needed).

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.

This problem typically happens after some years of use. Here are some questions to answer:
A glaze may look smooth visually, but is it really? If the surface has micro-cavities and surface disruptions this could give organics a place to attach and build up. Firing curves need to take into account the LOI of glaze materials (which can affect the microsurface), materials like calcium and barium carbonate, dolomite, talc and even clay.
Has the mirco-surface of the glaze changed? Many labs offer surface analysis services so it is possible to have a surface compared with one known to be good.
Is the glaze subject to leaching? This could happen if the fired matrix has excessive phase changes, possibly due to poor mixing, frit quality issues or materials of excessive particle size (e.g. use 400 mesh silica instead of 200 or 300). A change in the flow of a routine melt fluidity test can alert you to changes in materials or slurry preparation.
Zircon is implicated in cutlery marking, if your glaze is marking this puts suspicion on the zircon as at least part of the problem. What grade of zircon do you use? It should be 5 micron. Are you using high-energy mixing to separate the zircon agglomerates?
Is your glaze fired at the temperature to produce optimum hardness and durability? Fire it higher and lower than the production temperature and test surface properties and melt flow. If you determine, for example, that it is under-firing, then add flux to melt it a little more. If it is under-fired, add SiO2 or Al2O3 or reduce flux (KNaO, MgO or CaO).

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 glaze, "Bamboo Cone 10%", contains 50% potash feldspar. Don't do that. That much feldspar oversupplies K2O and Na2O, they have the highest thermal expansions of all oxides, by far. These are needed and valuable - but not too much. The result here: Crazing. This glaze used to work on this body, H550. The previous version of H550 was firing near the bloating point of the body, about 1% porosity, so the recipe had to be changed to provide more margin for error. The new recipe has a more practical 2.0-2.5% porosity, it has no danger of bloating or warping and still has excellent maturity and strength. This glaze was crazing before and pieces did not leak because the body was dense enough - so they were still water tight. But now it does not work. The solution is to do something that should have been done before: Use a silky matte base recipe that does not craze. We recommend our G2571A base (below right) - the Zircopax, rutile and iron oxide in the original can be added to it instead.

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!

This is a 3D-printed block mold of a medium-sized Medalta Potteries ball pitcher being prepared for filling with silicone rubber (to make a case mold for pouring working plaster molds). Although I used two different consumer 3D printers, a Prusa MK3 and MK4, the four pieces mate very well! I taped them together first and then welded them using an ultra-violet curing superglue from Home Depot (6 seconds to harden). The glue leaves a slight bump - that is not a problem - can be removed from final working molds. Notice I also made a 3D printed displacer (bottom inset) - I fill it with rocks as I fill the mold with PMC-746 rubber.