The only difference between these two cone 6 glazes is the silica. Both are the G2926B recipe, both were thickly applied and fired in the same kiln. The left one employs the 90 micron (or 200 mesh) grade silica and the right one uses 45 micron (or 325 mesh). These test tiles are about 6 months old. There was no crazing out of the kiln. The porcelain recipe is 25% silica, 25% nepheline and the remainder kaolin and bentonite. It appears the finer particle size silica is dissolving in the melt much better, this narrows the difference between calculated and actual behavior, especially relating to coefficient of thermal expansion. While this grade is better in glazes it is not better in bodies, they most often depend on the thermal expansion increasing effects of the larger particles in the 200 mesh grade.

A reader noted that it is also a matter of the reaction between glaze and body. The original glaze having coarser silica would have smelt and reacted with the body more, the extra dissolution sourcing Na2O - thus increasing the COE of the glaze. Conversely, when the finer silica dissolves it increases melt viscosity thus reducing reaction with the body.

This mold is for a Medalta Potteries ball pitcher having a closed top with a teardrop-shaped spout (lower right). This plaster mould does not need a spare - 3D printing makes it possible to create a pour spout that inserts perfectly into the angled hole. The size of the pour spout reservoir and the degree of insert can tuned so that when the level drops to the bottom it is ready to pour and the hole is perfectly formed.

This blistering problem is common in high-temperature rutile blue glazes - this is not saleable. The reason relates to what it takes to create this kind of vibrant variegated aesthetic: Melting the crap out of the glaze and cooling it just right. This particular one is being fired to cone 11 down to get enough melt fluidity to make it crystallize and phase separate. It seems logical that if the glaze is melting so well it should be able to heal any bubbles that form and break (these are more than usual because the body is being overfired and generating gases). However, the fluidity comes with surface tension that can hold the bubbles intact. Each of these holes in the glaze is a product of that - plus another factor: Cooldown is rapid enough that the melt is not sufficiently fluid to heal after bubble breakage. The potter has been using this glaze for many years with success, but a small change in process or materials has occurred to push it past a tipping point. Solutions? A drop and hold firing. Add a flux (e.g. a little lithium or a frit) to make it melt fluid at cone 10R (where the body generates less gasses of decomposition). Replace any high LOI materials in the glaze itself with other materials to source the same oxides.

I have not made slip casting molds for years because I dread the process, the mess, all the supplies and tools. I am not a mold-making expert either, but I found a way to do it that is fun, rewarding and effective.
-I am wasting less plaster (it is not a green material). And PLA filament is corn starch or sugar cane. And I am not using rubber.
-I spend most time on design, pouring the plaster takes minutes.
-Many fewer tools are needed, the process is less messy.
-No natches make sanding of flat mating faces possible (for better seams than I've ever had).
-No spare is needed, the 3D-printed pour spouts works better.
-More shapes are possible.
-My molds aren't right until at least version 3. 3D makes do-overs or changes in design as easy as a reprint and plaster pour. I can make a mold just to test an idea!

These bricks were being extruded in India and the plant was suffering drying cracks. A consultant recommended a high percentage addition of lignosulphonate, at a cost of $800/ton, as a solution. But before adding such a large expense, some obvious changes seemed in order first. The technician knew the plasticity index of the clay (a measurement used for soils) but he did not have records of its drying shrinkage, water permeability, drying strength or drying performance - when problems like this arise such data provides direction and help answer questions. For example, is cracking happening because of lack of drying strength or plasticity or because drying shrinkage is too high. The splitting along the corner of the extrusion is a clue that plasticity could be lacking - that could be solved by a small bentonite addition or reduction in grog. If permeability is low an increase in grog might be needed (if the pugmill can still extrude slugs with a smooth edge and corner). Notice the cracks that start from those splits (lower left). But also notice how the top edge has shrunk while the center section has not. That indicates the drying process is not tuned to subject all surfaces to equal airflow (sure enough, these are being dried outside in the sun and wind). Another factor is cross-section: The round holes create variations in thickness that exceed 300%, square holes with rounded corners would be better. Given the location, economic realities and past success this one change might be enough to make a big difference.

This is what typically happens when applying a dipping glaze over an underglazed bisque-fired piece - it does not stick. Commercial underglazes impede the absorptive powder of the bisque but it still might be possible to make it work. Here are some options:
- Bisque the under-glazed ware to a low temperature (e.g. cone 010 or lower) and apply dipping glaze to that.
- Make your own underglaze for dipping, leaving out the CMC gum.
- Make your glaze thixotropic - because it is gelled it may hang on in an even layer over the underglaze.
- Make a brushing glaze and paint that over the underglaze. 1%-1.5% CMC gum will make it paintable. 1% additional Veegum will gel it enough that it can tolerate more water and go on thinner, this enables applying multiple coats and gives good control of the final thickness.

This is possible because Polar Ice, the casting version, behaves like rubber after draining - because of 1% added Veegum. The rims can be peeled away from the mold and the piece can be completely collapsed to forcibly peel it out of the mold. They could then be opened and dried. To add more insult to the wonky shape large pieces can be broken away at leather-hard stage and then reattached using the slip as glue - the piece will still dry normally! The 1% Veegum is solely responsible for all of this, without it the New Zealand kaolin based slip would be too fragile to even cast this.

Orton says “90 angular degrees is considered the endpoint of cone bending”. First, let's assume the normal: Examination of cones on kiln-opening to verify controller operation. Consider the cone on the left: The tip is touching. But it is also beginning to buckle, which means it was touching for a while before the firing ended. Who knows how long! The second one is not touching but has still fallen a little too far. Why do we say that? The third one, positioned on the Orton guide, has reached the recommended 90 degrees. This demonstrates a good reason why self-supporting cones are much better than standard ones: They are not touching when considered done. And standard cones, when sent in a 3/4" plaque, have a less consistent bending behaviour.

This porcelain is 43% kaolin, 20% silica, 36% nepheline syenite and 1% Veegum. I cast it in a test mold that had been used for a very dark red burning body. The inner unstained section looks no different than the outside after firing to cone 6. This porcelain develops a vitreous surface, it is apparently able to dissolve and absorb the thin film of iron (unlike a stoneware). Kaolins in even very white burning porcelains always contain a small percentage of iron oxide as a contaminant, but as long as it is below about ~0.5% the fired color is not visibly darkened.

This test mold is thin-walled yet I can cast three thick-walled mugs in three hours. This clay is L2596G, a buff burning cone 10 stoneware - the mug on the lower right has been fired to cone 10 oxidation. Achieving 4-5mm thick walls is not a problem if the casting slip employs a large particle kaolin intended for this purpose (e.g. Opticast). The flared lip works as expected, keeping the rim nice and round. No cracks have appeared at handle joins, even for pieces left in the mold overnight. The mold halves mate with each other very well and the seam is easy to remove. The seam on the base is an issue - I have to be careful to line up the halves well before clamping the mold strap - this is a warning for accuracy during the mold production stage. And the possible motive for a three-piece mold if I get more serious about this piece.