This is a variation on the 50:30:20 cone 6 very fluid-melt pottery glaze recipe. I reduced the Gillespie Borate (GB) to 37% instead of the original 50% (thus bringing the B2O3 from 0.63 down to 0.5). My objective was to reduce the melt fluidity. But the crawling was so bad in this that it is almost unusable. The reason was not obvious until I fired a sample to 1550F and 1650F. At the former, the integrity of the glaze layer is great, but by 1650F it melts suddenly and does this. It is not difficult to see why these “puzzle pieces” with curled up edges might pull inward to create "glaze islands" characteristic of glaze crawling. This is happening even though the percentage of Gillespie Borate is lower. Not surprisingly, Ulexite mineral, which GB almost certainly contains, is also known for suddenly shrinking and melting.

I tried to solve another problem at the same time. GB is plastic on its own, and thus hardens the layer and suspends slurries well. Thus, the 15% kaolin in the recipe unnecessarily increases the drying shrinkage. So I substituted calcined kaolin. While it helped with that problem that was small consolation.

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This woman has quickly laid coils of plastic clay on top of each other, in a conical shape. Then she simply begins throwing, centering, compressing and even verticaling the walls on the first pull. Since joining stiffer clay elements, as done in typical hand-building, can be a time-consuming elaborate process, how can this potter just ignore that?

-The clay is quite soft, but very plastic (evident in that the potter dangles the coils like a rope, yet they don’t break, and that she can make such large pieces).
-The coils are rolled on a wet table, generating slip on their surfaces. The plasticity means the surfaces are sticky.
-The piece being made is large and walls are thick.
-The mere act of applying pressure and thinning the wall also joins the coils.

In industry it is normal to use frits whose chemistry is either unknown or approximate provided. The manufacturer has designed them for a specific use, so in many cases they comprise 80%+ of recipes used for that purpose. However potters more commonly use them as minor additions to recipes, they source needed oxides to the oxide formula (instead of raw materials).

Substituting a new frit into a recipe, without paying any attention to how its chemistry compares, is like adding a dog of unknown breed to a team. Knowledge of the breed is needed to know what good and bad it will contribute to the team. It’s a dog, but is it a sled dog? Ceramic glazes fire the way they do because of their oxide chemistry. Frits contribute oxides. Not knowing the chemical makeup of a key ingredient your recipe robs you of the single biggest DIY tool for understanding the fired properties: glaze chemistry.

Commercial brushing glazes are laced with CMC gum to make them paint on thin and dry slowly. Why would anyone want that? Layering. Brushing on layers takes time and it is difficult to get even coverage, but it justifies brushing up the prices also!

What if you are not a "layer slayer" and want the opposite of all of that: Go on thick enough at one go, dry in seconds and apply super even. DIY potters have that ability by making thixotropic dipping glazes. You cannot buy these because the gum kills thixotropy. Thixotropic glazes are fluid in the bucket but gel after a few seconds of standing. This enables really good dipping properties - the gelling enables the glaze to stay in place upon extraction from the bucket. This picture demonstrates how such glazes hang on to even a non-absorbent and wet surfaces.

Bottom: Extreme thixotropy. The spatula is held vertical by gelling only. Yet when this slurry is put in motion, it is fluid!
Top left and right: These spatulas were slowly extracted and the engobe and glaze just hang on in a perfectly even layer. On a bisque surface, the glaze dries quickly, within seconds. And the engobe hangs on to leather hard ware for perfect coverage, even around sharp contours.

Electrical insulators most often employ aluminous porcelains. Like sanitaryware and tableware (mullite porcelains), feldspar still forms some glass, but the microstructure of electrical porcelains is dominated by angular, size-controlled, alumina grains. Only a small amount of mullite forms. The result is a matrix having much better mechanical and dielectric strength, better insulating properties and resistance to thermal shock. How can this be affordable given that calcined alumina is many times more expensive than other common porcelain ingredients? When producers are already extremely careful to meet specifications, rejects are low enough that the added cost of alumina is acceptable given the performance gains.

What about the glossy brown glaze? Brown hides dirt, dust, and industrial grime. Slight variations in firing are less visible and the glassy finish causes rainwater to form discrete droplets rather than a continuous conductive film. The Iron-oxide-based brown is self-opacifying so it does not require zircon. And it is highly resistant to uV degradation and compatible with the chemistry needed to achieve glaze compression (to minimize crazing).

The original Italian majolica ware was red earthenware with a thick layer of tin-opacified glaze vibrantly brush-decorated using single-strokes of watery metal oxides. The water-color of ceramics. But tin oxide is no longer affordable. And ceramic stains are better. And no one uses lead glazes. So all majolica-like ware made today is actually “faux (false) majolica”. These test samples take the “faux” to the next level: Stoneware with a zircon-opacified white glaze. But almost all are crawling. If this happens for you ask these questions:

Is the glaze re-wetable? Dipping glaze recipes often are not, especially if they fail sanity check (e.g. are over-clayed or under-clayed).
Base coat dipping glaze better survive the rewetting of a second layer?
Mixing them as a brushing glaze give maximum insurance.

What did they look like when the overcolor dried? Cracks are sure indicator or crawling.
Were you painting pure stain or metal oxide (mixing with water only)? Don’t do that. Water color paint uses gum Arabic, pottery colors need to be in a stain medium (which often has CMC gum).

In terms of hardness, wear resistance, and high-temperature stability, alumina ceramic is far superior to even the strongest mullite porcelain. Such porcelains are mixes of kaolin, feldspar and silica. Alumina parts are just micron-sized calcined alumina powder fired to an incredible cone 30 or more, often held there for days! The powder is mixed with binders and formed by pressing or injection molding. Precision "green machining" is also used (while parts are chalky). With super fine particle size, high purity, dense packing and prolonged firing, surfaces can be very white and so smooth they are glossy (e.g. spark plugs are not glazed). While parts can even be translucent they are not vitrified, no glass is developed during firing. Rather, they are sintered - the fine particles fuse into a material approaching diamond hardness.

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. This inside glaze is crystal-clear on other bodies (and on this one without the black engobe). The black stain in the engobe appears to be the issue. How?

Underglazes (or engobes) become a semi-dense layer and impede LOI by slowing gas diffusion. If the glaze then melts early and lacks viscosity, remaining channels of escape are sealed (increasing bubbling dramatically). Double-melt interfaces can form between vitreous engobes and glaze when the former softens, the clear glaze begins melting. Gases get trapped at the boundary, being generated at the exact wrong time during the firing.

Look at the outside amber transparent glaze, GA6-B. Although also early melting and on the same engobe, it has very little micro-bubble clouding! Why? 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; these are likely acting as a fining agent that enables bubble merging. The larger bubbles break at the surface because of sufficient melt mobility and lower melt surface tension.

This green is not just a typical transparent cone 6 glaze with 2% copper carbonate added (and 2.5% tin oxide). That outer glossy glaze accommodates the copper without micro-bubbling or crazing because of its lower melt surface tension. In such glazes, significant MgO (a super low expansion oxide) can often be tolerated without losing gloss. This is a light bulb moment. Fully 0.15 molar of MgO are present here. This is the "matting oxide"! Yet the glaze is still hyper-glossy!

The above factors are enough. But if this were used in industry, technicians would fix additional issues: The very low initial melting temperature (from 37% very early-melting frit in the recipe). That traps LOI bubbles unnecessarily. Raising the ZnO and sourcing as much of the B2O3 and KNaO as possible from later melting materials and/or frits.

The porcelains are Plainsman P300 and M370. The liner glaze is G2926B, it is a gloss but has a much lower melt fluidity, it is a functional transparent whose main job is to fit the body and be hard and durable. The outer glaze is G3806C.