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.

Counterbalance a graduated cylinder on a 0.01g scale and pour in some slurry. Fill it to any level that does not exceed the weight the scale can handle. Divide the weight by the volume. In this case, it weighs 60.6g and the volume is 41. That calculates to about 1.47 specific gravity. The higher it is filled, the higher the quality of the graduated cylinder and the better you are at reading the level, the more accurate the measurement will be. In this case, I just need an approximate measure. After adding more water to this glaze, I will measure again, filling it to near the 100cc level. I have to use a plastic cylinder because our glass one is too heavy for this scale to handle (its max is 200g).

This inch-thick iron plate welded to a five-foot-long heavy pipe produces an ideal “mortar and pestle” style tool to break down dry clay lumps on a cement floor. I sometimes add side rails to contain flying lumps, but when crushing softer materials, like the clay shown here, they are not needed. On a heavy iron plate (instead of the floor), I can crush rocks and bricks. By incorporating appropriate sieves, I can effectively create granular material down to 50 mesh or finer.

I discovered the utility of this during the time the movie “Home” was popular, so I dubbed it “The Shusher” after Captain Smec’s control staff (more precisely the rock on the end of it).

Candling of kilns is the final stage of drying. Driers cannot achieve the temperatures needed to remove all water, so almost all industries rely on early stages of firing to remove it fully. Failures like this are part of the learning curve of every company (because there is always pressure to fire as fast as possible).

Although much more common in heavy clay industries, porcelain insulators are one of the less likely products for this to happen with. This is because machine-forming methods make it possible to use aluminous porcelain bodies having very little clay. Thus, faster drying (with less shrinkage and fewer residual internal stresses) also makes it possible for early stages of firing to be quicker. But there are limits. These insulators are solid, thick and heavy. And they have extreme variations in thickness (thin skirts to solid spindle). So, for even these, early stages of firing must be conducted carefully. For such products, periodic firings of days is often needed.