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.

The cone 6 mug on the left has the G3933A glaze, applied as a dipping glaze. It turned out poorly - crawling from corners and looking thin and washed out. I made a brushing glaze version of this (which adds 1.5% CMC gum), I keep it around for this very purpose. It has a high specific gravity (unlike commercial ones that have high water contents - they will run and go on too thin if you try this). Because of the gum, it dries hard, there is no shrinkage or cracking. On a second firing, using the C6DHSC schedule again, (mug on the right) the surface is transformed - thicker, more vibrant color.

It is possible to make a small brushing batch by simply dewatering some of the dipping glaze on a plaster bat (just for a minute or two, long enough to pull out the water). Then remix it with enough gum solution to get a paintable consistency. It doesn’t have to be precise, just get it to a point where it paints on and sticks in the thickness you need and does not crack on drying.

The top base glaze, G2926B, has enough melt fluidity to produce a brilliant functional gloss when used as a transparent. However, for this 2% copper carbonate addition, it has too little melt fluidity and/or too much surface tension to merge, pass and heal the entrained micro-bubbles (generated by the decomposition of the carbonate).

The lower glaze, G3806B, diversifies the fluxes (half the B2O3 in exchange for more Na2O and introduction of SrO and ZnO) and increases their total compared to Al2O3 and SiO2. The result is a more fluid cone 6 melt having lower surface tension. The mixed-oxide effect is also a factor here; the diversity itself improves the melt.

The above factors are enough to solve the problem here. But more can be done. More zinc (in exchange for KNaO) could produce later melting, especially in combination with sourcing some or all of the latter from a feldspar instead of the low-melting frit. The benefit would enable more gas escape until melt-sealing (and reduce the COE).

Industry, late-melting glazes are a must for fast fire because there is no time for glazes to debubble. The later they melt (while still melting well at the target temperature), the more LOI gases of decomposition (generated by the body, glaze materials, glaze & body additives) can be expelled first. What about potters? These melt flow tests are of specific interest to anyone making clear glazes using frit 3134. They compare four common Ferro products fired to 1750F: Frit 3249 (29% B2O3), frit 3124 (14% B2O3), frit 3195 (23% B2O3) and frit 3134 (23% B2O3). Surprisingly, the one having the most B2O3 starts melting the latest (more than 200F after 3134), this is because of the amount of MgO in the formula. So, if your transparent glaze contains any MgO (G2926S, for example, contains 0.15 molar), the more that can be supplied using this (instead of 3134), the later the glaze will melt. Likewise, frit 3124 is a better choice than 3134 in cases where the percentage of clay can be reduced (since it supplies much more Al2O3). Glazes containing high percentages of feldspar are least likely to benefit because the main alternative source of KNaO is frit 3110, and it melts even sooner than 3134 (an exception is cases where the glaze also has high MgO and B2O3).

This is a cone 6 transparent fritted glaze (converted from a Gerstley Borate one). Its B2O3 content is high, sourced by Ferro Frit 3134. Bubbles like this plague many potters, many just keep trying new glazes until one works, or give up on never finding one.

The first obvious problem is the frit, it starts melting at 1350°F, while plenty of gases are still being generated. Such a bubble-trapper is a non-starter in an industrial continuous fast-fire kiln. They need late melters. But potters have flexible firing, so what could be done? The firing could be slowed down, leading up to 1350. It could be held at top temp, then either slow cooled or a drop-and-hold.

Potters often encounter the problem shown here. These pieces are fired at cone 6. They are decorated with underglazes made from a mix of porcelain powders and stains. The transparent glaze works over certain colors but on others, it is full of microbubbles and pinholes. The potter has not had success finding a transparent overglaze that works consistently. Stain manufacturers do not mix stains with porcelain to making underglazes.

So, although closer control of the transparent glaze thickness or a more fluid melt glaze recipe might help, the real solution may lie with the underglaze recipes used here. An ideal bisque-stage underglaze is sinter-bonded but not sealed (therefore not accepting glaze water). An ideal fired underglaze also has controlled maturity: enough glass development to bond well to the body and promote glaze acceptance, but not so much that edge-bleeding and opacity loss occur. This state of 'controlled maturity' is also more likely to match body thermal expansion. The cost savings and the potential to fine-tune each color to your exact needs can be powerful motivations to use DIY underglazes.