Glazing black clay bodies stained with manganese is just about impossible with typical transparent glazes. The glaze over-fluxes the clay surface and ruins the color. Worse, if it accelerates surface maturity, the body can blister or generate LOI gases that blister the glaze. How about transparent glazes over a black engobe instead? At least the body color is not lost. But the wrong transparent glaze can do what you see here (inside left).

These mugs are a buff stoneware, Plainsman M340. A black engobe was applied by pouring the inside and dipping the outside two-thirds of the way down.

Left: A L3954F black engobe was applied inside and upper exterior at leather hard. After firing to cone 6 using the PLC6DS schedule, G2926B—which is crystal clear on M340 itself—became completely clouded over the engobe because bubbles generated during firing remained trapped in the melt.

Right: The entire mug was dipped in GA6-B. The Alberta Slip particles and the melt characteristics of GA6-B promote bubble coalescence and escape, producing an exceptionally glossy jet-black surface over the same engobe.

When you learn to make and use engobes correctly, they make magic possible. Here I am turning a dark rustic body into a smooth white one (rear mugs) and a white body into a dark one (front). The engobes have been applied at the leather-hard stage. That is the perfect time, the engobe and body are clay bodies, designed to fit each other; they dry together and fire together creating an inseparable bond.

Handles have been applied, and they have dried to stiff leather hard. Engobe was poured in, poured out, then the mugs were pressed, lip down, into it and extracted. No dwell time was needed. This dipping engobe is DIY thixotropic (not available commercially anywhere). That means I tuned it just before use, to just the right degree of gel (enough for it to drain to the right thickness, then gel just as the last few drops fall from the rim). Honestly, these are a beauty to behold at this stage, the silky, drip-free surface is just so perfect.

You have 147 glaze recipes. How can you get your head around all of them? Is glaze chemistry needed? No, that's a "maybe" way in the future. Right now, you need to start organized documentation. The recipe for each. A few pictures of each fired on different clay bodies, different thicknesses. Perhaps slow and fast-cooled firing. This is what an account at Insight-live does well. What it does even better is tracking your testing. The first step is to assign each recipe a proper code number (replacing these) and write that on all test specimens and buckets. From this point on, learn. Record every observation you make about each in its notes.

Through all of this, constant use in the studio (or factory) will never stop surfacing problems (e.g. settling in the bucket, crazing, running, blistering, material issues, etc.). The seriousness of each will determine the level of attack. First, identify the mechanism of the desired fired result. If it is a base recipe plus additions of colorants, opacifiers or variegators, then check if the base of one of the other glazes has a similar surface texture and character. If so, then could the additives in the troublesome one be used with the better base? If not, then it's time to sanity check the recipe and bring out the heavy guns of at least looking at the chemistry. But in Insight-live, you only need to turn on the display of the unity formula (there is nothing else to do). Next, make sure each material in the recipe links to one in the material database (so the calculated formula is accurate). Then compare the calculated unity formula with a limit formula (often a simple sanity check, like with the recipe, quickly spots oxides that are in excess or are short.

A potter reports that a switch from G-200 feldspar to Mahavir, and EPK to Imerys kaolin, has resulted in this transparent glaze becoming more satin. Is that possible? Yes. Because this glaze is on a unity formula tipping point.

To see it, you do not need to know how to do glaze chemistry, just how to display the calculated unity formulas side-by-side. My Insight-live shows them here. The material change has little effect. But there is an anomaly: 0.29 MgO. That is magnesia matte territory. The MgO is very likely there to help bring the thermal expansion as low as possible (to avoid crazing). For people who cool their kilns relatively quickly, this fires glossy. But a material change could well affect the cooling rate needed to maintain the gloss. That being said, the potter may also be firing slower, yet attributing the mattness to the materials. Or it could be a combination of both.

This is a popular glaze, among others in the book "Mastering Glazes". In Ron Roy's circumstances, and for many others, it is glossy. But for this potter, a small change (in the recipe materials and also likely in firing) has produced this issue.

The functional surfaces on these pieces all employ the GA6-B base honey glaze recipe. On Plainsman native stoneware clays, especially darker burning ones, typical transparents are very prone to micro-bubble and clouding issues. But not this glaze. The likely reason is that Alberta Slip contains coarser particles (it is only processed to 42 mesh), these act as a fining agent.

This glaze brings multiple other benefits:
-It fits Plainsman bodies, all of them.
-It is made from materials mined in Canada.
-The Alberta Slip base produces a thixotropic slurry.
-It acts as a very good base for dark colors and black.

The top mugs are GA6-B inside and out (MNS clay body). The top right has a black engobe under it.
Bottom left: GA6-B inside, GA6-C outside (MNS and Coffee Clay)
Bottom right: L4768E Coffee casting and M340 casting.

This Gemini-generated mug could conceivably exist yet carry these labels. Yet experienced ceramic technicians would immediately be suspicious. The glaze is highly fluid and heavily crystallized; both suggest low or very low Al2O3 levels (it is the key oxide that makes glazes durable). If the interior color were produced using a cadmium-containing encapsulated stain, cadmium-release testing would be essential before claiming the ware is food safe. This is clearly engineered for visual effects rather than durability. None of those characteristics prove it is unsafe, but they do mean that labels like "nontoxic" are not substitutes for actual leach testing. A glaze can be made entirely from materials classified as nontoxic and still fail to meet the durability standards expected of functional foodware.

This bisque mug has been glazed on the inside. But the bisque has absorbed water from that glaze, and this thin-walled mug is now waterlogged as a result (except at the thicker base). It does not have the absorbency needed to build up a thick enough layer of glaze on the outside. Even if it did, the water from the two glazes would wet the bisque so much that its drying time would be greatly extended. This is a problem because the mechanism of attachment of glaze to the body is fragile and works best when the glaze dries quickly. When drying is too very slow, bubbling and cracking often occur (leading to crawling in the firing).

The Yixing teapot craftsmen appear to break all the rules and yet produce impossibly delicate and symmetrical pieces. Hao-Tong Yan, one of those craftsmen, and I have been trying to understand the technical reasons for how this amazing craft is possible. It turns out not to be magic, but actually a highly evolved understanding of a very unusual material. Here are some of the things that we are coming to understand (which is making it possible to create a facsimile of the clay in North America).

-The Yixing ore can have the appearance of being like rocks, yet they make a workable clay body from it.
-The clay appears highly plastic yet is not; the workability is coming from surprising places.
-The clay is stiff enough to resist deformation, yet is cohesive enough to join seamlessly.
-Craftsmen flatten the clay with a mallet, instead of rolling it, yet it does not stick to the board.
-Sections are simply glued with slip, yet they hold.
-The clay burnishes, yet is not smooth.
-Fired ware is smooth, yet the soft clay appears sandy.
-The fired surface is glossy, yet there is no glaze.
-The fired clay appears super dense yet does have porosity.

I uploaded a screenshot of this recipe panel from Insight-live and asked ChatGPT why sanitaryware using this glaze is dunting. Its response is impressive, good enough to provide remediation ideas.

-It notes things about the recipe that are unusual. For example, that the Al2O3 of 0.69 and Si:Al only 4:1, plus 13% calcined alumina, make this glaze more like a refractory enamel than a normal glaze.
-Its observation that crystallization could mean the real thermal expansion may be different from the calculated one, maybe much lower.
-Barium carbonate decomposition does not seem like something that would contribute to dunting, but its presence is strange for such a glaze.

Based on its answer, I think the 13% calcined alumina is the wild card, which is way too much for any glaze to dissolve, something to deal with first.

The mug on the left, made by Holly McKeen, is a typical cone 10 Grolleg kaolin mullite porcelain (highly vitrified, low in residual quartz). Its glaze is crazed. Crystalline glazes are high in Na2O, making crazing virtually certain. Since most pieces are decorative, crystal glazers just accept this as part of the process. But these are functional mugs, the glaze needs to fit (if only for ware strength).

But what if the thermal expansion of the body could be significantly raised? The body on the right is Crystal Ice, it contains 40% silica (vs 20-25% in a typical porcelain). The percentage of Nepheline has been reduced, lowering vitrification to about 1.5% porosity. As a result, more quartz survives undissolved and less mullite develops, raising the body’s thermal expansion. The result is a body with a much higher thermal expansion, so it can not only relieve the glaze tension but actually put a squeeze on it. There is a downside: These are less resistant to dunting and thermal shock failure during use.

Could the glaze be adjusted instead? Yes. Some of the Na2O could be substituted for Li2O, the latter is also a strong melter but has a much lower thermal expansion. Glaze chemistry could be used to source it from Spodumene (to avoid solubility issues with lithium carbonate). However, zinc-silicate crystalline glazes are very sensitive systems, so the more lithia is introduced the more likely the effect on the firing window, crystal size/density and background clarity.