You don’t have to invent a glaze from scratch to start making glazes your own. Begin with a documented recipe, make one controlled change and compare the results. This is G3933, made by adding iron oxide, rutile and tin oxide to a 75:25 blend of G2934 matte and G2926B glossy base glazes. I like it—but this mug gives me plenty of reasons to keep testing:

A small test kiln makes these questions practical to investigate. I can fire a few samples when I need to, compare each change with the original and use what I learn to decide the next step.

The kaolin is implicated for this glaze crawling problem. 30% EPK is a lot, a tipping point to issues with shrinkage and cracking on drying. Although #6 Tile kaolin in its place (center) doesn't disrupt the overall chemistry, it does disrupt the drying shrinkage: it is just too plastic for that high a percentage in a recipe. The fix is to substitute part of it for calcined kaolin.

Calcined kaolin has no LOI (vs 12% for raw kaolins). Without overthinking it, I just consider that 10% less is needed. So 20% #6 Tile and 9% calcined kaolin will equal 30% EPK. The right panel shows the calculated chemistry of the recipe with this change; notice it is very close to the same.

There are other fix options also. NZ and Grolleg kaolins are less plastic choices than #6 Tile; using either might mean that no calcined material is needed. Also, the failure happened on multi-layering of dipping glazes; crawling is common with that. The base glaze could have been mixed as a base-coat dipping glaze.

It only takes her two seconds. But those two seconds are packed with details. To a typical potter, it might seem that this factory worker is doing the impossible. Although she is just dipping plates, there is so much going on here we need to stop and itemize it. There is more than one thing a potter can learn from this. Click/tap the URL below to see the video and learn more.

When we finally hit an iceberg, perhaps the big question should be: "How did we get away with doing that for so long?"

Tipping point glaze recipes and practices are like this. I want to use those reactive glazes, so I need them to work. A quick test or two confirms what I want to see. That's the visible part of the recipe iceberg. But below the waterline is all that pesky recipe logic, material sense and glaze chemistry that bring up questions about the recipe before I even try it. And the trigger-happy glaze nerds who raise alarm bells about safety about glazes even though they have never actually mixed them up. I don't really want to hear about those things; I just want to double or triple layer those colorful runny glazes that look so nice on that social media post.

This chart compares the decompositional off-gassing (% weight Loss on Ignition) behavior of six glaze materials (there are many more). Each was heated through the range 500-1700F at 350F/hr. It is amazing how much weight some can lose on firing. For example, 100 grams of calcium carbonate generates 44 grams of CO2! Magnesium carbonate even more. But the main message of this chart is that some late gassers overlap early melters. That can be a problem (causing glaze bubbles, blisters, pinholes). The talc in this test is an example: It did not finish gassing until 1650F. Yet many fritted and Gerstley Borate glazes have already begun melting by then.

There are two takeaways here. The biggest weight loser is not the biggest troublemaker. What matters is how gas release overlaps glaze sealing. Calcium carbonate illustrates this; its high carbonate content can still produce a clear glaze (when gases can clear ahead of any melting). And the talc, although having a small LOI, is a potential troublemaker when the glaze melt cannot effectively clear the bubbles.

This glaze has just been applied to a bisque tile. It contains wollastonite, a curious mineral because it has a shelf life (it can agglomerate in storage as a powder). Surprisingly, it can also form them in slurries. Other materials can also agglomerate; these include barium carbonate, iron oxide, cobalt oxide, clays, tin oxide, zircon and titanium dioxide.

This glaze was propeller-mixed, but that was not enough to break down the white lumps. Is sieving the answer? Maybe not. While screening can reduce the size of the lumps, it does not prevent their regrowth in the slurry (because smaller agglomerates that survive sieving can become starting points for larger clusters). A better option is blender-mixing. When that is done and re-agglomeration occurs, then the problem is more complicated.

Yes. The two glaze cones are both the same Grolleg-based porcelains. Both of them have the same base glaze: 1947U transparent. But the one on the left uses EP kaolin, and the one on the right uses Grolleg kaolin. The Grolleg glaze has a dramatically cleaner appearance. The Grolleg does not suspend the slurry quite as well as EPK; however, it responds to gelling better (using vinegar or Epsom Salts, for example). This ability to facilitate an easy-to-use thixotropic slurry more than compensates.

USG specialty plaster is entering a final-inventory phase in the ceramic supply chain, and rationing has begun at some suppliers even though the Southard plant does not close until November.

These have just been thrown on the wheel. I find it to be a foolproof method of comparing the plasticity of two clays. They were slurried up and dewatered to about the same moisture content and the same amount was thrown to compare the size achievable. While the Grolleg is stickier and dewaters a little slower, it is not as plastic as dark EPK (which itself is not that plastic compared to others). Curiously, New Zealand kaolin (halloysite) is quite a bit less plastic than the Grolleg but it responds to plasticity augmentation (in porcelain recipes) just as well as Grolleg (similar amounts of bentonite producing similar plasticities). And, bodies containing EPK also need about the same amount of bentonite to produce plasticity suitable for throwing large forms. So, the plasticity that a kaolin appears to have by itself is not completely indicative of what it will contribute to a body (if augmented with bentonite). The EPK used here is the darker and more plastic of the two varieties Plainsman receives.

Left: OptiKast, a coarse-particle kaolin intended for slip-casting. Right: Plainsman A2 ball clay. These DFAC test (drying performance) disks were dried with their centers covered, deliberately creating uneven drying. The exposed perimeter dries and stiffens first; the covered center dries and shrinks later. The resulting cracks reveal how each clay responds to that stress. OptiKast feels smoother, yet its individual clay particles are much coarser than those typical of ball clay. This kaolin has low drying shrinkage, so its disk remained intact. The ball clay shrinks more. As its center dried and tried to contract against the already stiff perimeter, stresses developed, splitting the disk and opening numerous cracks around the edge.

So why use ball clay? This particular kaolin has so little plasticity that even making this disk was difficult. And the dried disk is fragile. Yet it is very useful in casting bodies, where plasticity impedes permeability. Ball clays are highly plastic; high drying shrinkage comes with that. Adding some of this to poor-plasticity casting or forming bodies that can tolerate a darker fired color can transform them into ones that both form well and dry harder and stronger.