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.

This two-inch stack of clay body and glaze mix tickets represents only about half of my testing over the past year. Along the way, I also added thousands of photos to my Insight-live.com account. Looking at this pile, I realized why I am testing more: Organized records make it easier to keep multiple projects moving. I can see what I have tried, compare results and pick up where I left off. Each test builds on earlier work, and that visible progress motivates me to keep going. If you want to do DIY glazes and bodies, this is the secret is good record-keeping.

The underglaze is the 20x5 cone 6 base transparent glaze (G1214M with 5% Mason 6666 black stain). Overglaze left: GR6-H Ravenscrag Oatmeal. Overglaze right: GA6-F Alberta Slip oatmeal. Both produce a pleasant silky matte texture. Double layering using typical dipping glazes is prone to crawling (because of shrinking and cracking during drying). By keeping layers fairly thin, it is possible to achieve success, but with glazes high in clay content (as these are), more is needed. A mix of raw and roasted Alberta and Ravenscrag Slips is the answer here. Start with a 50:50 ratio and adjust to establish a balance between drying hardness and drying shrinkage. For more assurance in production situations, mix the underlayer black as a base-coat dipping glaze.

There are enough variables that a record-keeping system is needed. Start now, and you won't regret it.

Watch the G1214Z video to see me convert the G1214M 20x5 cone 6 clear base into G1214Z cone 6 calcia matte using simple glaze chemistry and recipe logic. This first appeared in the Digitalfire desktop Insight instruction manual 30 years ago. It is an understatement to say that this process is interesting if you want to know more about glazes, their chemistry and recipe logic. Watch this video and see me adjust the recipe of my high-calcium transparent cone 6 glaze to convert it into a calcia matte. In an Insight-live.com account, the process is easy enough for anyone. We'll cut the Si:Al ratio, increase the CaO, maintain the thermal expansion for glaze fit and make the recipe shrinkage-adjustable using a mix of calcined kaolin and raw kaolin. We will even compare it with the High Calcium Semimatte from Mastering Glazes.