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Tipping point

Why do ceramic processes seem to work for years and then suddenly fail? Learn how hidden tipping points in materials and methods can trigger production disasters.

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Hobbyists, potters and even manufacturers often "live on the edge" in their production process. This can be unintentional, or intended by so called “yolo potters”. For the former, when a problem hits the reaction is often: "Why did this not happen before?" Consider some reasons why it might be better to ask, "How did we get away with doing things this way for so long?"

A reliable ceramic process has margin. Temperature, material properties, thickness, drying rate, firing rate and other variables can move around somewhat without producing failure. A tipping-point process has little margin: production appears normal, but only because all of those variables happen to remain on the safe side of their limits. One seemingly minor change—or several minor ones together—can push the process across the line.

Potters, for example, may use drying techniques that subject ware to uneven shrinkage, yet they get away with minimal cracking. Or they might fire quickly through quartz inversion without dunting the ware. They may be tempted to see past success as a reason to continue a technique in spite of warnings that it should crack the ware.

Some hobbyists like to apply multiple layers of reactive and very melt-fluid glazes to outside surfaces of ware, and somehow they get away most often without glazes running onto the shelf or with surface defects (e.g. blistering).

Production departments of manufacturing facilities are under constant pressure to produce more - so short-cuts are common (especially in drying procedures and firing times). They also commonly apply glaze to ware that has not been bisque-fired, a very tricky technique to do successfully. After getting away with pushing the envelope for years, production staff may think their process is normal. Then, when multiple factors converge suddenly, reject rates skyrocket.

When everything in a production process stays the same (materials, processes, workers, weather, etc.), a 'breaking the rules' approach can come to appear as normal production. But finally, something does change. Or, more commonly, multiple things change. A waterfall of production problems then hits surprised workers and technical personnel are unable to adequately identify the cause. This is a cascading tipping point, a combination of multiple smaller tipping points.

"Tipping point recipes" are common among hobby ceramics products. Reactive glazes or highly vitrified bodies are examples. While these might work in the testing lab of the manufacturer, they don't travel well between the studios of potters and hobbyists (who often have very different process and firing habits). These recipes often have fragile mechanisms that depend on the consistency of the materials in the recipe. Recent years have seen many issues with material variation from batch to batch, even with frits. Potters sometimes fall into the trap of trusting a simple test under which no problem appeared, not realizing that the conditions of the test masked problems. Understanding materials and chemistry is a better approach, these can tell you why a problem might appear under conditions you didn’t test.

High feldspar glazes can also operate on a tipping point. One with 50% feldspar, for example, is almost certain to craze. But how about 40%? If it is used on a body having a lower thermal expansion (e.g. one having a low percentage of silica or a significant pyrophyllite addition) crazing may not occur. But if the body manufacturer changes the name brand or particle size of a body material, and that affects the particle size of the natural quartz in the material, then a glaze that has worked can start to craze. While this might surprise users, the glaze was already prone to crazing; it simply worked because of the body it was being used on.

An appreciation for just how magical the whole ceramic process is can help in creating realistic expectations for production processes. How is it even possible that a piece can dry rigid in one part and the rest of it continue to shrink without cracks forming? How can a vitreous body soften as if vitrifies and still stand up and hold its shape? How can a glaze melt enough to produce a super gloss and yet not run down off the ware. How can micro-bubbles in a glaze on a vertical surface find their way to the surface, break and the surface heal behind them? Ceramic-rigid ware cooling in the kiln must have temperature gradients across the matrix; how can different parts of it go through quartz inversion without dunting occurring? How can microscopic clay particles, through their surface charges and enormous collective surface area, bind a mass of otherwise nonplastic mineral particles into something that can be shaped? All of these seem impossible, yet they happen, and they are. That should encourage us to understand these processes, measure the variables that matter and build enough margin into them that normal variation does not become failure.

Related Information

Why are rutile blue glazes susceptible to blistering?

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This blistering problem is common in rutile blue glazes, especially high-temperature - this is not saleable. The reason relates to what it takes to create this kind of vibrant variegated aesthetic: Melting the crap out of the glaze and cooling it just right. This particular one is being fired to cone 11 down to get enough melt fluidity to make it crystallize and phase separate. It seems logical that if the glaze is melting so well it should be able to heal any bubbles that form and break (these are more than usual because the body is being overfired and generating gases). However, the fluidity comes with surface tension that can hold the bubbles intact. Each of these holes in the glaze is a product of that - plus another factor: Cooldown is rapid enough that the melt is not sufficiently fluid to heal after bubble breakage. The potter has been using this glaze for many years with success, but a small change in process or materials has occurred to push it past a tipping point. Solutions? A drop and hold firing. Add a flux (e.g. a little lithium or a frit) to make it melt fluid at cone 10R (where the body generates less gasses of decomposition). Replace any high LOI materials in the glaze itself with other materials to source the same oxides.

Crawling on sanitary ware:

Laydown is the first suspect

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This is glaze crawling and it underscores the need for attention to the details of all production parameters. This one small glaze defect makes this pedestal sink either a refire, a second or unsaleable. This is most common on abrupt surface contours but that is not the case here. The cause of this is likely several factors combining. The glaze is opaque white because it contains a high percentage of zircon opacifier. Zircon glazes tend to do exactly this so their successful use is doubly dependent on minimizing the percentage added and on attention to other details to compensate. This glaze has been applied thickly to ensure good coverage (thicker laydowns bring more crawling problems). The glaze is likely low in clay and thus the physical bond of the dried glaze layer depends on the binders being used, their percentages, the integrity of the way they were mixed in, and their shelf life. The ability of the glaze laydown to dry-bond with the body depends on the condition of the surface (e.g. water content, dry or bisque fired, smoothness, dustfreeness, quality of materials used in the body and integrity of body preparation, etc), the presence of surface contaminants (e.g. soluble salts) and the way in which it was applied and its thickness. The glaze melt's ability fire-bond and form an interface with the body that produces a smooth surface is dependent on its melt fluidity and ability to form an interface with the body.

There is another way to look at this problem: The process runs along crawling multiple tipping points: A viscous glaze melt, glaze application to dry rather than bisque ware, a thick glaze application, a large surface area intolerant of any defects and a glaze application technique (spraying) prone to irregularities of thickness. Rather than trying to identify the specific problem it might be better to simply make changes to move the process back from the tipping points.

Rutile in functional glazes:

Push the limit and you might get this

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The 80:20 GA6-A Alberta Slip base becomes oatmeal-like when oversaturated with rutile and/or titanium (left: 6% rutile + 3% titanium dioxide; right: 4% rutile + 2% titanium dioxide). The rough surface results from excessive crystallization of titanium-bearing phases from the melt during cooling. It is unpleasant to touch, readily shows cutlery marking and is less desirable for functional surfaces. In glazes of this type, rutile additions above about 4% can become increasingly difficult to keep smooth and consistent. Crystallization, rather than phase separation, increasingly dominates the variegation mechanism.

One way of avoiding problems is staying below the tipping point, keeping the percentage as low as possible while still getting the desired variegation (of course, that will vary depending on the melt fluidity of the glaze and kiln cooling). Surprisingly, making the glaze more fluid does not necessarily solve the problem. Greater melt fluidity increases ion mobility and can actually promote crystal growth during cooling, producing an even rougher surface.

Tipping-Point Glazes are Good. And Bad.

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Reactive glazes don't melt into a homogeneous melt and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) create a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the needed oxides, firing schedule, kiln atmosphere, cooling or heating cycle, etc. These glazes are at their best when each piece has a unique, artistic character. But, this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and not well understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

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