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Glaze Chemistry

Case studies where glaze chemistry was used to solve a problem.

Related Information

A high feldspar glaze is settling, running and crazing. What to do?

The original cone 6 recipe, WCB, fires to a beautiful brilliant deep blue green (shown in column 2 of this Insight-live screen-shot). But it is crazing and settling badly in the bucket. The crazing is because of high KNaO (potassium and sodium from the high feldspar). The settling is because there is almost no clay in the recipe. Adjustment 1 (column 3 in the picture) eliminates the feldspar and sources Al₂O₃ from kaolin and KNaO from Frit 3110 (preserving the glaze's chemistry). To make that happen the amounts of other materials had to be juggled. But the fired test revealed that this one, although very similar, is melting more (because the frit releases its oxides more readily than feldspar). Adjustment 2 (column 4) proposes a 10-part silica addition. SiO₂ is the glass former, the more a glaze will accept without losing the intended visual character, the better. The result is less running and more durability and resistance to leaching.

Is the N505 cone 6 matte glaze recipe what you think it is?

This recipe is from page 2 of the booklet: "15 Tried & True Cone 6 Glaze Recipes". Click the following code, G3955, to see more information on how it compares with G2934 and G1214Z1 mattes. This flow test and these test tiles were in the same kiln, fired at cone 6 using our PLC6DS schedule. The defining characteristic of N505 is its extreme melt fluidity - it runs because it is overtired at cone 6. Still, the surface on the tile (lower right) is arguably more interesting than G2934. Some felt pen marking reveals why: The micro surface is much rougher. To its credit, although it does stain easier, it can still be cleaned with effort.

From the chemistry, shown on Insight-Live side-by-side screenshot. It has very low Al₂O₃ and SiO₂ - that turns on some red and yellow lights. One could hope to have melt fluidity and great functionality, but they pretty well never go together. This glaze should fire glossy - the 6% magnesium carbonate is the mechanism of the matteness - MagCarb is super refractory, it may not be dissolving in the. melt. The glaze should cutlery mark (although it seemed hard in our testing). Most important, low Si:Al levels always carry the risk of leaching - exercise caution adding any significant percentage of heavy metal pigments. Crazing is another possible issue over melted glaze.

Sourcing Li2O from spodumene instead of lithium carbonate comes at a cost

Lithium carbonate is now ultra-expensive. Yet the reactive glaze on the left needs it. Spodumene has a high enough Li₂O concentration to be a possible source here. It also has a complex chemistry, but the other oxides it contains are those common to glazes anyway. Using my account at insight-live.com, I did the calculations and got a pretty good match in the formulas (lower section in the green boxes). Then I made 10-gram balls and did a GLFL test at 2200F (notice the long crystals in the glass pools below the runways). Not surprisingly, this recipe is very runny, that's why the tiny yellow crystals grow during cooling. The new version fires very similar, perhaps better. Our calculated cost to mix this recipe in 2022 was $17.84/kg vs. $10.40/kg. But there is a practical cost: Poor slurry properties. The spodumene sources so much Al₂O₃ that 70% Alberta Slip had to be dropped to accommodate it! How does one use this type of glaze without ruining kiln shelves? Using a catcher glaze is one answer.

High feldspar glazes craze. Don't put up with it.

This glaze, "Bamboo Cone 10%", contains 50% potash feldspar so or certainly qualified as a high feldspar glaze. K₂O and Na₂O are this over supplied. They have the highest thermal expansions of all oxides, by far. These are needed and valuable - but when grossly over supplied the result is crazing. This glaze used to work on this body, H550. The previous version of H550 was firing near the bloating point of the body, about 1% porosity, so the recipe had to be changed to provide more margin for error. The new recipe has a more practical 2.0-2.5% porosity, it has no danger of bloating or warping and still has excellent maturity and strength. This glaze was crazing before and pieces did not leak because the body was dense enough - so they were still water tight. But now it does not work. The solution is to do something that should have been done before: Use a silky matte base recipe that does not craze. We recommend our G2571A base (below right) - the Zircopax, rutile and iron oxide in the original can be added to it instead.

How I calculated a feldspar-to-frit replacement in a cone 10R clear glaze

A screen shot of side-by-side panels in my account at Insight-live.com. On the left is the original G1947U recipe. On the right I have substituted frit 3110 as a higher-concentration-source of KNaO. This enabled actually increasing KNaO to get a better gloss and melt. I have introduced calcined kaolin (to ratio with the raw kaolin to control slurry and drying properties). I added frit 3249 to introduce low-expansion MgO to counterbalance the higher levels of KNaO. That frit, like the 3110, will not only melt things better simply because it itself was pre-melted, but it also brings along a little boron. That supercharges melting and enables an enhancement: The addition of more SiO₂. The low thermal expansions of MgO, SiO₂ and B₂O₃ counterbalance the increase that will occur as a result of the higher KNaO (0.15-0.26). Sound boring? When you see the unexpected results you might think differently (see the linked post below). I never considered using frits at cone 10R before, this success led to an improvement in my main silky matte glaze also.

Sanity checking a cone 6 purple pottery glaze

A customer was having serious trouble with this cone 6 glaze recipe shivering. A quick check of its chemistry reveals the reason: It has the lowest calculated thermal expansion we have ever seen! The reason is the high spodumene and talc levels. Adding the 3% cobalt also makes this among the most expensive we have seen. To say this recipe looks non-typical is an understatement. And, it raises flags on working properties and susceptibility to leaching in both limit recipes (e.g. very low clay content, high talc and spodumene) and limit formulas (stratospheric levels of Li₂O and MgO coupled with plenty of cobalt).

The hard panning problem can be fixed easily: Supply the same amount of Li₂O from lithium carbonate (only 10% is needed so the overall recipe cost is reduced), which makes room in the recipe for clay (to supply the lost Al₂O₃ and SiO₂ from the spodumene). Second, introduce KNaO at the expense of MgO and Li₂O, which will greatly increase the thermal expansion and reduce or stop the shivering.

Use a frit blend ratio to control the amount of kaolin in a glaze recipe

These are the recipes and calculated oxide chemistries of two pottery glazes (as shown in my account at insight-live.com): The original problem recipe and an adjustment to fix it. Recipe #1 sources boron from a soluble material and three plastic materials are combined to increase drying shrinkage enough to cause cracking when drying (and thus crawling). Recipe #2 solves these problems while producing the same chemistry. It sources boron from two frits (one having almost no Al₂O₃) whose ratio to each other can be altered to supply more or less Al₂O₃ to the melt. That enables removing two of the plastics: Ball clay and Gerstley Borate. The remaining 20% EPK is perfect to create a creamy slurry that suspends, applies and dries well.

Imagine realizing that pottery you're selling is all going to craze!

What if every single piece you have been making is all eventually going to craze? Do you refund all the customers? All because you trusted a liner glaze recipe found online. At a reputable site. This recipe is called "Classic White". And it has "classic crazing". On going back to the source web page you find there is no mention of even the possibility of crazing. Then you type the recipe into your account at Insight-live.com and are horrified to see how high the thermal expansion calculates to (see red circles on picture). Then you realize that, based on simple recipe limits, you could have steered clear of this just by looking at it. Then you find there are other well-documented base glossy and matte recipes that do not craze (like G2934 and G2926B). Even if they do they are adjustable to enable fixing the issue. And you can even blend them to create any degree of satin desired. And they use a mix of tin oxide and Zircopax to avoid crawling. And they are durable, don't cutlery mark, don't leach, don't run, don't settle in the bucket.

Substitute Ferro Frit 3134, using glaze chemistry, in three glaze types

Can't get frit 3134 for glaze recipes? Can you replace it with frit 3124? No, 3124 has five times the amount of Al₂O₃ (the second most important oxide in glazes) and half the amount of B₂O₃ (the main melter). This ten-minute video presents a glaze chemistry approach that is easier to do than you probably think. On three different recipe types, you will learn to source the needed oxides from two other Ferro frits, 3110 (or Fusion F-75) and 3195 (Fusion F-2) and end up with at least 15% kaolin in each (to suspend the slurry). Each requires a unique approach. Two of the calculations produce improved slurry properties and one yields a recipe of significantly lower cost. If you have a recipe that needs this, get an insight-live.com account, enter it there and I can help you do the calculation.

Pottery glaze peeling when multi-layered

Dipping glazes can, in very controlled circumstances, be multi-layered. If you have done it for some time, with success, you may have been just lucky. These pieces demonstrate one of many factors that can produce failure: The top glaze contains 7% bentonite and 5% zinc oxide - that is 12% hyper-fine particles, perfect to create the drying shrinkage to make this happen. The recipe author must have reasoned that it could "pinch hit" for the inadequate clay content. But 7% bentonite in any glaze is highly unusual. And, it is actually not even necessary here. Why? The high percentage of Ferro Frit 3124 is sourcing needless Al₂O₃ (alumina), that should be coming from kaolin or ball clay instead. Frit 3134 is the perfect stand-in, it contains almost no Al₂O₃, but otherwise is quite similar. The equivalent recipe we calculated on the right has the same chemistry, but does pass a sanity check. It is not guaranteed to work, but has a better chance than this one. For assurance of success, the base should be mixed as a base-coat dipping glaze.

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