Ravenscrag Slip used by itself as a glaze shows no crazing on this body at cone 10R. The mug in the photo was fired with a blend of 90 parts Ravenscrag Slip and 10 parts whiting (calcium carbonate). It crazed. Why would that happen when CaO has a relatively low expansion compared with Na2O and K2O?

The unity formulas show what changed. Whiting supplies CaO without supplying silica or alumina. With the fluxes normalized to one mole, SiO2 drops from 3.89 to 2.74 and Al2O3 from 0.47 to 0.33. That’s about 30% less of each relative to the total flux, even though their ratio stays at 8.3:1. The calculated expansion rises from 6.4 to 7.0, consistent with the crazing on this mug.

The 90:10 talc blend goes the other way in the calculation. Talc supplies silica along with MgO, so SiO2 drops only to 3.25. MgO also has a lower expansion contribution than CaO. The result is a calculated expansion of 6.1, below that of Ravenscrag Slip alone.

The speckled iron stoneware recipes I created in the past had a major problem: Color variation as a product of natural material changes. To illustrate, zoom in on the bare clay bases on these mugs. The differences in the variegated light and dark color patching is a product of differences in fired maturity. Small differences which I can measure as porosity (water absorption on an SHAB test).

The solution is to create a more refractory base body and add some feldspar to reduce its porosity to 3%. Why? The feldspar percentage can then be tuned to compensate for material changes in each production mix. These mugs illustrate. The base recipe for them is a two-part mix of red stoneware and two buff fireclays.

• Mug #1 measures 2.8% absorption and #2 3.5%. Both have 12% Custer feldspar (which I need to replace), but the #2 clay base employs the more refractory and plastic fireclay.
• Mug #3 has the same clay mix as #2, but has 12% Nepheline Syenite, dropping absorption to 1.5% and the color to solid brown.

The next move: I want the more plastic base of #3, but with less fired maturity. It is just a matter of testing to reduce the Nepheline percentage.

In a clay body: In a clay body: Compare the original body with one containing the proposed replacement using SHAB test bars fired together. Adjust the replacement percentage to approach the original absorption and firing shrinkage (and fired color, deformation and glaze fit). Nepheline syenite or a suitable soda feldspar may achieve the required maturity at a lower percentage.

In a glaze: Start by matching KNaO—the combined molar amounts of K2O and Na2O. This gives more freedom to use available feldspars without requiring an exact match of potassium and sodium separately. Confirm the result by firing tests. The following are practical starting points, not fixed percentage rules:

1. About 20% feldspar or less: Try a one-for-one substitution with a comparable feldspar recommended by your supplier. Compare fired tests before changing a production batch.
2. About 20–40% feldspar: A chemistry adjustment becomes more worthwhile. Use the replacement feldspar to match KNaO, then adjust the other ingredients to bring Al2O3, SiO2 and the remaining oxides into line.
3. More than about 40% feldspar: Check glaze fit carefully. If the glaze crazes, this may be an opportunity to fix both issues (reduce KNaO and increase suitable lower-expansion fluxes, such as CaO or MgO). Test for changes in melting, surface and color.
4. Special-effect or reactive glazes: Identify what produces the effect and preserve the relevant chemistry, materials and firing conditions. Matching total KNaO is a useful first step; if the effect changes, investigate the K2O:Na2O ratio as well as other differences.
5. Glazes with too little clay to suspend or adhere well: Consider fixing this issue at the same time, supplying some KNaO from a low-alumina frit such as Frit 3110. This can make room for more clay while maintaining the Al2O3 target. Rebalance the other oxides and test both slurry behavior and fired results.

The GBMF test compares Custer Feldspar with 15% Frit 3110 and 15% Ferro Frit 3134 (when this feldspar is fired without additives at cone 10, it barely starts to flow). This tester is fired at cone 6. This shows how much help the feldspar needs to melt at cone 6. And it illustrates how effectively the boron-rich frit promotes flow in this feldspar at cone 6. Frit 3134 supplies much more B2O3 and CaO, and less SiO2, than 3110. The latter supplies more alkalis.

This melt-flow test compares two feldspars at cone 10, with no additives. Feldspars are among the most common flux sources in glazes, yet by themselves they flow surprisingly little. How is this possible?

Feldspar supplies potassium and sodium (K2O, Na2O), but these come packaged with substantial alumina and silica (Al2O3 and SiO2). The result is a viscous melt: These specimens have softened and become glossy, but barely moved. In a glaze, feldspar interacts with other materials to create a different oxide balance, one that can melt more readily and flow much farther. Its usefulness as a flux source depends on the chemistry of the whole recipe—not simply on how far it runs by itself.

These are L4005C and L4005D unglazed mugs fired to cone 6 oxidation (same schedule). Shrinkage and absorption data are shown. They contain 35% Redart and a 2% red iron oxide addition. The redder body is not merely a different color because of less iron; it is less vitrified (if it were fired a cone higher, it would also be brown).

D simply adds 5% silica and uses a different ball clay (click the above code numbers for details)! This demonstrates that Redart can be a tipping point material when used at significant percentages for fluxing stonewares. The above linked page shows the sudden color transition just below cone 6 for L4005C). So if the recipe or firing schedules change a little, the appearance of the body can change a lot. But what if the alkali content in the Redart changes? That would do the same. Is the Redart the problem or failure to test incoming shipments and adjust recipes (assuming they are adjustable)?

A change in Redart’s chemistry or mineralogy could shift this balance too. That makes incoming-shipment testing especially important for bodies formulated close to a maturity threshold. The challenge is not just restoring the red: it is preserving absorption, workability and glaze fit at the same time.

These G1916Q and G1916J low fire clear glazes contain 85% frit and 15% kaolin, so they begin to soften as low as 1450F. I made these 10 gram GBMF test balls to compare the degree of melting across a broad range. I did not expect to see carbon or the degree to which these balls could trap it. Notice that unoxidized carbon is still obvious at 1650F, still visible at 1750F and leaves the residual effect of micro-bubbles that opacify the matrix even at 1950F (cone 04). While a thin layer of these glazes on ware will most often tolerate this anyway, when bubble clouds do appear an option is to hold the firing at 1400F to fire away as much of the carbon as possible (before melting begins).

Redart has been so reliable over the years that many have just trusted it. But it is a high-iron terracotta clay. This is ceramics; we've just been lucky. Such materials are variable by nature.

While many clay body makers put Redart into their products as soon as it arrives, Freddie doesn't do that. He notes the batch code(s), takes multiple samples, and makes SHAB test bars. He even mixes up a test body and makes ware and test bars from it too. He has a well-developed testing program and knows Redart well. Freddie also understands the recipes it is used in. The low-temperature bodies depend on its iron content for their red color (he is ready to adjust the iron oxide percentage in recipes). But Freddie has to be really diligent with stoneware bodies that also depend on Redart for fired maturity, being ready to adjust their recipes (to increase or reduce the degree of vitrification).

The "Q" in the code number signifies that it was preceded by tests A, B, C, ... P; thus, it is the product of a lot of work to develop a flexible recipe that anyone can make and adjust. This could be your path into low-fire pottery.

The photo shows G1916Q on L215, L212, L210, L213, Buffstone at cone 03. It uses common Ferro frits and can be made to fit any low-fire clay body (it is thermal expansion COE adjustable). It melts well down to cone 05. And we have a strategy to reduce clouding and micro-bubbling. These five test tiles were fired using the 04DSDH (drop-and-hold) firing schedule. All exited the kiln without crazing. The L215, L213, L210 and L212 samples subsequently survived a 300F/Icewater test without crazing, but the Buffstone did not (reduce its COE by increasing Frit 3249 and reducing Frit 3110, e.g. 15:5 instead of 10:10). The L213 could shiver (increase COE by increasing Frit 3110 at the expense of Frit 3249).

These two GBMF test balls were melted on refractory tiles to compare their melting behaviour. Their calculated oxide chemistries are almost identical, but their recipes are very different. G2931F (left) contains 29% Ulexite, driving its calculated LOI to 14.3%. G2931K (right) sources its boron from frits, reducing the LOI to just 3.1%. The difference is striking! The high-LOI glaze is full of large bubbles that were unable to escape or clear before the melt stiffened during cooling. Although ulexite releases most of its chemically bound water before significant glaze melting, that early gas evolution can contribute to foaming and trapped bubbles as melting progresses. The fritted glaze is much clearer, although it still contains fine bubbles. These may be associated with the kaolin or other sources of entrained gas.

The GBMF test exaggerates gas-retention behaviour, which is precisely why it is useful for comparing recipes, but it does not necessarily predict the appearance of a thin glaze layer on ware.

This is G1916Q3, a version of G1916Q with added 45-micron-grade silica, fired at cone 04. Despite the extra silica and low firing temperature, the glaze remains glossy, transparent and well melted. The finer powder promotes dissolution into the melt, where the added silica can improve chemical durability. Notice the shivering at the right-hand rim of this Plainsman L215 mug. The encouraging part is that the recipe provides a straightforward way to raise its thermal expansion: replace some low-expansion Frit 3249 with high-expansion Frit 3110, then retest.

For stoneware potters exploring low fire, a recipe like this offers a practical starting point and a clear way to make adjustments. It can be opacified with zircon, colored with compatible stains, and prepared for dipping or brushing. This surface shows how much brilliance and clarity are possible at cone 04. Fire to cone 02 and you have stoneware.

Frit manufacturers face a daunting challenge: Converting carbonates, hydrates, borates, and nitrate materials, which off-gas during firing, into a homogeneous melt, quenching to a glass of zero LOI and then grinding that back into a consistent powder. You face a challenge also.

Shown here is a GLFL test that reveals a combination of improper mixing and inadequate smelting. This melt fluidity test compares the previous good shipment, L, with the latest faulty one, S. S appears to have not been fully reacted during manufacture (as exhibited by the foam blanket). But there’s more: a glaze containing 50% of this had inadequate melting. That could be chemical drift or inadequate batch mixing. The latter seems most likely because S was from a full two-pallet batch (the likely capacity of their smelter), from which a dozen bags were tested. These showed the most typical results at the top of the first pallet and the worst at the bottom of the second.

A bad batch of frit can ruin a lot of ware. Buy the largest amount you can afford and do the GBMF test is almost as good as these. Character changes in the melted balls put up the red flags you need to see.

This cone 6 vase was made from a coarse-grained stoneware body typical of those long produced by Plainsman Clays. These materials performed well at cone 10 reduction, but when similar bodies began being fired at cone 6 oxidation, pinholing often became a challenge. Why? The coarser particle structure creates fewer but larger pathways for escaping decomposition gases, concentrating gas flow through localized vents and leaving behind pits like those visible here. The gas is generated from fewer larger particles, concentrating it.

Yet today many potters fire these same clays with little or no pinholing. Electronic kiln controllers deserve much of the credit, making drop-and-hold and slow-cool schedules practical. Equally important are glazes with enough melt fluidity to heal LOI gas-release craters during the hold, while maintaining sufficiently low surface tension to avoid trapping gases in bubbles that later become blisters. Commercial glaze manufacturers have quietly optimized for these same properties.

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 in this glaze crawling problem. 30% EPK is a lot - a tipping point for 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, LOI-adjust by considering 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.