This recipe is pretty far out on the edge for a glaze rescue. But the potter loves the appearance, so it is still worth preserving for decorative or non-food-contact use. Of course, one must go in with eyes open about its application, fit and durability issues. Consider the obvious red flags:

• Almost 30% barium carbonate, combined with SiO2, this raises serious durability and barium-release concerns.
• Only 5.7% kaolin means poor slurry suspension and troublesome application.
• There is no added silica, and the calculated SiO2 is very low for a high-fire glaze.
• Custer feldspar is no longer available to many users.
• More than 43% nepheline syenite supplies a lot of KNaO, and the very high calculated thermal expansion strongly suggests crazing risk on many bodies.

But this glaze has one thing the potter values: it appearance (see image). So rather than discard it, we reworked the material sourcing while keeping the oxide chemistry very similar. I reduced the nepheline syenite substantially and introduced Ferro Frit 3110 to supply much of the KNaO without bringing much Al2O3 with it. That enabled the kaolin percentage to almost triple, from 5.7% to 16.2%, while calculated Al2O3 stayed at 0.37. The result should be a much more practical slurry while giving the glaze a reasonable chance of retaining the character that made it worth rescuing in the first place.

However, the new recipe still has 29% barium carbonate and a calculated expansion of 9.4, so this is not a “fixed glaze.” It is a more usable version of an intentionally problematic glaze.

A potter uses a grogged body that contains a mix of three clays (with small amounts of feldspar, bentonite and silica for tuning fired maturity, plasticity and COE). It fires to a dense light tan at cone 10 (as shown). Although his pugmill does not have a vacuum chamber, until now the body has worked well. For whatever reason, it now has become insufficiently plastic, so some "recipe engineering" seems like the best option.

The obvious response is adding more bentonite. But bentonite works best when the mixing system can thoroughly disperse it, and this pugmill likely cannot do that reliably, even with the 3.5% already in the recipe. Fortunately, physical testing I did years ago on Hawthorn Fireclay, Goldart and OM#4 ball clay provides another route: compare their plasticity and fired maturity and change the proportions of these clays in the recipe (see the full report by clicking/tapping their code numbers: L2950, L3320, L3254).

The three fire to a similar color, which simplifies things. Test #1: Increase the ball clay by 20% at the expense of Hawthorn, the least plastic of the three. Drop the bentonite. This will make the body more vitreous; compensate by trading 5% feldspar for 5% more ball clay. An SHAB test comparing the original and the new mix will give direction to test #2. An adjustment in the silica can be done then if glaze fit is an issue.

This mug blends the stages of ceramic production into one and builds appreciation for how magical the ceramic process is. That can help set realistic expectations for production. Consider:

• How can a minority of microscopic clay particles, through their surface charges and enormous collective surface area, bind a mass of other nonplastic mineral particles into something that can be shaped? Using just water as the glue?
• How is it even possible that a handle can become rigid during drying while the mug body continues to shrink and dry, yet no crack occurs?
• How can a clay-containing glaze powder, which must shrink on drying, stick to a smooth bisque surface and not crack off?
• Temperature gradients are inevitable, meaning different parts of the mug are not passing through the same stages of quartz inversion, yet it does not dunt.
• How can a clay body soften as if it vitrifies in the kiln yet still stand up and hold its shape?
• How can two ceramic powders, neither of which melt, melt if they are mixed together?
• How can a glaze melt enough to produce a brilliant 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 yet the surface heals behind them?

These things seem impossible or improbable, yet ceramics does them routinely. That should encourage us to understand the mechanisms, measure the variables that matter and build enough margin into our processes that normal variation does not push them over the edge into failure.

A potter uses a 70% potash feldspar, 20% whiting, 10% kaolin glaze for a crackle cover over iron oxide brush strokes. While it fires crystal clear, it has a problem: it crawls. While it can be flocculated with Epsom salts, and the application and dry adherence are fairly good; this crawling problem is occurring. By mixing it as a DIY brushing glaze we can fix three of its problems without changing the recipe: Its tendency to go on too thick, the entrained bubbles in the clear glass and the crawling. Don’t dismiss this idea immediately. This is a special-purpose surface and brushing can be done more quickly than you might think. Careful control of thickness is possible and the CMC gum in the glaze will make it adhere much better (eliminating the crawling). The consistently thinner coverage will also make it possible to produce crystal clarity with fewer micro-bubbles in the glass. A hybrid approach is also possible: Pour glazing the inside and brushing the outside.

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 two cone 6 glazes are being compared in a melt-flow tester. Their calculated oxide chemistries are essentially the same, but G2934Y4 sources KNaO from Ferro Frit 3110 rather than feldspar, and much of its MgO from Ferro Frit 3249 rather than the talc used in G2934 (talc is so refractory that it is a key ingredient in making kiln shelves!). The Y4 recipe even sources Al2O3 from calcined alumina (another kiln shelf material!). Yet the fritted glaze flows much farther.

How can the frit make such a difference in the melt, despite the burden of the Al2O3? The feldspar and talc require decomposition/reaction before the oxides become available to the melt, whereas the fritting process has already combined the oxides into a glassy, highly reactive homogeneous form.

Example of a whiteware clay bloating at cone 10 oxidation. While it appears stable at cone 8, it isn't. This is a variation on the 25x4 porcelain recipe. It employs 200 mesh silica, kaolin, ball clay and feldspar. Whiteware bodies like this often soften and melt without ever bloating, but not this one. These fired bars appear to show that this vitrifies at cone 8. However, ware fired at cone 8 exhibits the classic evidence of overfiring: It warps. Overfire it still more, to cone 10, and we enter bloating territory by a new development: The ball clay in the recipe contains soluble salts; these become a thin glaze that glosses and seals the surface, trapping LOI gases which create internal voids (top bar). This line graph shows it as a massive spike in absorption/porosity going into cone 10. It also shows that cone 6, the bottom bar, provides the lowest porosity and highest fired shrinkage, while not being on a volatile part of the curves.

This phenomenon is a reminder that the percentage of feldspar (or feldspar-containing clays) is best tuned such that the body is just reaching (or has just reached) its minimum achievable porosity (assuming it is for functional ware). This can be done by making SHAB test bars like this and firing them at a range of temperatures up to and beyond the target to get a broad view of how maturity progresses.

This reduction celadon is crazing badly. Why? Its chemistry is dominated by feldspar, which supplies lots of K2O and Na2O fluxes. Scores of recipes being traded online contain 50% or more feldspar; they benefit from these powerful fluxes but inherit their high thermal expansion (COE). Many work only because the body happens to have sufficiently high expansion - often aided by significant residual quartz.

Crazing is more than a cosmetic problem in functional pieces. It reduces ware strength and durability, can compromise food safety and turn into a customer complaint. Let's fix it, starting without chemistry:

• Use recipe logic: Transplant the color mechanism into a transparent base glaze that already fits the body. In this case, the celadon color is largely produced by an iron addition under reduction, so there is no need to preserve the high-feldspar base itself.
• Use material sense: A 2:1 silica:kaolin mix supplies an Si:Al ratio similar to typical glossy glaze chemistries. A 10% addition thus shouldn't change the degree of gloss but will likely lower the COE, reduce melt fluidity and improve durability.
• Use glaze chemistry. Substitute some of the high-expansion KNaO with much lower-expansion MgO. Both are fluxes, and even a 0.1-mole shift can make a significant difference. Couple that with an increase in SiO2 where the melt can tolerate it, and glaze fit can improve dramatically.

These pure material powders were applied to bisque as slurries, suspended by gelling with Epsom salts, and fired at cone 10R. On the left is Custer feldspar on a Grolleg porcelain (P700); on the right is Covia nepheline syenite on a whiteware body (P600).

Feldspars and nepheline syenite supply large amounts of K2O and Na2O, oxides having very high thermal-expansion contributions in glaze chemistry. When these materials are melted essentially by themselves, the resulting glasses have very high thermal expansion relative to typical ceramic bodies - thus the spectacular crazing seen here. Glaze recipes containing high percentages of them can likewise be prone to crazing unless enough lower-expansion oxides, especially SiO2, Al2O3 and MgO, are present.

Here is what Redart fired like in 2010 when evaluated for physical properties using the SHAB test and LDW test. These bars are fired from cone 5 down to 06 (top to bottom). The clay reached zero porosity between cone 1 and 2. Beyond that, it was stable to cone 4. Its drying shrinkage was only 5%, indicating low plasticity (compared to a typical pottery clay). It is thus an ideal candidate for a ball clay addition to create a plastic body for handbuilding or throwing.

However, its plasticity is just right for slip casting; it can be used as is. It has no soluble salts and a perfect score on the drying performance DFAC test. Cone 04 and 06 porosity and fired color are very typical of terracotta clays. The fired color of this material really is stunning. And the stability of that color across such a wide temperature range.