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.

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.

On this M390 red clay test tile, the titanium-variegated cone 6 calcia matte glaze G1214Z1 looks excellent (rutile has a similar effect). But that appearance depends on a combination of factors: glaze chemistry, 5% titanium dioxide, the titanium grade, application thickness, the red M390 body and the PLC6DS firing schedule. The relief of the tile especially flatters it - forcing the glaze thin over high points and allowing it to pool in recesses, displaying a whole range of effects in just a few centimetres.

The tile tells you what effects a glaze is capable of; it does not necessarily tell you what the glaze will look like on ware. These mugs demonstrate that. They were fired using the C6DHSC schedule and are made from buff-burning M340 (left) and M390. The mug on the right also has a heavier, less-even application. Surprisingly, some of the thickest runs become more transparent rather than more opaque. That is a reminder that the pale, variegated character of this glaze is not simply the result of titanium acting as an opacifier; it depends heavily on melt thickness and titanium-driven crystallization during cooling.

The objective is to discover the ratio of Redart to ball clay that reaches near-zero porosity by cone 10, with minimal bloating and rich color. And with high plasticity. This is stage 1 in creating a new body; nine more tests eventually achieved L3118J.

These test bars were fired at cone 10R and 11, 10, 9 and 8 oxidation (top to bottom). It reaches zero porosity by cone 8 and blistering is already happening there (which makes measuring fired shrinkage or porosity beyond that pointless). However, the cone 10R bar is not blistering. It clearly needs less fired maturity. A real bonus: Plasticity is fantastic yet drying shrinkage is well below 7%! In the final body I ended up using 20% Redart, it still imparted good color and near-zero porosity by cone 10.

This is L3118J, a chocolate brown burning super-plastic base clay (to which 25% coarse grog is added). The base matures at cone 6, yet the grog stabilizes the fired matrix enough that it stands up in the kiln at cone 10R, firing to a dense vitreous product (its porosity of 4%, comes from the grog). Only 20% Redart clay provides both the color and the K2O flux (no feldspar is needed). There is a second factor that makes this clay functional: An engobe to smooth the surface under the glaze. These two mugs employ L3954N (on the inside applied at the leather hard stage), they enable the smooth surface needed for the glaze. The glaze is brown and blue colored versions of G2571A.

GA6-C (left) and GA6-E (right) at cone 6 oxidation. GA6-E adds 4% spodumene to GA6-C, which contains 4% rutile (the base is 80% Alberta Slip and 20% Frit 3134). Notice how the spodumene suppresses the large pale, overly variegated areas that commonly develop in GA6-C, producing a more uniform blue.

Why? The small spodumene addition introduces Li2O along with Al2O3 and SiO2, changing the melt chemistry and therefore the crystallization and phase-development behavior of the rutile/iron-bearing glaze during cooling. Thus it is safer to regard spodumene as modifying the titanium-related crystallization rather than simply reducing it. Like GA6-C, GA6-E requires a slow-cool firing schedule to develop the rich blue (e.g. C6DHSC). Both look best on darker-burning bodies.

Here are the three common lithia source materials in a side-by-side calculation to compare their potencies. I have adjusted the material weights so that all are delivering the same amount of Li2O (assuming a recipe calling for 5% lithium carbonate).

By weight it takes 42 parts of petalite to deliver the same amount of Li2O as 5 parts lithium carbonate! But notice how much SiO2 and Al2O3 come along, major surgery on the host recipe is needed to compensate for this (if even possible). Spodumene is complicated too, it brings along so much Al2O3. That’s a problem because the most frequent existing Al2O3 supplier, clay, may have to be greatly reduced or even eliminated (producing a recipe that does not suspend in water). The other baggage both materials bring is lots of SiO2, especially petalite. Thus, replacing lithium carbonate with petalite or spodumene is not really a material substitution—it is a glaze reformulation.

Crackle glazes are often thought of simply as recipes that somehow produce a crack pattern. However, crackle is a glaze/body fit phenomenon, not simply an intrinsic visual property of the glaze. They have very high levels of Na2O and/or K2O (collectively, KNaO). These two have the highest coefficients of thermal expansion, by far, of oxides commonly found in pottery glazes. Feldspars, and particularly nepheline syenite, can supply them in abundance.

Shown here is the 77E10A recipe, originally from Luke Lindoe. It is simply Nepheline Syenite and water (shown on three types of clay bodies). Of course, this won't suspend in water so special measures are needed to make it suitable as a dipping glaze or brushing glaze. Think of it as an on-ramp to understanding more about this type of glaze. 77E10A1 and 77E10A2 are improvements on this; both provide a path to develop and adapt it for your needs.