The objective was to discover the ratio of Redart to ball clay that will reach near zero porosity by cone 10. With minimal bloating. And rich color with high plasticity.

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 was clear I needed less fired maturity. However, 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.

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 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 co-efficients 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). 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.

This slurry is 100% traditional Redart. This deep red color is a product of the high percentage of iron oxide in its chemical analysis. But that chemistry provides a good example of why using the classical ceramic rational-analysis method to derive a theoretical mineralogy can be misleading for an illitic clay (which this is).

The traditional calculation assumes that K2O and Na2O belong to feldspars, then assigns the remaining Al2O3 to kaolinite and the remaining SiO2 to free quartz. However, Redart is known to be an illitic sedimentary clay. And illite itself contains substantial K2O. That 4.1% K2O is a strong chemical confirmation of Redart containing a lot of illite/mica-type clay. The 1.6% MgO is another clue that some of the chemistry belongs to smectite minerals rather than discrete feldspar and MgO phases.

On the left I applied pure Ravenscrag slip, as a glaze, inside and out, using only a small brush. I did the same to the bowl on the right, using the G1947U transparent glaze. Both of H550 clay. We almost always use these as dipping glazes. But here I mix them as brushing glazes, adding 10g CMC gum powder to one liter of a water-reduced version of the slurries. Blender mixing makes it possible to mix in the powdered gum and tune water content for the best brushing experience. The gum slows down the drying speed dramatically so there is plenty of time to brush it into place (while the wheel is turning). The gum also greatly increases the cohesion, enabling pouring out of a pitcher in a long thin stream. In this firing I also glazed a leather hard 40-inch tall vase and a 35 lb bone-dry bowl using the same technique. The evenness of coverage was the best I have ever gotten (of any technique) on large sizes and shapes.

It might surprise you that handmade tile is often glazed using brushing. A glaze can be squirted onto a tile and spread with a large brush in seconds. This can be done on a passing conveyor or in batches on a table.

This is a cone 6 transparent fritted glaze (converted from a Gerstley Borate one). Its B2O3 content is high, sourced by Ferro Frit 3134. Bubbles in transparent glazes often plague potters; many just keep trying new glazes until one works, or give up on never finding one. Why not try to fix this problem?

The calculated thermal expansion of this glaze is 8.8, very high because of its high KNaO and low SiO2. These plates are unglazed on the underside. The upper one has a well-fitting glaze and its base remains flat. On the lower one, the high-expansion glaze on the inside contracts much more on cooling and, because it is bonded to the body, ends up in tension. The resulting stress is enough to pull the plate out of shape, bowing the base upward. This issue is common with ceramic tile, where an otherwise flat tile bows as the two layers contract differently on cooling.

A glaze this far into tension will also be highly prone to crazing. And its combination of high alkali and low silica raises another red flag: chemical durability. The solution is to reformulate, reduce KNaO in favour of lower-expansion fluxes (among which MgO is most effective), and increase SiO2 as much as the desired melting behaviour and surface will permit.