Plainsman Clays extracts 6 different sedimentary clays from this quarry (Mel knows where the layers separate). The dried test bars on the right show them (top to bottom). The range of properties exhibited is astounding. The top-most layer, A1, is the most plastic and has the most iron concretion particles (used in our most speckled reduction bodies). The bottom one, 3D, is the least plastic and most silty (the base for Ravenscrag Slip), it is also the cleanest and most vitreous. The middle two, A3 and 3B, are complete buff stonewares (e.g. M340 and H550). A2, the second one down, is a ball clay (similar to commercial products), it is refractory and the base for Plainsman Fireclay. The second from the bottom, 3C. fires the whitest and is the most refractory (it is the base for H441G).

Reactive glazes don't melt into a homogeneous melt and they don't freeze as a typical glass. The physical nature of the material powders (e.g. their particle size and the individual nature of how they respond to heat, soften, melt and interact with their own kind and others) create a melt that does not solidify into a homogeneous glass. These glazes are said to be dynamic. And unpredictable effects often occur during firing, like color variegation, speckles, streaks, mottled and flowing textures, crystallization, pooling, etc. The outcome is influenced by factors such as the materials chosen to source the needed oxides, firing schedule, kiln atmosphere, cooling or heating cycle, etc. These glazes are at their best when each piece has a unique, artistic character. But, this is also their worst feature, making them "tipping point glazes", ones whose visual character is a product of fragile and not well understood features of the materials and process. Small changes typically produce big changes in fired appearance (often to the chagrin of the potter).

Here it is fired to cone 8 where the melt obviously has much more melt fluidity! The photo does not do justice to the variegation and crystallization happening on this surface. Of course, it is running alot more, so caution will be needed.

These fired bars are nepheline syenite (NS) fired to Orton cone 03 (~2000F). The top bar, L4526C, is 95% NS and 5% Veegum. Its porosity is 3%. The bottom bar, L4526B, is 90% NS and 10% raw bentonite. Its porosity is 19%! The top bar has much higher fired shrinkage, it looks and feels like a porcelain, that is clearly what is densifying it so much (the bottom one looks and feels like Plaster of Paris). Veegum is a plasticizer, not a flux. But it is acting as the latter here. Or perhaps its surface area enables acting as a catalyst to initiate the melting of the nepheline syenite. Imagine what Veegum can do in a porcelain-like Polar Ice.

It took a lot to be able to throw this moderately bellied vessel because the clay is pure calcined kaolin. It has zero plasticity. Actually it is worse than zero. That is why 25% bentonite was needed to make it barely plastic enough. That 25% would have done much better with other non-clay materials like feldspar or silica! How can this be? In its natural state, kaolin’s plasticity comes from its layered crystal structure, the water both bonds the plate-like particles together and lubricates their lateral movement against each other. The chemically bound water in the natural kaolinite crystals, which are tiny water magnets, is the secret to their ability to create plasticity - calcining drives it off. This dehydroxylation also changes the crystal structure, converting kaolinite into non-crystalline metakaolin, a particle that is actually hostile to plasticity. Calcined kaolin is also subject to shear thickening, a thin slurry thickens when propeller mixed - the particles form structural resistance, the opposite of what raw kaolin does.

We find the 0.047 relief depth shown here is best (K152). Shallower ones will stamp a crisper design but K152 is better if pigment is used to highlight the recesses. For some things, it can be valuable to put a border around the outside of a design so that when the stamp is pressed hard into the clay, the edges do not smear outward. These do not need to be stuck to a piece of wood, just lay them face down on the clay and use a wooden block to press them down (because they are flexible it is easy to peel them out). When the clay is stiff enough no parting agent is needed. The cost: In 2024 the minimum charge is $37.5 for 50 square inches. They accept PDF and AI vector files and the shopping cart enables previewing. The cart might generate all four CMYK plates, remove the CMY ones and keep the K (black). The most common mistake is having too much detail or too small printing. Or, forgetting to make them reverse-reading. It is best to make your images using vector graphic software like Illustrator or Inkscape.

G2917, which I mixed as a brushing glaze, is on the right. This is not sold in jars, I make my own labels as part of the demonstration that it is possible to make your own brushing glazes (ink-jetted onto regular paper, cut 62mm wide (2 7/16") and held securely on with 2 7/8" transparent packing tape). This glaze is less runny but lacks some of the floating white colouration. But that can be achieved using Alberta Slip floating blue L4655, it employs titanium instead of rutile (and relies on the rutile/iron mechanism for the blue color).

Stains can and do influence the degree of vitrification of a porcelain. Some stains will make a porcelain more refractory (decreasing fired shrinkage), others will make it more vitreous (increasing the firing shrinkage). Obviously, the greater the percentage of stain the greater the effect. Stained porcelains having differing fired shrinkages will stress at boundaries in accordance with the degree of difference in their fired shrinkages. In this piece, you can see how the boundary between the red (more vitreous) and green (less vitreous) porcelains is the point-of-failure. The only solution is to adjust the porcelain recipe to move the fired maturity in a direction that counterbalances the effect of the stain. For example, you could employ three recipes (regular, more vitreous, less vitreous) and use the indicated one for each stain added.

In this screenshot I am comparing the chemistry of a cone 10 clear glaze being developed (left) to the one it is being patterned after (right). The G1947U on the right is a long time staple but I want to see if it is possible to duplicate its chemistry using a raw clay material, Plainsman 3B (also known as MNP). The intention is to source the maximum of Al2O3 (the red box) from this instead of feldspar and kaolin. That enables using almost 60% in the recipe! Sourcing the KNaO from this near zero-alumina frit, rather than feldspar, is what enables almost all of the Al2O3 to come from the 3B clay! Only silica and calcium carbonate are needed to bring the SiO2 and CaO up to match.

This black engobe, L3954F, is on a cone 6 buff stoneware (at leather hard stage). It contains only 7.5% Mason 6600 black stain. How is that possible? Why do people add so much more to their underglazes? Because this recipe has been tuned to have the same degree of maturity as the body - it therefore fires totally opaque. This contrasts with underglaze/engobe recipes containing significant frit, among other issues, their vitreous nature renders them translucent. Thus, up to 40% stain is needed to crowbar their opacity enough to intensify color. And a thicker application (that carries other issues).

Notice how thinly and evenly this is applied. This was possible because of another key factor: The slurry was adjusted to be thixotropic. The thinner layer enables drying more quickly. The body-compatible engobe recipe also means fewer issues with flaking during drying, better fire-fit.