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.

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.

These are pure Custer feldspar and Nepheline Syenite. The coverage is perfectly even on both. No drips. Yet no clay is present. The secret? Epsom salts. I slurried the two powders in water until the flow was like heavy cream. I added more water to thin and then started adding the Epsom salts (powdered). After only a pinch or two, they both gelled. Then I added more water and more Epsom salts until they thickened again and gelled even better. The result is a thixotropic slurry. They both applied beautifully to these porcelains. The gelled consistency prevented them from settling in seconds to a hard layer on the bucket bottom. Could you do this with pure silica? Yes! The lesson: If these will suspend by gelling with Epsom salts then any glaze will. You never need to tolerate settling or uneven coverage for single-layer dip-glazing again!

G2931 Worthington Clear is a popular low to medium-fire transparent glaze recipe. It contains 55% Gerstley Borate (GB) plus 30% kaolin (GB melts at a very low temperature). GB is also very plastic, like a clay. I have thrown a pot from this glaze recipe! This explains why high Gerstley Borate glazes often dry so slowly and shrink and crack during drying. When recipes also contain a plastic clay like this one the shrinkage is even worse. GB is also slightly soluble, over time it gels glaze slurries even in smaller percentages. Countless potters struggle with Gerstley Borate recipes.

This is L4023F (a test body like our H440 cone 10R body). The polygons on this surface are produced when the 3D CAD software converts from its native format to an STL file that slicer software can use. These are the product of the default settings (which can be changed but increase file size). The precision of the 3D printer is evident in that it can reproduce these. Since the polygons are not visible in the final glazed piece, neither the PLA surface on the 3D printed block mold, or the surface of the plaster case mold made from it, were sanded.

Feldspars are employed in glaze recipes as melters. So comparing their melt fluidities should be helpful in deciding if one can substitute for another (of course, if possible a soda predominant feldspar should be substituted for another soda spar). Feldspars don't melt alone at cone 6 (2200F) so we mixed each with 15% Ferro Frit 3195. Nepheline Syenite is obviously the champion melter here. Other similar ones can be spotted easily. In the end, degree of melt is a valid consideration in determining if one feldspar is a viable substitute for another in a recipe. Even if the feldspar you want to substitute does not melt as much a little frit can be added to the recipe to make up for the difference (e.g. 3-5%).