Texas talc (left) quickly absorbs all the water poured on it. Montana talc (right) resists whetting of the particles much more, the water is just sitting on top and has not penetrated at all.

The Montana material is more pure, and it is a platy talc. Talc platelets expose broad basal water repellent surfaces that are difficult for water to wet. When these fine plates settle parallel to the surface, they can form a tightly packed, water-repellent layer with very small pore entrances. Surface tension and trapped air then prevent water from entering, so it remains pooled on top.

The Texas is chemically and physically different. It contains appreciably more calcium-bearing material and other non-talc components. It also has rounder, fluffier and substantially coarser particles. These characteristics provide larger, better-connected pore passages and more readily wetted mineral surfaces. Once water enters those pores, capillary suction quickly pulls it through the powder.

This is M340, it is not made from a mixture of 200 mesh refined minerals and ball clays, rather, from minimally processed natural mined clay. It is ground at 42 mesh, so the material contains a wide range of particle sizes. Potters who have only used this type of body are at home with it. Its advantages include excellent dry strength, pleasant texture, natural fired character, easy glaze fit and low cost. But downsides are lower plasticity, higher drying shrinkage and splitting. But a case this bad is not normal, it only happens when there is a "perfect storm" of factors:

• An unstable bowl shape (flared out to the edge of the clay's ability to support itself).
• Cutting the rim off with a needle tool and leaving that flat surface uncompressed and with water on it.
• Applying slip just after throwing the piece (rather than waiting until leather-hard). Or, simply leaving excessive throwing slip in place.
• Not wedging the clay (or not wedging it well).
• Using the clay when it is too soft.

The best way to prevent this from ever happening is to do the opposite of all the factors just mentioned.

A light coating of ordinary ceramic dust is made of the same minerals as the ware and glaze itself. During dipping, the incoming water instantly penetrates and incorporates surface dust. Glaze are dust mixed with water! However, washing trades a tiny and often harmless surface contaminant for a more serious problem: partially saturated pores. Bisqueware is a porous ceramic sponge. Its job during dip glazing is to suck water out of the slurry quickly and evenly, leaving behind a stable layer of particles on the surface. If the bisque is damp from washing, the suction is weakened. Of course, there are common-sense exceptions, but compressed air, a soft brush, or a barely damp sponge used sparingly are better cleaning solutions to preserve the essential dryness.

These thin-walled mugs demonstrate: Just pour-glazing the inside has waterlogged them, they must be dried before applying the outside glaze. Imagine if they had been washed also!

This L3954F engobe is tuned to have the same degree of vitrification as this P300 porcelain (using EBCT test). I made a pint of a brushing version by mixing a 500-gram batch with 75g of Laguna Gum Solution and 280g water (it does not contain CMC gum but not VeeGum because I want a lower specific gravity than would be typical for a brushing glaze). Blender mixing gets all the lumps out and makes it paint beautifully onto leather-hard ware. One coat covers. This enables presenting this normally white-burning body as a black porcelain to match the glaze.

I was so excited about this engobe that I made my own label. It shows the code number I assigned in Insight Live. Subsequently, the piece was bisque-fired, black-glazed, and fired at cone 6. The band painted on the base, which I did as a fix-up for a few tiny white bare spots, demonstrates something unexpected: It can dry on a vitrified surface without cracking, and this thin layer can even fire on without peeling.

Left: The base recipe of GA6-C (80% Alberta Slip, 20% Frit 3134, 4% rutile). That recipe is stable; it does not run. Right: A 2% addition of copper carbonate transforms it into a runny, melt fluid glaze. It also produces a pleasant green even on this red-burning body (a good green is not easy in oxidation ceramics). Using a catch glaze (e.g. a frit-reduced version), this could be made safe on vertical surfaces. However, there are red flags here. Having only 2% copper would not destabilize a typical glaze (making it leachable). But phase separation is occurring here, likely because the base contains significant rutile. That means the phases are copper-bearing and possibly copper-concentrating. And, as noted, the copper has a greater-than-expected impact on the melt. Third, commercial variegated copper glazes can even leach in acids (as happened with the mug inset after a night with lemon juice). While the GA6-C recipe does have a 0.3:0.7 R2O:RO ratio, other indicators make it a course of wisdom to consider the GLLE test before use on functional surfaces.

Mason is unusually information-open for the ceramic stain market. Their public reference guide gives compositions by listing oxides such as Co, Cr, Fe, Mn, Ni, Sb, Ti, Zn, Zr, etc., and then marks which oxides are involved in many stain families. It also gives practical compatibility notes, especially about zinc, calcium, firing limits and body-stain use. Their product pages often identify the crystal system/pigment class. For example, Mason 6630 Black is described as a chrome-iron-nickel black spinel formed by high-temperature calcination of chromium, iron and nickel oxides into a spinel matrix. Mason 6274 is described as a nickel silicate green olivine pigment formed from nickel oxide and silica. It is certainly helpful to know if your stain is, for example, a cobalt aluminate spinel, chrome-tin pink, zircon-vanadium blue, nickel silicate/olivine green, rutile yellow, etc. Mason’s technical table also lists CICP/color-index numbers, CAS numbers, chemical names, specific gravity, oil absorption, mesh residue and pH for some pigment lines. Of course, they don't publish oxide percentages, mineralizers, firing schedule, milling procedure, soluble salts, frit additions, trace impurities or batch tolerances. This being said, they are not completely alone; some others also publish such information.

Shown here are mugs I glaze using the G2934Y recipe with added Mason stains.

I did this batch-to-formula glaze chemistry calculation to help a potter with a transparent cone 10 glaze for a Plainsman P500 (a 25x4 porcelain). This version of Desktop Insight ran on the TRS-80 Model I and III, they were the first popular consumer microcomputers for business (outselling Apple 5-to-1). Notice the report uses capital letters; the machines did not support lower case! The dot matrix printers of the time lasted forever on an ink ribbon. Fanfold paper fed from a box, I could tear off only as much as was needed for a report. Boot time was less than 5 seconds. Here is what is amazing: In 2021 I found this same recipe in my Insight-live account (the green screenshot)! The results are a little different; I had the chemistry of talc wrong in 1980. Through the years, I wrote code to migrate from one system to another, and eventually it got to Digitalfire.

This is a cone 6 oxidation test mug I made in 1981. The speckling in the glaze was made by adding iron stone concretion particles. But this also has a story. Most potters at the time were firing cone 10R or low-temperature, cone 6 electric stoneware was a new development. Note the incised code number: "81-R-5". Digitalfire data archival was already well underway on my TRS-80 computers. The base also has a "60#" marking. I was trying a finer 60 mesh particle size to alleviate the glaze pinholing problems that plagued Plainsman customers at the time (their products were made at 42 mesh and kilns did not have controllers that enabled drop-and-hold or even hold-at-temperature firings that are used now). Pinholing was one of the first glaze problems that I studied; many glaze chemistry projects, using my new Desktop Insight, were aimed at making glazes having melts of lower surface tension and higher melt fluidity (using frits).

The clay here is called jiani, it’s found in various layers along with other yixing clay (but not used for teapots). The translation of this video screen capture (below), provides a fascinating insight into how they judge the suitable temperature at which to fire. First, technicians measure the porosity and firing shrinkage over a range of temperatures, likely looking for a firing "sweet spot". Notice shrinkage reaches a maximum at 1100C, then drops off as the clay begins to expand. But this is not the only thing considered. Notice, in the comments, that they are also looking for "surface luster" (which is not found). They also comment about a "dull sound" and "crisp/clear sound" (so they must create a sounding vessel of some sort). They also break a fired piece and comment of the nature of the cross section, revealing something else interesting: The clay holds on to a dense cross section for 100C degrees after reaching maximum fired shrinkage.

The Yixing teapot craftsmen appear to break all the rules and yet produce impossibly delicate and symmetrical pieces. Hao-Tong Yan, one of those craftsmen, and I have been trying to understand the technical reasons for how this amazing craft is possible. It turns out not to be magic, but actually a highly evolved understanding of a very unusual material. Here are some of the things that we are coming to understand (which is making it possible to create a facsimile of the clay in North America).

-The clay is not highly plastic; the workability comes from surprising places.
-The clay has impossibly low water content, yet can be formed.
-Craftsmen flatten the clay with a mallet, instead of rolling it, yet it does not stick to the board.
-Sections are simply glued with slip, yet they hold.
-The clay burnishes, yet is not smooth.
-Fired ware is smooth, yet the soft clay appears sandy.
-The fired surface is glossy, yet there is no glaze.
-The fired clay appears super dense yet does have porosity.
-The Yixing ore can have the appearance of being like rocks, yet they make a workable clay body from it.