I am getting closer to reduction speckle in oxidation. I make my own speckle by mixing the body and a glossy glaze 50:50 and adding 10% black stain. Then I slurry it, dry it, fire it in a crucible I make from alumina, crush it by hand and screen it. I am using G2934 cone 6 magnesia matte as the glaze on this mug on the left. To it I added 0.5% minus 20 mesh speck. Right is a cone 10R dolomite matte mug. Next I am going to screen out the smallest specks, switch to a matte glaze when making the specks (they are too shiny here), switch to dark brown stain. Later we will see if the specks need to bleed a little more. The next step is to tune the degree of matteness in the glaze and add a tiny amount of blue stain. I am now pretty well certain I am going to be able to duplicate the reduction look in my oxidation kiln. Methodical testing with good records are the key to fine tuning the color, speckle density and size distribution and glaze surface character.

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.

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.

Two potters in the tiny town of Enderby, BC, realized something amazing: Clay is the ideal material from which to make water filters (CWFs). Its internal pore space and structure (and thus filtering efficiency) can be tuned by clay selection, percentage and type of organic combustible and firing temperature. The fired ceramic is also an ideal carrier to host colloidal silver. When optimized, this forces bacteria on a torturous journey that ends in their demise! The result was "Potters Without Borders" and their own journey to facilitate the research, development and commercial application of CWF technology that permits people in developing countries to draw water from unreliable sources. As master organizers and prioritizers, they have enabled factories in Africa, even under sub-optimal conditions, to create teams capable of commercializing the production of these in factories or by local potters. And they have standardized testing to measure capacity and performance. It has been a David and Goliath story of being listened to above the noise from multinationals.

So, what about the clay? Is it special? No. A wide range of clays can be made to work. As noted, they do testing to determine how much combustible to add (typically 40%), what temperature to fire (typically 850-925C) and how well colloidal silver is absorbed. Target performance is 1-3 liters per hour.

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.