These are table salt crystals on a 60 mesh sieve. It has an opening of 250 micro meters (or microns). Half of the crystals passed this sieve, half are retained. Notice on the right, several crystals are in the openings, about to fall through. Imagine that an particle (or crystal) of bentonite or ball clay can be sub-micron in diameter, they can actually be 2500 times smaller on a side than these salt crystals! One-tenth-micron ultimate particles would thus fit 2500x2500 on the flat side of a salt crystal. And, since the clay crystal is much thinner than wide, perhaps ten could stack to the same dimension. That means theoretically 2500x2500x25000 (or 1 with eleven zeros) could pack into a grain of salt!

The body is Plainsman L215. Both were thinly applied and fired using the 04DSDH schedule. The glaze has 2% iron oxide added and sieved to 80 mesh, this reddens the color and acts as a fining agent to reduce micro-bubble population. The one fired to cone 03 (left) is considerably stronger, better surviving the stress of successive impacts with a hammer. However, it has minute dimples in the surface, likely because it is having to clear bubbles originating from decompositions occurring in the body below (as it fires well above bisque temperature). The mug on the right fired about 40F cooler, between cone 04 and 05, only slightly above bisque. The glaze surface is much better, almost crystal clear. Since the glaze fits well the mug has surprising strength, much better than a stoneware piece with a poorly fitted glaze that shatters with one tap of a hammer. This one survived about ten whacks before a piece broke out! A big advantage of cone 04 and cooler is that ware can be fired on stilts, meaning you can glaze the whole thing, no bare clay is exposed. Again, I can only achieve this kind of glaze surface using the above-mentioned firing schedule.

Iron oxide is an amazing glaze addition in reduction. Here, I have added it to the G1947U transparent base. It produces green celadons at low percentages. Still transparent where thin, 5% is producing an amber glass (and the iron is showing its fluxing power). 7% brings opacity and tiny crystals are developing. By 9% color is black where thick, at 11% where thin or thick - this is “tenmoku territory”. 13% has moved it to an iron crystal (what some would call Tenmoku Gold), 17% is almost metallic. Past that, iron crystals are growing atop others. These samples were cooled naturally in a large reduction kiln, the crystallization mechanism would be much heavier if it were cooled more slowly.

Tin is super expensive, so how much should you use in a clear glaze to get a white? It is a trade-off of cost and whiteness. Nothing else can make a glaze this white and opaque and these low percentages. Consider this: A tin-opacified glaze may only need to be half as thick as a zircon opacified one. Tin has other advantages over Zircopax. First, the percentage required could be half or one third. That takes us down to four or six times less tin being needed (Zircopax is five times less expensive at time of writing). These two factors mean thermal expansion mis-fits between body and glaze start off four or six times less likely to produce shivering or crazing! And tin affects the melt fluidity and thermal expansion half as much. On these samples, the higher percentage of tin seems to produce an even better glossy surface. Crawling is a classic issue with high-zircon glazes (because it impedes melt fluidity, that is what holds super thickly applied majolica glazes on the ware). Tin is the opposite; even though this recipe is high in strontium, and thus has a high surface tension, there is no indication of crawling with the tin addition. A final issue is cutlery marking, a common problem with zircon-opacified glazes. But not with tin oxide.

This was a fast firing. The glaze is G2934, a silky matte. But that does not mean it is pinhole-prone, it has good melt mobility. The clay on the right is Plainsman Coffee Clay. It contains 10% raw umber, that generates plenty of gases during firing. The centre one is Plainsman M390, not normally difficult to fire defect-free. The left one, M332, should be the worst, but is the best! What is needed to fire these without pinholes? The drop-and-hold and slow-cool C6DHSC firing schedule. It is extra effort to program your kiln controller, but well worth it. If you don't have a kiln controller then by a little experimentation you can develop a switching pattern to produce the same effect.

The firing crack from the rim down has released the stresses produced by uneven thermal contraction during cool-down in the kiln. Any factor that contributes to a temperature gradient within a piece will contribute to the likelihood of dunting. Cooling too quickly through quartz inversion, for example, can cause this in almost any piece. Pieces that are thick and heavy, or have uneven cross section (with thick foot and thin walls, for example) will certainly suffer gradients, even in slow cooling. A wide, flat bottom (that is heat-sunk by the a heavy shelf) will also increase the temperature gradient between the outer walls and the inner foot. If that wide piece has vertical walls that get direct radiant heat, especially if one part is more exposed to the elements, it will start a gradient during the up-ramp in the firing. And, on the down-ramp, it will "come back to bite you" with a crack.

Using my account at Insight-live.com I calculated a frit-based recipe having an "evolved" chemistry from the original G1947U feldspar-based one. Only after seeing the fired results did I fully realize I made a discovery as well as an improvement. My original approach was just theoretical: Shift KNaO-sourcing from feldspar to frit to get a better melt (just because the frit is a premelted source of KNaO). As calculations took shape it became clear that I could increase KNaO (it is a super-flux for cone 10 brilliant surfaces) because of the multiple options to counterbalance its high thermal expansion. Those options would theoretically supercharge melting more, that gave me confidence the melt could even dissolve additional SiO2 (which would improve durability). When the kiln opened I got the surprise with the original G1947U: It never looked white before! But when seeing it this thick in comparison to the improved version, it looks really cloudy. Why? Likely the melt is not completely dissolving the particles of quartz! The "lead glaze surface brilliance" of the new G3910 blew me away at first, but now that I realize it is also melting all the silica I see how much better it potentially is. One issue: The transparency of G3910 brings with it the amber color of the body:glaze interface.

Here is an example of our lab firing schedule for cone 10 oxidation (which the cone-fire mode does not do correctly). To actually go to cone 10 we need to manually create a program that fires higher than the built in cone-fire one. Determining how high to go is a matter repeated firings verified using a self supporting cone (regular cones are not accurate). In our lab we keep notes in the schedule record in our account at insight-live.com. And we have a chart on the wall showing the latest temperature for each of the cones we fire to. What about cone 6? Controllers fire it to 2235, we put down a cone at 2200!

On the right is the G2934Y matte base recipe with only 8% Cerdec Orange encapsulated stain. G2934Y employs a frit-source for the MgO (as opposed to G2934 which sources the MgO from dolomite). If this was a glossy glaze the required percentage of stain would be higher. Other colors (like yellow, red, blue, black), are equally vibrant. But not all (e.g. purple), testing is needed. The porcelain is Plainsman Polar Ice.

Left is Plainsman M340. Right is M390. Each mug has been white-engobed inside and half-way down the outside. The insides have been glazed using G2926B clear. The inside surface has more depth and has a richer appearance than could be achieved using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6-B. The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and striking visual contrast and character on the outside of the dark body mug.