The mug on the left, made by Holly McKeen, is a typical cone 10 Grolleg kaolin mullite porcelain (highly vitrified, low in residual quartz). Its glaze is crazed. Crystalline glazes are high in Na2O, making crazing virtually certain. Since most pieces are decorative, crystal glazers just accept this as part of the process. But these are functional mugs, the glaze needs to fit (if only for ware strength).

But what if the thermal expansion of the body could be significantly raised? The body on the right is Crystal Ice, it contains 40% silica (vs 20-25% in a typical porcelain). The percentage of Nepheline has been reduced, lowering vitrification to about 1.5% porosity. As a result, more quartz survives undissolved and less mullite develops, raising the body’s thermal expansion. The result is a body with a much higher thermal expansion, so it can not only relieve the glaze tension but actually put a squeeze on it. There is a downside: These are less resistant to dunting and thermal shock failure during use.

Could the glaze be adjusted instead? Yes. Some of the Na2O could be substituted for Li2O, the latter is also a strong melter but has a much lower thermal expansion. Glaze chemistry could be used to source it from Spodumene (to avoid solubility issues with lithium carbonate). However, zinc-silicate crystalline glazes are very sensitive systems, so the more lithia is introduced the more likely the effect on the firing window, crystal size/density and background clarity.

Crazing is one of the most common glaze defects. AI image generators can produce this really convincing photo, but AI explanations often still recycle oversimplified glaze-fit advice from the web. Let's work in reverse to see why using this speckled stoneware, it has lots of ball clay and quartz, it is easy to fit glazes to. What would it take to craze the glaze on the right? A lot. Glazes that craze out of the kiln on quartz-rich bodies are not "slightly misfit"; they are "hugely misfit". Under or over-firing, or holding less time at temperature, would not be enough to craze it. Reducing the silica enough to start severe crazing like this would fundamentally alter the glaze character and functionality.

This is not a recipe-level problem (e.g. reducing feldspar for silica or zinc). This is a material problem; it is an oxide chemistry issue. By far the best way to put the glaze under tension, to craze it, is to trade low thermal expansion oxides (not materials) for high ones. In this case, shift some of the flux unity away from CaO/MgO and toward KNaO (the latter being the highest thermal expansion oxides, by far).

All are fluxes and this is a transparent, so minimal change it character should occur.

When you learn to make and use engobes correctly, they make magic possible. Here I am turning a dark rustic body into a smooth white one (rear mugs) and a white body into a dark one (front). The engobes have been applied at the leather-hard stage. That is the perfect time, the engobe and body are clay bodies, designed to fit each other; they dry together and fire together creating an inseparable bond.

Handles have been applied, and they have dried to stiff leather hard. Engobe was poured in, poured out, then the mugs were pressed, lip down, into it and extracted. No dwell time was needed. This dipping engobe is DIY thixotropic (not available commercially anywhere). That means I tuned it just before use, to just the right degree of gel (enough for it to drain to the right thickness, then gel just as the last few drops fall from the rim). Honestly, these are a beauty to behold at this stage, the silky, drip-free surface is just so perfect.

Left is G3933A, it is an 80:20 mix of our matte and glossy cone 6 base recipes (plus a mix of iron oxide, tin oxide and rutile). The body is Plainsman Coffee Clay. Because of repeated issues with crawling a project was started to create the same effect using Alberta Slip to supply as much of the chemistry as possible. Along that road, the opportunity arose to add lithium (to duplicate Amaco PC-32, a classic Albany/Lithium recipe). That is the glaze on the mug on the right, G3933G1, it has 6% lithium carbonate. Lithium is a super powerful melter, turning this into a very reactive glaze! To make a 500ml jar of brushing glaze, in 2023, required about $7 worth of lithium carbonate.

These SHAB test fired bars are 95% nepheline syenite (5% Veegum added). By cone 02 (bar stamped #4) is self-glazing and glass-like with a total shrinkage (plastic to fired) of 15% (less than some porcelains). At cone 03 (the #5 bar) the porosity is 3% (a stoneware). This is not an absolute indication of the materials' melting profile because of the Veegum, it behaves as a powerful flux and melting catalyst.

The Blue Mountain nepheline syenite deposit in Havelock, Ontario, is a major, high-purity industrial mineral source mined since 1955 for glass, ceramics, and filler applications. This 99% pure, iron-poor deposit consists of albite, microcline, and nepheline. It has less than 0.1% free SiO2 and Fe2O3! The deposit is approximately 400 feet deep.

This is Plainsman Buffstone with G2931L glaze fired at cone 06. A hotter bisque not only produces a stronger body but also eliminates crazing (these specimens were glaze-fired one month ago). Firing the bisque just one cone hotter has made the ceramic into a denser matrix having a higher thermal expansion. That has the power to put the squeeze on the glaze, preventing it from crazing. Hotter bisque temperatures can be problematic for dipping glazes (they reduce bisque absorbency and lengthen dip and drying times). But for low-temperature hobby ware, this is not as much of a problem since glazes are gummed and multi-coated (with air drying between each).

Note that bisquing higher has no effect on glaze fit for stoneware, since the glaze firing is done at a far higher temperature.

This cone 04 flow tester compares two commercial low-fire transparent glazes. Their different chemistry strategies are revealed by the shape of these melt flows. While 3825B appears to have the higher melt fluidity, it also has much higher surface tension. This is evident in the narrow, rope-like stream and the way the flow meets the runway at a high angle before pulling into a rounded bead. A, by contrast, spreads and wets the runway, meandering downward in a broad, flat and relatively bubble-free river.

This difference is important in low-fire ware because these glazes must pass far more gases and bubbles than high-temperature glazes. The lower surface tension of A aids bubble release and healing after bubbles break. A is Amaco LG-10. B is Crysanthos SG213 (Spectrum 700 behaves similarly, although flowing less). Both approaches have advantages and disadvantages and are worth testing in your application.

The left two leather-hard mugs were made from a 100% Lincoln 60 Fireclay (from Gladding McBean). By itself, the clay matures into a stoneware at around cone 8. While the pure material has a pleasant, smooth, soapy feel and can be thrown on the wheel, the plasticity is lower than that of typical pottery clay. The mug on the right adds 2% bentonite. That simple addition transforms it into a delight to throw! And only increases the drying shrinkage by about 0.5%.

Numbers on data sheets simply do not convey the difference the bentonite makes. But an experienced potter can feel it immediately. That makes a potters wheel (and throwing experience) a valuable laboratory testing instrument for a comparative assessment like this. There is no absolute measure for plasticity, so we most often simply say that one body is more or less plastic than another.

The white slip on the left, L3685Z2, (applied to a leather hard cup) is dripping downward from the rim (even though it was held upside down for a couple of minutes!). Yet that slurry was very viscous with a 1.48 specific gravity. Why? Because it was not thixotropic. The fix? I watered it down to 1.46 (making it runny) and added pinches of powdered Epsom salts (while mixing vigorously) until it thickened enough to stop motion in about 1-2 seconds on mixer shut-off. But that stop-motion is followed by a bounce-back. That is the thixotropy. It is easy to overdo the Epsom salts (gelling it too much), I add a drop or two of Darvan to rethin it if needed. When the engobe is right, it gels after about 10 seconds of sitting, so I can stir it, dip and extract the mug, shake to drain it and then it gels and holds in place. Keep in mind, this is a pottery project. In industry, they deflocculate engobes to reduce water content and then impose thixotropy, but that is more technical than the average potter would want.

This is a variation on the 50:30:20 cone 6 very fluid-melt pottery glaze recipe. I reduced the Gillespie Borate (GB) to 37% instead of the original 50% (thus bringing the B2O3 from 0.63 down to 0.5). My objective was to reduce the melt fluidity. But the crawling was so bad in this that it is almost unusable. The reason was not obvious until I fired a sample to 1550F and 1650F. At the former, the integrity of the glaze layer is great, but by 1650F it melts suddenly and does this. It is not difficult to see why these “puzzle pieces” with curled up edges might pull inward to create "glaze islands" characteristic of glaze crawling. This is happening even though the percentage of Gillespie Borate is lower. Not surprisingly, Ulexite mineral, which GB almost certainly contains, is also known for suddenly shrinking and melting.

I tried to solve another problem at the same time. GB is plastic on its own, and thus hardens the layer and suspends slurries well. Thus, the 15% kaolin in the recipe unnecessarily increases the drying shrinkage. So I substituted calcined kaolin. While it helped with that problem that was small consolation.