Tech-Tips

You found some recipes. Their photos looked great, you bought $500 of materials to try them, but none worked! Why? Consider these recipes. Many have 50+% feldspar/Cornwall/nepheline (with little dolomite or talc to counteract their high thermal expansion, they will craze). Many are high in Gerstley Borate (it will turn the slurry into a bucket of jelly, cause crawling). Others waste high percentages of expensive tin, lithium and cobalt in crappy base recipes. Metal carbonates in some encourage blistering. Some melt too much and run onto the kiln shelf. Some contain almost no clay (they will settle like a rock in the bucket). A better way? Find, or develop, fritted, stable base transparent glossy and matte base recipes that fit your body, have good slurry properties, resist leaching and cutlery marking. Identify the mechanisms (colorants, opacifiers and variegators) in a recipe you want to try and transplant these into your own base (or mix of bases). And use stains for color (instead of metal oxides).

2% zircon also was also added, it helps prevent micro-bubbling. The PLC6DS (drop and soak) firing schedule was used. The G3806 base clear glaze is normally better than G2926B for really bright colors but this stain is an exception.

If your reclaim is short and non-plastic you can make it better-than-new by using an additive of 50% ball clay and 50% bentonite. While only a few percent bentonite supercharges the plasticity of any clay body it is almost impossible to get it to mix into a wet slurry or plastic clay. But thoroughly shaking it together with ball clay (in a plastic bag) separates the super-tiny particles of bentonite between the almost-as-tiny particles of ball clay, that new powder will easily mix with water. And it fires to a tan-buff stoneware at cone 6 so it won't change the fired appearance of most buff or brown cone 6 stoneware bodies. There is one downside: I can leave a scum on your plaster batt if your bentonite is high in soluble salts, so test on a small bat first. Or dewater by another method. Or use a dedicated batt whose surface you can scrape periodically.

This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time. When ware is glazed inside and the compressive forces the glaze finds itself under keep it crack free and also significantly strengthens the piece (like pre-stressed concrete). But here there is no outside glaze to be counteract the inside one pushing outward. When suddenly heated it pushes even harder. Structural weak points, outside surface imperfections or pronounced contour or thickness changes provide crack-initiation-points to relieve the stress. The only way to make this inside-only-glazed technique work is carefully tuning the thermal expansion of the inside glaze. That means a lot of testing and a lot of broken pieces.

Four degrees F. I was consistently getting the cone on the left using a custom programmed firing schedule to 2204F. However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I changed the temperature to 2200F and got the cone on the right.

In 2015 we did a project comparing common cone 6 fluid-melt base glazes, picked a favourite (Panama Blue) and fixed it's slurry issues and crazing. Fluid-melts almost run off ware when applied thick, but they host stains & opacifiers to produce super-gloss, super-brights. That recipe, G3806C, has been among the most popular pages on our site. In 2019 we moved the thermal expansion much lower (from 7.3 to an incredible 5.7, it can survive a 325F-to-icewater test on our toughest-to-fit porcelain). The best variation, G3806N, is fluidity-controllable (by adjusting kaolin content), durable (double the Al2O3/SiO2 of other common ones) and employs lithium, strontium and magnesia frits. This version, G3806E, has 4% added copper oxide and sources SrO, Li2O and MgO from strontium carbonate, spodumene and Frit 3249. Follow the link here to see the entire history of this development effort (beware, there are multiple pages, each with many columns).

The cone 6 one is on the left, it contains about 25% frit. Both are colored using a black stain. That low fire glaze on the right has a high percentage of frit, likely more than 80%, that is the main reason for the beautiful surface. Frits are really fantastic, and standard practice in industry. However potters have been slow to adopt them, thinking they are more expensive. But from a "total cost" viewpoint, they are cheaper.

This was left for 24 hours. Wrapped in stretch wrap. Then the surface of the glaze was inspected under a lamp to detect any differences between the lemoned and non-lemoned surfaces. Lemons are highly acidic. This glaze passed because the base recipe, G3806N, was methodically developed so that it has plenty of Al2O3 and SiO2 (in the fired chemistry) to build a stable glass.

The body is Plainsman M390. These are commonly used base glazes. The top one is an MgO matte, next down is a calcium matte. They behave very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.

M370 is now our second-most-popular clay body. Are you curious of the recipe? It is really quite simple. It has enough silica to resist crazing. It has enough Nepheline Syenite to mature to 1% porosity at cone 6. The rest is a mix of #6Tile kaolin and M23 ball clay with a little added bentonite to push up plasticity.