Tech-Tips

This is based on one of our most popular base glaze recipes, G3806C. This version, G3806E, has 4% added copper oxide. The piece is made from Polar Ice casting. The glaze is designed to have a fluid melt (almost enough to run off the ware if too thick) but enough to produce a super-gloss brilliant surface. It solves a problem with pretty well every other similar glaze you will find: crazing. It is difficult to produce a super-gloss surface yet still have a low thermal expansion. This is still under development, if you want to follow our work on this project click the link.

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.

G3806C has been our recommended base recipe for reactive glazes (by the addition of colorants and opacifiers). It excels for copper blues, for example. But its thermal expansion is high enough that it crazes on some bodies (e.g. Plainsman P300). To adjust it (via glaze chemistry) I introduced some low expansion Li2O (from Spodumene) at the expense of high expansion KNaO, this dropped the calculated COE from 7.1 to 6.6. The melt fluidity, shown here at cone 6 (its most important feature), is exactly the same. The color is bluer. But not as dark, so copper oxide might be better. Or a higher percentage of copper carbonate. The base recipe (without the copper and tin) is potentially very valuable to create other reactive effects that depend on melt mobility. Why? Because it is very difficult to create a high gloss melt fluid glaze that also has a low thermal expansion.

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.

These are made from a 50:50 mix of bentonite and ball clay! The drying shrinkage is 14%, more than double that of normal pottery clay. It should be impossible to dry them, the most bentonite bodies can normally tolerate is 5%. Yet notice that the handle joins with the walls are flawless, not even a hairline crack (but the base has cracked a little). Remember that the better the mixing and wedging, the smaller the piece, the thinner the walls, the better the joins, the more even the water content is throughout the piece during the entire drying cycle and the more damp of a climate you live in the better your drying success will be. What did it take to dry these: 1 month under cloth and plastic! I changed the cloth every couple of days. So by implementing these same principles you will have better drying success.

Both of these were glazed by brushing. The inside transparent and white glazes are fairly easy to apply evenly but the bright color on the outside left one certainly is not. The problem is a combination of things. It is difficult to apply it evenly with a brush. It is difficult to get it on thick enough. And this commercial glaze does not contain enough of the purple stain (so I added 6 grams of Mason 6304 Violet stain powder to the 2/3 of a jar I had left, and mixed thoroughly). That, more careful brushing, and an extra layer produced the piece on the right!

This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.

Fire-Red is an unusual material for several reasons. It has a high iron content yet is a fireclay (the iron percentage is so high that it fires black at cone 10R). It is also non-plastic. Most important, it is not ground to 200 mesh like industrial materials. This body demonstrates it well: 42.5% Fire-Red, 42.5% ball clay and 15% feldspar. All that ball clay gives it awesome plasticity. The feldspar gives control of the degree of vitrification (just raise or lower it for more or less). This recipe produces good density and strength yet still exhibits deep red color! Look closely at the surface: It is covered by thousands of tiny iron eruptions, these will bleed through an over glaze to give "speck-city" like no other!

Orange is a very difficult color in ceramics. Inclusion stains are the only reliable method and universally used in industry. But you could ignore that and try a bunch of recipes online, buying exotic materials to complete each one. Maybe one will be orange enough, but will it craze or run or blister or leach or cutlery mark or crawl? Or you could put an orange stain into a transparent glaze you already know works on your clay. Or, how about trying a premixed orange at low fire? Ware can be amazingly functional and there are so many other bright colours available.