Clear glazes - Matthew Blakely
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Most of the glazes from the celadon biaxial will prove good starting points for developing a good clear stoneware glaze. The key glaze qualities that your ideal glaze will have depend on your personal taste or the effects you are after, but the general requirements for a clear glaze are:
- Glaze to be fully matured.
- Broad maturation range to give consistancy.
- Clarity not too dependant on glaze thickness (though clear glazes are always applied thinner than celadons).
The glaze needs to be fully matured so that it is fully melted at top temperature, and should not be on the borderline of the eutectic, where crystallization (and therefore opacity) will develop on cooling. Looking at the celadon biaxial, glazes around the centre of the grid will probably provide the best starting points for a standard glaze. Glazes at corner C are higher in fluxes and will be slightly runnier and glassier. These clear glazes can be nice and bright, but can cause underglaze decoration to blur as they move on the pot. If the glaze is too high in fluxes, it may well partially crystallize on cooling causing mattness and opacity. Glazes on the A-C line are lower in silica and again give good bright clear glazes, though they are likely to craze. Glazes around corner B are high in alumina and silica, are therefore stiffer and tend to produce static well behaved clear glazes that do not cause underglaze decoration to run. Standard industrial glazes fall in this category. They are reliable and durable but can be criticised as being dull and uninteresting.
It is essential that the glaze is fully matured to produce a clear glass. The rich 'smoky' look of many celadons is caused by millions of tiny bubbles trapped in the thick glaze, giving a semi-opacity. As the temperature increases (or the glaze becomes more matured) the bubbles escape the glaze leaving a clear glass. This is more pronounced when the glaze is applied thinner - clear glazes should always be applied thinner than coloured glazes. To assist with the maturity of the glaze it could either be fired to a higher temperature, a cone say, or a low temperature flux can be added. Adding 5% Borax Frit to these glazes will greatly improve their maturity and clarity, and has the added advantage of broadening the maturation range so it should be clear in all parts of the kiln.
Looking agian at the celadon biaxial, you will be able to see (depending of course on what clay/s you have put the glaze tests on) that as the glazes increase in alumina (heading towards line A-B) and silica (heading towards line B-D) their crazing decreases. A simple way to decrease a glaze's propensity to craze is to add silica (flint) and/or alumina (clay) to it, though it must be said there is a limit to this. But, you must note that as you are adding flint and clay you are heading into corner B and beyond, where the glaze becomes increasingly stiff and bland.
The other way of reducing crazing in a glaze is to change the ratio of fluxes in the glaze. Some fluxes produce a glass that shrinks a lot on cooling and will therefore craze readily eg. potassium and sodium. Other fluxes produce glasses that shrink a lot less on cooling and reduce crazing eg. lithium, magnesium and to a lesser extent calcium.
Looking at the Seger Formula for the celadon biaxial - the flux ratio is Calcium 0.7, potassium/sodium 0.2, magnesium 0.1. To reduce crazing the glaze you are after could be recalculated by substituting some of one flux for another. For example, magnesium could be increased to 0.2 and calcium or potassium dropped by 0.1 part. Another way would be to intruduce a new low shrinkage flux, such as lithium (in the form of petalite or spodumene) to replace some of the potassium sodium eg. lithium 0.1, potassium/sodium 0.1, magnesium 0.1, calcium 0.7.
Either choose a glaze from the biaxial that you like and add 5% Borax Frit to it and retest, or the biaxial could be repeated with a 5% addition of Borax Frit to all glazes. If your chosen glaze crazes on the clay you are using and you don't want it to, then you will need to do further tests - increasing silica and/or alumina in lineblends, or recalculating the glaze with slightly different flux ratios. It is important to note that additions of silica and alumina, or the intoduction of other fluxes will eventually change the quality of the starting glaze, but it is often possible to eliminate crazing with relatively small additions or changes so the glaze quality does not change.
A good clear stoneware glaze, with relatively good glaze fit, for cone 9 - 10 is: