Celadon glazes
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Blue celadon

A wide range of glazes, with varying fluxes and alumina and silica levels, will give a blue celadon type glaze in reduction firing. The most important factor in promoting the blue colour is purity of materials, specifically very low levels of titania. Most materials have significant impurity levels. In Australia the problem was getting hold of a clay with low titanium content. Here in the UK tha china clays are low in titanium so are perfect for use in a blue celadon glaze. However, I have found that silica in the form of quartz seems to be rather coarsely ground and impure. If flint is used as the silica source good blues can be obtained. I have also found quite large variation in the purity of talcs available. It is essential that all materials used are extrememly low in contaminants.

A celadon glaze is quite a simple glaze, though difficult to get a particularly good one. The application of the glaze is important - it has to be quite thick to get the right effect. Essentially celadon glazes are thick, semi-transparent glazes, fluxed mainly by Feldspar and calcium (whiting) and coloured by small percentages of iron. The semi-transparency is created by millions of microscopic bubbles remaining trapped within the glaze. This is responsible for the jade like quality that was so prized by the Chinese.

Traditionally celadon glazes had a satin or slightly textured surface. This was acheived by underfiring the glazes slightly. To acheive this effect, you would need to fire a glaze to 2 cones or so below its maturation temperature (or the temperature at which the glaze becomes clear) and held at that temperature in the kiln (soaked) for at least 2 hours. The lower the temperature and the longer the firing the better the effect. Test rings would need to be used to assess glaze development.

My own blue glazes are not traditional. They are fired high and are quite fluid, bubbles only being present in the thicker runs of glaze.

The best way to develop a good celadon glaze is to make up a biaxial blend using the Seger formula,with the flux equivalents KNa 0.2, Ca 0.7, Mg 0.1. This is a basic feldpsr/whiting glaze type, with a little magnesium to help smooth the melt and decrease crazing. 0.5% red iron oxide should be added to the glazes.

 
A   B
  5 x 5 glazes  
C   D

The corners A,B,C,D having the following alumina and silica levels-

A: alumina 0.5, silica 3.4

B: alumina 0.5, silica 5.0

C: alumina 0.34, silica 2.4

D: alumina 0.34, silica 3.4

 

Unsure of what the Seger Formula is? Click here for A,B,C,D recipes.

It's important to note that the corners of this biaxial have the following properties:

A - High alumina, low silica.

B - High alumina, high silica.

C - High flux.

D - High silica, low alumina.

 

The corner glazes should be seived through at least 120# seive, preferably 200#. Once blended they should be applied thickly to test tiles made from the clay you are going to use. Mark the test tiles with ceramic pencil or iron oxide brushwork so that you know which is which.

The results will show a variety of pale blue glazes with varying texture, gloss and transparency when fired to cone 9-10. Glazes at corner C will be increasingly fluid and may show some crystals at cone 9. Increasing silica to corner D will stiffen and 'fatten' the glaze up. As alumina and silica are increased from corner C to corners A and B the eutectic point is reached (the lowest temperature of glaze maturation - therefore a glaze with a wide firing temperature) and passed, crazing decrease and eventually the glazes become increasingly stiff and static. There are a couple of nice orange peel glazes on the A - C line.

Glazes should be seived through 120 or preferably 200 mesh lawns and fairly thickly applied, on porcelain to get the best blues. They need to be fired in reduction, but NOT necessarily strong reduction, as is often cited. In my experience a light but constant reduction will give good clean blues, anything further is a waste of fuel and unneccessarily polluting.

This biaxial is a good starting point as it leads to the development of a variety of glaze qualities and an understanding of the effect of varying a couple of parameters. One-off glaze tests are hit and miss. Doing a biaxial gives a spread of results, which can be easily understood and prove much more helpful eg. in finding a craze free glaze or particular surface texture.

A glaze that gives a good soft blue with a textured surface at cone 9, becoming smooth an glossy at cone 10 and above is:

Potassium feldspar 38.1, Whiting 20.2, Talc 3.8, China clay 19.0, Flint 28.9, Iron oxide 0.5 - 0.7

Green celadon

Take a glaze from the biaxial and add 0.2% titanium dioxide to turn the pale blue to a green. Alternatively recalculate the glaze with HVAR (or another) ball clay rather than the pure china clay. If the iron oxide content is increased above 1% the colour of the glaze will deepen and become increasingly green. Try a line blend from 1 to 3% iron oxide in half percent incements. With iron at 3% and above glazes tend to be increasingly 'dirty' green and olive in colour. Fluid glazes are often gorgeously bright emerald where thick but tending to brown where thin.