Clays and Glazes

Clays

Electric Kiln Clays

I use two purchased clays a grogged terracotta from The Clay Cellar and a toasted stoneware currently from Potterycrafts.

Very wet recycled toasted stoneware clay drying in front of the fan heater

The grogged terracotta is grogged with sand and tolerates firing to 1260C. It is therefore a dual purpose clay and I do sometimes make plantpots for my own use fired to 1050C. I usually fire it to 1240C and glaze it with my chun glaze. The underlying maroon clay gives, what is essentially a white glaze, a pronounced bluish cast which is attractive and popular. The maroon colour is also attractive when unglazed. Most clays are rather lifeless and dull when fired in an electric kiln.

The toasted stoneware contains a certain amount of iron. This burns through on edges with some of my glazes and enhances impressed or incised decoration. Unglazed areas take on a nice orange/brown colour if the glaze gives off sodium at high temperature. To achieve this effect I either base the glaze recipe on Nepheline Syenite or add a small percentage (<5% by weight) of sodium bicarbonate to the glaze.

I also occasionally use a clay I dig myself from my sister-in-laws garden in Kent. It is pretty rough and ready and contains bits of rock, root and vegetation and chunks of iron ochre. The amount of ochre in each batch is quite variable and I never quite know what colour it will fire – anything from white to maroon. I only use this for my experimental explorations or to make items for my sister-in-law and her family. Sometimes when I feel the purchased clays are too refined for the work I am making I’ll mix in some of this clay to add some life and unpredictability.

I also make other additions to the purchased clays. Currently I am experimenting with volcanic sands, biscuit fired granules of earthenware clays and tiny glass beads. These melt at stoneware temperatures and interact with the glazes to give interesting results.

 

Wood fired kiln clays

I am still experimenting with clays to use in the wood-fired kiln. Although my target top-temperature is the same as I use in the electric kiln, the capricious nature of a wood firing tends to result in some parts of the kiln getting much hotter than the target temperature. I am therefore evaluating a number of clays which can tolerate 1280C. My favourite to-date is Valentines B17C white stoneware which has not warped even in the hottest parts of the kiln, is a superb throwing clay for small to medium (30cm high) pots, gives a very clear colour response with slips and glazes and can be made to look like other types of clay by coating it in a slip of that clay.

 

Glazes

I experiment with glazes all the time and have far too many buckets of glaze to be sensible. I’ll just list a couple of recipes here but am happy to take requests for more. All but one of my glazes is self formulated from raw materials which helps to keep costs down. I don’t use lead or barium so they are all non toxic. The one glaze I don’t make myself is sold as a Turquoise Raku glaze and for several years I enjoyed making Raku pottery in an Ian Gregory style flat pack ceramic fibre kiln. I now use the glaze for decorating the outside of pieces at stoneware temperatures. It significantly adds to the flux content in the area where it is applied and causes an area of glaze which moves on the surface of the pot, running down vertical surfaces or pooling in hollows.

Recipes

FFF Chun – Glossy, mottled, semi-transparent white. Pale blue over red clay.

Feldspar 260
Blue Ball Clay 60
Whiting 230
Calcium Borate Frit 80
Bone Ash 50
Quartz 330
Titanium Dioxide 10
Red Iron Oxide 10

Reactive Iron Slip – this tends to burn through overglazes giving a purple/red/brown colour. It can be changed to an orange slip by doubling the amount of Titanium Dioxide.

Soda Feldspar 1400
Blue Ball Clay 1200
Whiting 600
Calcium Borate Frit 400
Bone Ash 400
Titanium Dioxide 80
Crocus Martis Iron 800

 

Posted February 3, 2012 by damiankeefe

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