Monday, January 19, 2009
ABOUT TILES AND GLAZES by Peter Gregory
In 2006 I was commissioned to produce handmade glazed tiles that were to make a splash back behind a sink bench and electric stove. The tiles were for a new building that is located in Central Otago.
The property is in a valley that runs like a trough between two waves in the sea. Mountains, with a cap of snow, are behind the nearer hills, the sky seems vast, and the silence can frighten some people. Underfoot the ground is stony, hard, and usually dry. Wind weathered formations of schist rock make abstract fists and figures on hillsides.
The landscape, and the sky inspired the glazes. Where possible, I gathered actual rock and wood ash from the property to supplement commercially obtained glaze materials.
I made the tiles out of Southstone clay from Southern Clays in Dunedin. This clay is quick drying due to its open, sandy nature, a characteristic very helpful for tiles, which can be problematic to dry properly at the best of times.
The tiles had to be easy to keep clean, but my client gave me a generously free hand as to design and colour.
One evening, my client presented me with a box full of leaves that he had collected on the property. I pressed these into some of the tiles to form textures and patterns. Later, after bisque firing, I used some of the leaf tiles as stamps, and pressed “positive” leaf pattens from them, which were highly successful and had amazing detail.
I took samples of schist and other rock from the property, samples of wood ash from the wood burning stove, and also wood ash from a bonfire which had been used to burn weeds. The stove ash was predominantly poplar and was well burnt without much charcoal or other contamination. The weed ash was full of all sorts of things including garden soil, and had a great deal of charcoal.
I was particularly interested in the schist rock. Schist looked to me like layer after layer of dry muddy river bed pressed together in thin layers. You can snap and crumble schist easily and dustily with your bare hands.
Materials for the New Zealand Potter by J. C. Schofield (Published 1977 by E. C. Keating, Government Printer) contains an analysis of schist rock that had been taken not far from my client's property.
The analysis of the schist was hauntingly similar to that of ball clay and china clay (In fact I later read that schist is fossilized clay).
I also noted high levels of Titanium were recorded in the sample taken near my client's property. Titanium is a material that can sometimes add an interesting opalescence to glazes.
Clay is often used in glazes to add silica, the glass former, and alumina, the stabilizer, to the glaze. I speculated that I might be able to replace all or most of the clay content of a glaze recipe with schist.
The schist had to be ground up somehow. I heated the schist to 700 degrees Centigrade in my electric kiln. Once cool, I could laboriously reduce the schist to powder with a mortar and pestle. As an experiment, I wrapped some heat treated schist in canvas and carefully wound it through my etching press. With a satisfactory crunching sound, the press rapidly reduced the heat treated schist to a fine powder that I was then able put through a fairly coarse sieve and then add to my glazes.
Wood ash can act as a glaze in its own right. In addition to silica, the glass former, wood ash contains several fluxes, typically: calcium, potash, magnesia, phosphorus, and soda. Wood ash is usually low in alumina, so a pure wood ash glaze at stoneware temperatures, will merrily flow down the side of a pot with stringy run marks. A far more useful glaze can be made with a mixture of wood ash and clay (clay supplying both silica and alumina).
Some of the tiles used in the splash back were glazed with a 50/50 by weight blend of poplar ash and schist. At nearly 1300 degrees centigrade this made a pleasing satin glaze with a red/brown colour and granite-like appearance. I did glaze one or two tiles with a simple glaze made from the very contaminated bonfire ash and feldspar. The bonfire ash had a strong tendency to turn everything into a orange/brown, bubbly, gloop. With more time I might have found a use for it, but I mostly ended up using the stove ash, as the pure poplar worked well.
A great number of glaze tests were done. In addition to making glazes from natural substances from my client's property, I had to formulate reliable clear glazes to go over the leaf patterned tiles without obscuring the delicate detail. Also I wanted to develop some opalescent glazes to give delicate blues and grays without the use of cobalt or blue stains. Such glazes reflect back the short wave light and absorb long wave light. They are rather like the lovely opalescent blue atmosphere of our planet.
I wanted to make glazes like that to say something about snow, water, and sky.
The idea of such glazes is not new, Chinese potters were making them hundreds of years ago. They are called Chϋn glazes. I had a feeling that a small quantity of schist either in the glaze or under the glaze could assist the development of a glaze of this sort, due to the titanium content.
Most of the tiles have a coating of slip over them. You could think of clay being skin, slip being underwear, and glaze as being the clothes on top.
Slip was used to modify the colour and texture of the clay, and to give extra complexity to the appearance of the glaze that went over it. Most of the slip I made for the tiles contained schist, often up to 55 percent, the rest was usually clay. Sometimes iron oxide or copper carbonate was added for extra colour.
In the case of the leaf tiles, I used a dark slip rubbed into the leaf pattern to make the pattern more visible.
The schist slip dramatically promoted the formation of fine yellow pollen-like crystals in a number of glazes that were applied over the top, schist slip also appeared to have a slight fluxing effect on glazes. When put under a Chϋn glaze, schist slip deepened and strengthened the blue colour of the Chϋn glaze. It was possible to paint schist slip pattens, and have them show up clearly as dark blue in the pale blue/white Chϋn glaze that covered the tile.
A starting point for a stoneware temperature schist slip for leather hard clay was schist 55, ball clay 45. Sometimes I added up to 10 percent potash feldspar to assist the slip to make a good bond with the clay. For applying to dry clay I would substitute half the ball clay for china clay, and consider calcining the ball clay if the slip was shrinking too much.
Some glazes began by just simply trying two or three likely ingredients in about equal proportions, then further developing the glazes according to how they performed in initial testing. I also tested and modified glazes that appeared as recipes in books by potters, Emmanuel Cooper, and Janet DeBoos. I modified them by substituting schist for clay and wood ash for fluxes such as calcite and got some interesting and useful results.
The commission proved to be quite challenging and highly educational. It was a good opportunity to test lots of glazes, and to move outside the comfort zone of using commercially obtained raw ingredients. Schist is a very interesting material, and I will certainly use it in glazes and slips. Some of the modifications to DeBoos glazes have been of use in glazes I use every day.
You may notice small animal footprints in some of the tiles. These were obtained from my client's elderly, and much loved, dog.