Pottery: Firing, Glazing and Making Clay Plates and Dishes
Earthenware These clays have a range of colors but are porous when fired.
Stoneware This fires to a hard, non-porous texture.
Porcelain Difficult to use because of their poor plasticity anbd the high temperatures needed to firem them, porcelain clays fire to a translucent white.
Self-hardening Beginners can start with this natural clay because it needs no firing; a glaze or varnish will make it non-porous, but it is not suitable for vessels intended to hold liquids.
Preparing and modeling the clay
Pottery clay can be bought wet or dry from crafts shops. The less bulky dry clay must be mixed and sprinkled with water to make it workable, then allowed to sit, in a sealed container or wrapped in plastic for a few days.
Add plenty of water to wet clay to break it down and then mix it well to remove any lumps. Let the clay sit for several days before draining off excess water. The remaining clay must dry out until it is firm enough to use.
Clay must always be stored in an airtight container. Put a damp towel in with it to stop it drying out completely.
You will need a few modeling tools, a large sponge or cloth for mopping up, and a board to work on. Have a plant spray or bowl of water ready to damp the clay as you work. Make your own wedging wire for cutting the clay, by attaching metal wire or strong nylon to two wooden handles.
Work on a very strong flat surface and place a board beneath the clay. ‘Wedge’ it by cutting the block in half, putting one half on top of the other and banging the two together. Do this until the clay is of even texture, cutting in different directions. Then knead the clay for at least ten minutes, until it is easy to handle and not at all sticky.
Simple pots or dishes can be fashioned by hand. The items made can be textured in various ways. Use everyday objects such as nuts, bolts or shells, or roll twigs or leaves onto the clay to create patterns.
If the clay has to be fired allow the pots to dry out thoroughly for at least a week beforehand. Self-hardening clay will harden fully after two days. Learn to make or ‘throw’ pots on a wheel at evening class before buying equipment.
Firing
Clay is fired at temperatures usually exceeding 1090 C (2000F) to make it rigid and non-porous. This requires either an electric or fuel burning kiln: new ones are expensive but second hand kilns are advertised in specialist pottery magazines.
If you consider buying a kiln, make sure the manufacturer is reliable and accessible for servicing and parts. Check an electric kiln’s capacity to avoid over loading your domestic system.
Most fuel burning kilns use natural gas, but some burn oil, coke or wood.
Clay is fired in two stages : biscuit firing, then glaze firing.
Biscuit
This is done at relatively low temperatures, usually 890-900C (1634-1652F), and stiffens the clay, making it msafer to handle and ready to absorb glaze. The temperature has to be brought up slowly to allow any moisture that remains in the clay to evaporate gradually, rather than as an explosive steam which could crack the pot. Cooling must also be gradual.
Pots can be stacked on top of each other for biscuit firing, but should stand at least 6mm (�¼ inch) apart for glaze firing.
Glaze
A variety of ‘glazing’ materials is avavilable from craft shops – follow the maker’s instructions. low firing glazes come in a greater range of colors than those for high firing temperatures, but are softer and more liable to scratch.
Before glazing wipe any dust off the biscuit fired pots with a damp sponge or cloth. Apply the glaze either by pouring it into the pot and rotating it until all areas are covered or by dipping the pot in glaze and shaking off the excess. Use a brush to touch up the patches where you were holding the pot. Do not glaze the underside of a piece, or it will stick to the kiln shelf.
Do not touch a piece after glazing. Let the glaze dry completely, then fire it again, at the temperature specified for the glaze you are using.
