Earthenware glazes are often borax based. Some earthenware glazes are lead based, but most potters using them these days would make them from a lead frit, which is lead that has been made safe to use by combining it with silica to form a glass, much like lead crystal used to make high quality wine glasses. This glass is then powdered, and is usually mixed with some clay and some water to make a glaze that can be applied to the pot.
Stoneware glazes are mostly made from powdered rock and clay. At 1300 degrees Centigrade the kiln is hot enough to melt rocks, such as basalt or feldspar, on their own; and some clays such as estuarine mud, and garden clay, need very little done to them to turn them into a high temperature glaze, usually all that is needed is an addition of calcium carbonate or ordinary wood ash.
Various metals can be used to colour glazes, copper and iron, being my personal favourites.
Glaze materials are first crushed into a fine powder, then are added to water to form a thin creamy consistency. This is sieved repeatedly to remove lumps. The glaze is applied to the pot by pouring, dipping, brushing, or spraying. In my case I usually dip the pot into the glaze or brush glaze over it.
Shino glaze is a Japanese style of glaze
Shino glaze suits wood firing very well, in fact it probably needs a wood fired kiln to be at its best (Shino glaze fired in an electric kiln looks rather like half dry PVA glue!). The passage of the flame around the pot, the variations in atmosphere in the kiln, the fall of wood ash, all leave their signature on the glaze. Variations in the thickness of the glaze have a major effect on the colour. In general the thicker the glaze is, the paler the colour, as most of colour is from traces of iron that are in the clay that the pot is made of working through the glaze, so if the glaze is thin, you tend to see more of that iron.
I am currently exploiting the colour variations of Shino by trailing lines of Shino glaze over Shino glaze. Trailing glazes is similar to decorating a cake. The glaze is squeezed from a hand held container of some kind through a thin tube.
In a wood fired kiln a glaze that has a small amount of copper in it can turn out to be green or red or both at the same time, depending on how much oxygen was present in the kiln as it was being fired. When I am firing copper glazes in my wood fired kiln and I want the glazes to be red, I have to starve the kiln of oxygen as I fire from when the kiln is at around 800 degrees Centigrade, until the top temperature is reached. This is called, firing in reduction. To do this I carefully add more wood each time I stoke until the flame is long, hazy and turbulent. This hazy flame is starved of oxygen and will look for oxygen from anywhere it can find it, including the glaze and the pot. When the flame takes oxygen from the metal oxides in the glaze, it dramatically changes the colour. If all the oxygen is taken from the copper oxide, the result is a thin, lustrous layer of copper metal on the glaze surface, this is often found on Raku fired pots.
Often a cloudy blue, chün glazes get their colour by scattering the light within the glaze. The colour is often more pronounced if it is over a dark background. I usually put chün glaze over a dark brown iron rich glaze, but have found them to work over copper red.
Chün glazes are very sensitive to thickness and to temperature. If they are put on too thinly they will not produce a colour, if they are not fired high enough, they will be an opaque white, if they are over fired, chün glazes can simply become clear and shiny.
What Sort Of Wood?
People often ask, “What sort of wood do I use to fire my pots to 1300 degrees Centigrade?”. I use almost any dry wood that I can get my hands on. Dry pine is thought to be the best. Most people find this odd, but pine burns with a very long flame, and releases its energy quickly, this is usually more effective in heating the whole kiln, than a heavy dense hardwood that burns with a short hot flame.