|Monthly Tech-Tip |
These were only fired at cone 04, but they are durable enough to last some time with normal use. The insides have a transparent glaze (Spectrum 700), it is leadless and completely safe. These are great insulators, they keep coffee warm longer than porcelain or stoneware. The feet are glazed so they are fine for the dish washer. They are super-light, the body is made from ball clay and talc and throws really well. These shrink very little on drying (in this case less than 2% compared to porcelains which can be 8% or more). These are inexpensive to fire, only four hours to cone 04. They withstand impacts better than you think (a thin porcelain mug propagates cracks and can shatter). Colour, glorious color! These are Spectrum Opaque low fire glazes, dozens of wild colors are available.
Not so fast! Cone 10R ware is only strong if the glaze fits. Fortunately this does. But either one will break if dropped on the floor. The stoneware clay has a porosity of 2.5-3%, the earthenware 12%. But the entire base of the orange one is glazed (it was fired on a three-pointed stilt), and the glaze fits well, so there is no point-of-entry for water. The stoneware mug has bare clay exposed on the foot so it will absorb some water. The stoneware mug weighs 432g, the other 281g. It holds only 1.16 times its weight in water, the orange one 1.92! The earthenware is a much better thrower. It is also a much better insulator, you can hold your hand around it with scalding hot coffee inside. And only two pint jars and a brush were needed to glaze it. And it fired in four hours. I needed to make two pails of glaze for the other and took 22 hours to fire! My favourite to use right now: The orange one.
Low Temperature Glaze Recipes
In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).