|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.
Comparing these two firing ranges and their clays is like comparing apples and oranges. But for hobby pottery, there are some compelling advantages of the earthenware process (compared to stoneware) that you might not expect. Cone 10R ware is certainly stronger - but only if the glaze fits (crazed glazes dramatically weaken ware). But either one will break if dropped on the floor. The clay of the bamboo colored stoneware mug has a porosity of 2.5%, the clay of the orange earthenware mug has 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 bamboo mug has bare clay exposed on the foot so it will absorb some water. Consider another factor: The bamboo one weighs 432g while the orange one is only 281g. And the orange one will hold 1.92 times its own weight in water while the bamboo one only 1.16! That is because the orange one is made from an artware clay that throws much better - so walls can be squeezed thinner (and many more pieces made from the same amount of clay). The porosity of that clay also makes it a better insulator so it keeps coffee just as warm despite the thinner walls. For hobby potters, the difference in the effort and energy cost needed to glaze these two really favors the artware: It was just a matter of getting out two jars of prepared glaze, brushing on three coats and firing to cone 05 in four hours (while our ability to make the stoneware mug depends on mixing pails of dipping glaze and firings that take almost 24 hours from cold-to-cold).
Low Temperature Glaze
In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).