From the Kiln Room
We are on 18th firing for the year. Right now, there is a back up on stoneware as the kiln is not firing to temperature – so waiting for kiln refurbishment. Just leave your work on your shelf as there is no room in the kiln room. We should have an idea of what needs to be done to the kilns on 29 March when the kiln repair man is coming to check them out. Hopefully they won’t be out of commission for long. We will keep you posted.
Now for a little pottery trivia.
There are three main types of clay:
Stoneware
Earthenware
Porcelian
Each of these is different and has qualities that distinguish them from one another.
What is stoneware?
Stoneware ceramics are hard and strong and can hold liquid. Stoneware after bisquing is usually fired over.
1200°C to about 1280°C. Different glazes mature at different temperatures within that range. They can also change colours when fired to different temperatures.
Vitrification happens during bisque firing which changes clay particles. Particles move closer to each other during firing and surface particles bond together. This is why it is easier to handle pieces that have been bisqued without breaking them. Bisque temperature is about 1000°C.
Then the next step is to glaze and the temperature of the firing is increased to 1260°C to 1280°C and in some cases higher. The clay after bisquing remains porous enough to accept glazes which can absorb into the clay. During the higher firing the silica, which is used in making glass and becomes molten and moves into the porous holes created during bisque process. The silica should start to become molten around 800°C. Think of a sponge with holes and those holes fill up with a solid material. This next statement is important to remember. Silica is also one of the ingredients which causes Silicosis which is why when sanding pots, or around clay dust, we should always wear a mask, sand outside and clean up with water.
It is this process which creates stoneware pots which are watertight, strong and resilient.
Basically firing in the kiln creates a physical and chemical process that matures the clay from a malleable substance into your durable creations.
More next month on other types of clay.
Linda, Gus, Jean, Katrina, Lynne, Douglas
And Catherine and Gabriel from the glaze room