Preparing rocks for glazes

Unlike when using commercial materials, and you can just open a bag of powdered ingredients, there is a long process involved in preparing rocks for use in glazes. This marks the second stage in my making cycle. The first being preparing and pugging the clay bodies ready for making.

First of all I sort out samples of rocks that I am getting low on in powder form - generally around 2 - 5kg, though less if it is a particularly rare rock, and up to 10kg if it is one that I use regularly. If the rocks are in large pieces I will break them up into more manageable pieces with a steel mallet.

The next stage depends on what type of rock I have to prepare. Some, like the ones shown here, benefit from calcining. This involves heating them to above red heat and allowing them to cool down. I generally put them on the floor of the kiln in a normal bisque firing. This is particularly useful for rocks with big crystals like the granite shown here. The pale white/clear crystals are quartz and these expand and contract at a different rate to the others, on heating. As a result, when you take the granite out of the kiln, it can often be crumbled by hand into its individual crystals.

Other rocks that benefit from calcining are granodiorite (left) and diorite (right). These rocks, like gabbro have a coarse crystal structure that breaks up on heating.

Other rocks, have to crushed to a powder without calcining, as it has no effect, such as with the quartz to the left. the volcanic tuff (right) has such a fine crystal structure that heating makes no difference in its hardness, while clay (far right) will just get harder.

I will go into more detail about this in my forthcoming book, Rock Glazes Unearthed.

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