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Glaze components: raw material tests

As part of our MA course Dave has started teaching us the very basics about glazes. He has asked us each make some test tiles on which are to put one basic material and fire one version in the gas kiln to reduction and the other sample in the electric kiln to oxidation.

My basic ingredients are a bit unusual (from left to right):

No. 53 - Bicarbonate of Soda

No. 52 - Table Salt

No. 51 - Demerara Sugar

and No. 50 - Plaster.

I mixed them with water so that they would stick to the tiles, which are made by using the reclaimed stoneware clay in the studio.

In order to assess the opacity of a glaze we have painted a strip of red slip over one half of a tile.

These are my results. Top pieces are fired in an electric kiln in oxidation to about 1265°c. Bottom row are fired in a gas kiln in reduction to about 1270°c.

Left - No. 50 Plaster functions mostly as a mask. It didn't fuse with the surface but simply crumbled off.

No. 51 Sugar has a slight sheen but not very much has changed.

No. 52 Salt is rather black and has probably fumed off onto most of the other pieces causing the unexposed clay areas to turn red. Salt glazes are well known but the resulting fumes in the kiln are very toxic.

No. 53 Bicarb of Soda has the most astonishing and pleasing result in both oxidation and reduction. I did not expect such a beautiful glazed surface and in such a pleasant green. However, it does flux quite a bit and a fair amount of it ran off the tiles onto the kiln shelf. I think there is some potential here. I'm just not sure how toxic it is.

Here are all the other test pieces done by us our MA group:

Without going into each ingredient they are arranged (from left to right) in the following groupings:

Top: Clays (Alumina) > Glass formers > Primary Fluxes > Secondary Fluxes.

Middle row are all Colouring agents.

Bottom: Opacifiers > Frits > other non-standard materials.

From here we will go on to test combinations of two base materials. This whole process is very interesting and a great way to make us understand some of the basics of glazes, a field that is totally huge and mind boggling.

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