Steve Irvine on wed 15 jun 05
Hello,
I've received some off list requests for more details about the beach pebble and shale glaze
posting of May 9:
http://lsv.ceramics.org/scripts/wa.exe?A2=ind0505b&L=clayart&F=&S=&P=6546
so I've put a composite photo online that shows both the clayey, shale deposit where I dig the
clay, and a close-up of the pebbles I used:
http://www.steveirvine.com/clayart/pebble_shale.jpg
The little crayfish claw was about 2 cm long for scale. Both photos were taken within 1 km of my
house.
The make-up of the pebbles in your area is probably different, depending on your local bedrock.
The pebbles that I used, as you can see in the photo, are mostly dolomitic limestone (there are
several limestone quarries in my region) with a bit of feldspar, granite and quartz. The presence of
feldspar, granite and quartz is glacial in origin and they were brought here from several hundred
kilometers north, from the Canadian Sheild, about 12,000 years ago during the last ice age. There
are also some coral fossils in the pebbles from when this area was a tropical sea, about
440,000,000 years ago. The first year and a half that we lived in our home we had salt water in
our well, a remnant of the tropical sea.
The clayey shale has a fairly high iron content, as you can see, and that is where the green colour
comes from in the cone 10 reduction glaze. The shale was laid down in the region at the same
time as the tropical sea. It is a geological feature that sweeps diagonally across Southern Ontario
and is known as the Queenston Formation. I believe it extends into New York state as well. A
Google search will give several pages with information about it. It has been used for about 200
years in Ontario for making bricks, and in fact my own home is made from Queenston Formation
brick. By itself at cone 10 it melts into a dark brown glass, the addition of the dolomite from the
pebbles fluxes it a bit and also makes it more of a green colour.
I pulverized the pebbles in a ball mill, and simply rough sieved the shale to get out the bigger
stones, rootlets etc. The glaze was made up of shale 60% and pebbles 40%.
I also use this red clay/shale as a sort of wild card addition to several of my other glazes, shinos
and wood ash for instance, to give them more character, a less refined look.
Steve Irvine
http://www.steveirvine.com
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