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cone 6 oil spots

updated tue 8 nov 05


Mark Tigges on mon 7 nov 05

About a month ago John Post started a discussion by posting this url:

I love the look of oil spot glazes, so I tried it out. My results
were identical to his. I wouldn't call it oil spot, rather oil dots.

Not bad, just not prominent enough to really use it.

Ron Roy joined in the conversation, he suggested that strontium
carbonate might be used in place of the whiting. His reasoning was
that the strontium disassociates later than any other carbonate.
Hopefully the bubbles from the disassociations from the strontium
would happen at point in the firing to cause aesthetic depositing of
iron on the surface, producing oil spots at cone 6.

The revised recipe from RR is as follows:

Custer - 19.0
Strontium - 5.5
Talc - 4.5
Frit 3134 - 23.5
EPK - 27.0
Silica - 20.5


2 Bentonite
9 Spanish Red Iron ox
2 Cobalt Carbonate

I tested it yesterday. I don't have pictures of it. I don't think my
photo setup is good enough to capture the detail required. I tested
two pieces, one on the bottom of the kiln, which is usually half a
cone below the top, and the other at the top. I generally fire to
about 6.5 at the top of the kiln.

The piece on the top shows no oil spots at all. The piece on the
bottom shows spots very similar to the original John Post glaze.

I remember John Hesselberth mentioned that he feels one should pursue
trying catalyze thermal reduction of iron earlier than would o/w
happen. Any more thoughts John?