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In the photos below there is a tenmoku bottle. The recipe for the
tenmoku is below. My question relates to the blue matte areas of the glaze.
There is one area in this picture to the front of the opening which is not very marked but obvious nonetheless. It is slightly more matte than the surrounding
to the extent that you can feel it as you run your finger over it.
In the next photos there are closeups of the foot of the bottle. There a thicker roll of glaze shows substantially greater evidence of this blue crystallisation. Bear in mind that the real colour is a much darker blue than it appears. Here the surface has a definite matte feel.
The glaze is :
custer spar 1200
ball clay 200
whiting 400
flint 600
RIO 250
bentonite 1%
and there was some wood ash wash painted on the top flat section above
the shoulder. The clay body is Glacia porcelain. I had other pieces
with the same glaze combination on the same clay in the same firing
which do not exhibit this effect. That said many seem to show hints of it
where the glaze is thick.
The firing was cone 10 reduction with a 1 hour hold at
peak temperature. The only other thing of note was that there was copper
flying about the kiln. I know this, as some of my celadon pieces picked
up a copper red blush.
So the question is, what caused the blue crystallisation, or what tests could I do to narrow it down ?