Tony Hansen on wed 1 oct 03
Check this page on glaze blisters, it lists all kinds
of possible causes and solutions:
http://www.digitalfire.ab.ca/cermat/education/112.php
In industry they try to minimize boron because of the
bubbling and blistering problems, but we potters work
with glazes high in boron anyway because we like what
they do visually. I wish I knew more about the exact
firing schedule that makes them blister.
Anyway, the 20x5 recipe is quite low in boron so it
should be better than most. The fancy reactive
rutile glazes, like the one on the cover of Mastering
Glazes, can have more then three times as much boron.
-------8<--------
When I fired his clear glaze to cone 7 I got bubbles not pinholes on the
outside of a vase. It was worse on the inside. I used the same base glaze
with cobalt, rutile, and tin and everything fire ok on another piece.
What went wrong with the clear base?
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Tony Hansen, Digitalfire Corp.
John Hesselberth on thu 2 oct 03
On Wednesday, October 1, 2003, at 09:48 PM, Tony Hansen wrote:
> In industry they try to minimize boron because of the
> bubbling and blistering problems, but we potters work
> with glazes high in boron anyway because we like what
> they do visually. I wish I knew more about the exact
> firing schedule that makes them blister.
Hi Tony,
I wish I did too; however, I have become convinced boron is not the
only (and perhaps not even the primary) culprit--although it probably
is a contributor. For example, the glaze in our book that is most
prone to blistering is Waxwing. It tends to blister when it is fired
too hot or when it is applied to thickly. While it has a high level of
boron (.4+), it is nowhere near as high as Waterfall Brown (.7+). Yet
virtually no one has reported trouble with Waterfall blistering. The
only time I see a tendency for it to blister is when I cool it very,
very rapidly. Then it is also much less attractive. It does, however,
blister some when copper or cobalt is used as a colorant instead of
iron.
I conclude that blistering results from a complex interaction between
ingredients, firing conditions, and cooling conditions that we haven't
figured out yet. But I usually have good luck reducing it or
eliminating it in a given glaze by firing a cone or two lower, cooling
more slowly and/or applying more thinly.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
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