Dave Murphy on sun 12 nov 00
Ivor:
I had always attributed these with the inclusion of whiting(calcium =
carbonate). Do you think this could still be a possibility. Also, =
because this is not always occuring, it seems to be a problem I have not =
dealt with. It often happens during a busy production period when new =
materials are purchased and time to test is just not feasible. I am =
still digesting your article on static charge on pots. Well done. My =
education is not of a technical nature but I am surrounded by engineers =
of various stripes.
Barbara Murphy
Waterloo County Pottery
Waterloo Ontario
Canada
Snail Scott on sun 12 nov 00
I have heard that fritted glazes are often high viscosity and
also melt early, trapping bubbles which would have escaped in
a glaze which was still powdery later in the firing. I also
see more bubbles in areas where the clay was scraped but not
smoothed ( a rough, more 'open' surface) I once ground these
areas down - no craters, but closed bubbles like foam all the
way through, only in those areas. I reglazed those areas and
refired, slow: same result, plus slight bubbling in a few new
areas. This seems consistent with a 'sealed over by glaze' effect.
I reglazed those areas again with a different glaze - no problems
that time.
This probably isn't the effect you are discussing, but I though
I'd mention it.
-Snail
Fraser Forsythe on mon 13 nov 00
I've had this discussion with many people on and off the list. There are several
theories I know of that offer explanations for bubbling. I suspect they are all
valid at times.
First theory: combustible material in the substrate(clay) creates lots of gas
during sintering. Solution: slow and long organic burn during the bisque and
slow(er) glaze firing.
Second theory: some glaze materials give off tons of gas and this is directly
related to LOI - in most cases. ie. something like whiting would be a more likely
culprit than a frit (extreme example I know). Solution: reduce amount of culprit.
Third theory: high viscosity traps air bubbles. Change recipe in order to affect
overall viscosity.
Fourth theory: boiling, ie some glazes 'seem' to boil at certain temps regardless
of how many times they are fired. Solution: find a melt temp that is either above
or below the 'boiling point' of the glaze.
Comments: first there is no such thing as boiling points in glazes. I suspect there
are chemical reactions with gaseous byproducts that occur at various temps. However
I've had glazes that exhibit this behavior -over 2 and three firings- and its very
frustrating. Changing the firing temp is a quick fix that could throw the rest of
your glazes out the window - so to speak. Best to change recipe until you find the
culprit(s). In some cases you will find bubbling as the result of carbon burning
off the clay and a longer bisque will solve the problem, but not always. You can
confirm by refiring the piece. If the bubbles persist burn off is not the culprit
(my reasoning is simple: after a piece is fired to cone 6 or 10 the clay is
vitrified and anything that could burn off has burned off). I've done this test and
some glazes will keep bubbling after 3 firings. You can either add materials that
reduce the viscosity and or surface tension or go back to the drawing board and
look for materials that dont give the headache.
I like the theory of boiling the best. seems to explain what I've found. I dont
find changing viscosity ever helps when I've got a glaze that's 'boiling'. Besides
high viscosity can create tiny bubbles within the glaze and create amazing effects
- e.g.. blue celadons.
One final note and I'll stop droning. I also suspect iron, esp in the clay body. If
you're using a dark body and there is more than 22% this alone could cause you
problems with bubbling and bloating.
that's my 2 cents worth. Please refute me if I've missed something or you think Im
mistaken. As you can probably tell I'm not 100% sure but have some hunches.
Fraser
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