Chris Schafale on fri 18 apr 03
Tony Hansen said:
Considering the important function of alumina
> in glass structure, the lack thereof would be an agravating
> factor in the separation. Phase boundary phenomenon and the
> differences in surface tension and melt fluidity of the
> phases could breed blisters. This process likely continues
> in a second firing (this accounts for blistering getting
> worse). Ferro Frit 3134, for example, has no alumina, lots
> of boron and plenty of CaO/Na2O, glazes high in it make
> ideal candidates for this phase separation.
I note that the glazes I am having trouble with are actually quite
high in alumina, for cone 6. One glaze which starts with the 50-30-
20 formula, but has additional alumina added, is even more prone
to blistering than the original. On some of my Currie tiles using
100% Frit 3134 in the C corner, there is dramatic blistering in the
top row, where the alumina is highest, and on some the same
phenomenon occurs in the bottom right corner, where there is little
or no alumina and high silica.
(http://www.lightonecandle.com/tn_currie.html) My assumption is
that these glazes are more viscous, therefore the craters don't
smooth out.
I also note that firing a bit lower does tend to reduce the blistering,
and firing hotter, longer or refiring increases it. I tend to believe
that the problem is related to body-glaze interaction -- could it be
that the high boron glaze is "attacking" the clay body in some way
as heatwork increases, causing release of additional gasses?
On the face of it, it would seem that Currie-style blends would be
an excellent way to study whether alumina/silica balance is related
to this problem. I think one would need to put the 35 glazes on
pots, rather than tiles, however, in order to see the problem show
up reliably.
Chris
--
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh, NC)
www.lightonecandle.com
candle@intrex.net
Tony Hansen on fri 18 apr 03
Very interesting.
Maybe alumina stiffens the melt in one of the phases and
it has the higher surface tension.
> Tony Hansen said:
> Considering the important function of alumina
> > in glass structure, the lack thereof would be an agravating
> > factor in the separation. Phase boundary phenomenon and the
> > differences in surface tension and melt fluidity of the
> > phases could breed blisters. This process likely continues
> > in a second firing (this accounts for blistering getting
> > worse). Ferro Frit 3134, for example, has no alumina, lots
> > of boron and plenty of CaO/Na2O, glazes high in it make
> > ideal candidates for this phase separation.
> I note that the glazes I am having trouble with are actually quite
> high in alumina, for cone 6. One glaze which starts with the 50-30-
> 20 formula, but has additional alumina added, is even more prone
> to blistering than the original. On some of my Currie tiles using
> 100% Frit 3134 in the C corner, there is dramatic blistering in the
> top row, where the alumina is highest, and on some the same
> phenomenon occurs in the bottom right corner, where there is little
> or no alumina and high silica.
> (http://www.lightonecandle.com/tn_currie.html) My assumption is
> that these glazes are more viscous, therefore the craters don\'t
> smooth out.
> I also note that firing a bit lower does tend to reduce the blistering,
> and firing hotter, longer or refiring increases it. I tend to believe
> that the problem is related to body-glaze interaction -- could it be
> that the high boron glaze is \"attacking\" the clay body in some way
> as heatwork increases, causing release of additional gasses?
> On the face of it, it would seem that Currie-style blends would be
> an excellent way to study whether alumina/silica balance is related
> to this problem. I think one would need to put the 35 glazes on
> pots, rather than tiles, however, in order to see the problem show
> up reliably.
> Chris
> --
> Light One Candle Pottery
> Fuquay-Varina, North Carolina, USA
> (south of Raleigh, NC)
> www.lightonecandle.com
> candle@intrex.net
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========
Tony Hansen
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