Fara Shimbo on wed 28 jul 04
Hello, Jim,
The comments you found in "Glass-Ceramic Technology" throw some
light on a phenomenon I've found. It happens frequently in
glazes with enough titanium to make the ground (where there are
no crystals) opaque. Whether the same effect happens in more
translucent/transparent glazes I don't know. But what one
observes is a dark line around the very edge of the crystal,
that's wider the thicker the glaze. Curiously enough, when
I looked at these lines under a microscope, what they showed
was that there isn't a dark line at all: there's an area of
transparency right up against the edge of the crystal. What
one sees as a dark line is the shadow of the crystal on the
porcelain below. I wonder if titanium from the glaze is migrating
into the crystal at that point. Hmmm, something to think about!
Hang in there,
Fa
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Fara Shimbo, Master Crystalliere, Certified Public Nuisance
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Jim Murphy on wed 28 jul 04
on 7/28/04 8:22 AM, Fara Shimbo at shimbo@CRYSTALLINE-CERAMICS.INFO wrote:
< ... there's an area of transparency right up against the edge of the
< crystal. What one sees as a dark line is the shadow of the crystal on the
< porcelain below. I wonder if titanium from the glaze is migrating
< into the crystal at that point. >
Hi Fara,
Very interesting.
In glass-ceramics, I think the technical term for what you've observed is
"phase separation". At least 3 separate "phases" appear to exist: the
Titanium-laced opaque "ground", the larger macrocrystals (Zn2SiO4 ??), and
an apparently Silica-saturated high-alkali transparent "glass phase" around
the edge of the macrocrystals.
So, there's Titanium suspended in the "ground" but not in the "glass phase"
around the macrocrystals.
I'm inclined to think there's something about the "glass phase" composition
- perhaps high-alkali content - which is limiting its Titanium-level
compared to the ground's Titanium-level.
Additionally, it's my understanding, in general, temperature and hold-time
control the Silica-level and corresponding quartz dissolution level of a
glass phase.
Best wishes,
Jim Murphy
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