Shelley S on fri 26 mar 99
Hi all -
I have a question that I will try to express clearly---
If you have two cone 9-10 matte glazes, with essentially the same fluxes,
but one has an alumina/silica ratio of 1 to 4.59 and the other 1 to 8.59,
will one melt before the other? That is, will the one with higher alumina
take longer to mature (i.e., length of firing), or does it need to mature
by reaching a higher temperature? Does viscosity influence the melt, or
does it occur after the glaze is melted?
I hope this makes some sense.
Thanks in advance,
Shelley
Michael Banks on sun 28 mar 99
Hi Shelley,
My short answer to your question is that whichever glaze melts first,
depends not on their respective alumina/silica ratios but a completely
unrelated factor: The grainsize of the ingredients and the respective bond
strength of the ingedients.
For example: The ingredient supplying the alumina could be nepheline
syenite, fine-grained ball clay, china clay or alumina (aluminum oxide).
Also for example: the ingredient supplying the silica could be: petalite,
minus 600 mesh diatomite or minus 200 mesh quartz flour. All these
ingredients have widely different fusion rates caused by their physical
grain size and whether accompanying fluxes are intimately bonded into their
constituent crystal lattices or not. This is the reason that fritted glazes
can melt many cones lower than a glaze with identical chemistry, but made
with raw materials.
Generally speaking however, I've found that given flux unity, the higher
alumina/silica glaze with reach the transition point earlier, but will take
longer to completely dissolve the other ingredients. The lower transition
point is due to the fact that high alumina ingredients are generally
fine-grained compared to the ground quartz which usually is added to most
glazes to provide silica. However, the slow diffusion rates in high
alumina melts can result in either slow dissolution of other glaze
ingredients, or incomplete melting before the kiln soak period finishes.
Therefore, like the hare and the tortoise, the lower alumina/silica (and
lower viscosity) glaze might well be more mature when fired under the same
conditions and will win the race.
In other words, the higher alumina/silica glaze could melt first, but it
will take longer to mature, no it doesn't need a higher temperature to
mature - just longer and yes viscosity does influence a melt. One proviso,
higher temperature would help the high alumina/silica glaze clear itself of
defects such as pinholes and bubbles though, faults exacerbated by high
viscosity.
I hope this makes some sense!
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
Hi all -
I have a question that I will try to express clearly---
If you have two cone 9-10 matte glazes, with essentially the same fluxes,
but one has an alumina/silica ratio of 1 to 4.59 and the other 1 to 8.59,
will one melt before the other? That is, will the one with higher alumina
take longer to mature (i.e., length of firing), or does it need to mature
by reaching a higher temperature? Does viscosity influence the melt, or
does it occur after the glaze is melted?
I hope this makes some sense.
Thanks in advance,
Shelley
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