Jeff Lawrence on tue 2 jun 98
Hello all,
I am puzzled by what silica does in glazes. A little makes glossier. Too
little makes more matte (just by lowering the alumina:silica ratio?). Too
much, ah there's the question. I deduce that it increases the refractoracity
of a glaze, because it is by itself a refractory material.
I've tried the advice to "just add silica until the crazing stops" and
learned that it is not that easy. But I also learned that too much silica
will induce a very fine, sugar-crystalline crazing effect (someone else was
commenting on this effect a couple of weeks ago). Looks like sheet diamond
where thick on a dark body, but doesn't look like functional to me.
A possibly related pheonomenon: as my glazes get more refractory, they tend
to craze and crack ware more. I'm still puzzling over this one; my current
theory is that since the glaze doesn't melt that thoroughly, there are areas
of weaker glass that didn't get mixed. Looking at the surfaces closely, they
often have a grainy or pebbly look, for the want of better descriptive
terms, as if the melting were arrested before the glaze had a chance to
smooth out. If I were a crack, I'd definitely move along the road of low
resistance. One of these refractory glazes, that cracked 8 out of ten
fixtures at ^04ox, cracked none when fired to ^03ox.
I wonder if the fine crazing I've seen in silica-rich glazes is related to
this phenomenon. Interestingly, my test samples of local silica-rich tuff
seem to melt with oodles of silica (~10 m.p.) and nowhere near the same
multiple of fluxes. At the same time, they craze, which according to the
expansion calculations, ought not to be. Their surfaces tend to be still
bumpy a bit, though very shiny. According to my geologist contact, the
silica in the tuff is glassy in state, which seems to me ought to melt more
readily than a nice tight quartz crystal. Maybe the glass form softens at
lower temperaturs but the high silica content prevents a thorough mixing?
Looking forward to another test firing at ^6 with this tuff; almost a month
since the last one.
Please shoot these theories down.
Jeff, never daunted by the absence of data when there is theorizing to be done.
Jeff Lawrence
jml@sundagger.com
Sun Dagger Design
Rt 3 Box 220
Espanola, NM 87532
ph 505-753-5913
David Hewitt on thu 4 jun 98
My way of looking at this is to consider a phase equilibrium diagram.
If you consider a simple one of two materials A and B (which could be
Al2O3 and SiO2), the vertical axis is temperature and horizontal axis
has A 100% on the left decreasing to 0% on the right, and B 100% on the
right decreasing to 0% on the left. The graph will show a temperature
for each mixture at which the mix becomes all liquid. Typically this
line decreases to a point and then increases. This point is the eutectic
temperature for that mix and all components have gone into the liquid
phase. For a clear shiny glaze with this mix we need to fire just above
this temperature.
In practice we will have decided what temperature or cone we wish to
fire at and will have selected a recipe which will give us this clear
shiny glaze. If we then increase or decrease the silica in this recipe,
we are going to continue to fire it to the same temperature or cone.
Let us then draw a horizontal line at this eutectic temperature on our
graph. If we then consider a mix to the left of this point, and firing
to the same temperature, not all components of the mix will have melted.
You will have A + Liquid. If the mix is to the right of the eutectic
point you will have B + Liquid. The A + Liquid will look different to
the B + Liquid when cooled.
This is a very much simplified explanation of what I think happens with
a glaze having a number of different materials, but if you consider that
increasing the silica in a glaze takes it past the eutectic mix, as in
the example above, then a different phase mixture will result and the
different crystalline effects will result.
David
In message , Jeff Lawrence writes
>----------------------------Original message----------------------------
>Hello all,
>
>I am puzzled by what silica does in glazes. A little makes glossier. Too
>little makes more matte (just by lowering the alumina:silica ratio?). Too
>much, ah there's the question. I deduce that it increases the refractoracity
>of a glaze, because it is by itself a refractory material.
>
>I've tried the advice to "just add silica until the crazing stops" and
>learned that it is not that easy. But I also learned that too much silica
>will induce a very fine, sugar-crystalline crazing effect (someone else was
>commenting on this effect a couple of weeks ago). Looks like sheet diamond
>where thick on a dark body, but doesn't look like functional to me.
>
>A possibly related pheonomenon: as my glazes get more refractory, they tend
>to craze and crack ware more. I'm still puzzling over this one; my current
>theory is that since the glaze doesn't melt that thoroughly, there are areas
>of weaker glass that didn't get mixed. Looking at the surfaces closely, they
>often have a grainy or pebbly look, for the want of better descriptive
>terms, as if the melting were arrested before the glaze had a chance to
>smooth out. If I were a crack, I'd definitely move along the road of low
>resistance. One of these refractory glazes, that cracked 8 out of ten
>fixtures at ^04ox, cracked none when fired to ^03ox.
>
>I wonder if the fine crazing I've seen in silica-rich glazes is related to
>this phenomenon. Interestingly, my test samples of local silica-rich tuff
>seem to melt with oodles of silica (~10 m.p.) and nowhere near the same
>multiple of fluxes. At the same time, they craze, which according to the
>expansion calculations, ought not to be. Their surfaces tend to be still
>bumpy a bit, though very shiny. According to my geologist contact, the
>silica in the tuff is glassy in state, which seems to me ought to melt more
>readily than a nice tight quartz crystal. Maybe the glass form softens at
>lower temperaturs but the high silica content prevents a thorough mixing?
>Looking forward to another test firing at ^6 with this tuff; almost a month
>since the last one.
>
>Please shoot these theories down.
>
>Jeff, never daunted by the absence of data when there is theorizing to be done.
>
>
>Jeff Lawrence
>jml@sundagger.com
>Sun Dagger Design
>Rt 3 Box 220
>Espanola, NM 87532
>ph 505-753-5913
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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