Ron Roy on sun 12 jul 98
Hi Autumn,
I have some interest in this aspect of ceramics but find it heavy going - I
have done some experiments and plan more in the future.
The American Ceramic society has published many volumes on the subject
which give many examples of this fascinating aspect of melting and phase
equilibrium. The latest is "Introduction to Phase Equilibrium" by Bergeron
and Risbud. It is not easy going - at least for me - but I am determined.
Perhaps an example will help make this a little clearer.
The combination of CaO, Al2O3 and SiO2 which melts at the lowest
temperature is as follows. You need to set you calculator on % to use these
numbers. This mixture is supposed to melt at 1170C - there is no heating
time given - at least I have not found it.
CaO - 23.3
Al2O3 - 14.7
SiO2 - 62.2
This is as close as I can get (given my limited time tonight) using raw
materials - it contains other oxides as well so - in theory it should melt
at an even lower temperature. I have tried this before and did not get a
proper melt at cone 6 - I surmise I would have to do some ball milling to
reduce the particle size and/or extend the firing.
Wolastonite - 50.0
EPK - 33.5
Silica - 16.5
Total - 100
Ratio 7.31
It should not craze on most cone 6 bodies - will not be a durable glaze (it
would need some other oxides to balance the CaO.)
All that CaO is going to lead to some of it coming out of the melt on
cooling I suppose so fast cooling would be best.
Any adjustment (up or down) in any of the three oxides will result in a
higher melting temperature.
Any different amounts will result in "extra" material being left over (out
of the melt) at that temperature.
It would seem to me that being near a eutectic would be the best way to get
a clear glaze - I am also concerned that this would result in a glaze with
a very short firing range - can anyone confirm those two statements?
Most of our glazes have at least 5 oxides in them so the process of
determining what is going on becomes very complicated - in most cases - for
us - because we use less than pure materials - so complicated we can never
really find out what is really going on. Never the less - we can still use
this information. Keep in mind that moving away from a eutectic is how we
get many of the effects we want. It would be most helpful - I think - to at
least know when we are moving away from or towards the eutectic point of a
glaze.
I do welcome any comments on what I have said - in the interest of finding
ways to use this type of information.
>----------------------------Original message----------------------------
>In message , Autumn Downey writes
>>----------------------------Original message----------------------------
>>I have been reading a bit about eutectics and that's left me somewhat
>>confused. If some of the glaze theory people have time to comment, I'd
>>really appreciate their thoughts.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Grimmer on mon 13 jul 98
Ron,
Interesting stuff. Not to argue with either you or the Acers, but I was
under the impression that calcia will not melt silica at normal stoneware
temperatures, that it needs an alkali to 'get the ball rolling.' Does
alumina actually assist in the melt in this case? Why does that sound
familiar? I just got the Magic of Fire Reference from IMC. Good stuff in
there. Have you seen it?
Being near a eutectic point does seem a good starting point for a clear
base, a point made in an earlier posts by Craig Martell and John Baymore. In
those it was put forth that glazes right on these points can be very touchy
WRT firing temp, etc. It seems that the problems would be more pronounced
as the slope of the graph increased. ie a sharp eutectic would leave little
room for errors in mixing or firing or for variation in materials.
Maybe what we want is a phase diagram for these elements (assuming C10):
Al2O3, SiO2, CaO, K2O, and either SrO, ZnO, BaO depending on firing
atmosphere and personal taste for auxilliary melter. Hmm, a Pentaxial blend.
I suppose it would be easier to pull off four quadraxials holding, say, the
SrO at a constant .05, .075, .10, .15 for each. Then one could see how
changing that oxide moved the eutectic around.
Am I making any sense here or just babbling?
steve grimmer
marion illinois
----------
>From: Ron Roy
>To: Multiple recipients of list CLAYART
>Subject: Re: Eutectics -an even simpler explanation!
>Date: Sun, Jul 12, 1998, 10:28 AM
>
>----------------------------Original message----------------------------
>Hi Autumn,
>
>I have some interest in this aspect of ceramics but find it heavy going - I
>have done some experiments and plan more in the future.
>
>The American Ceramic society has published many volumes on the subject
>which give many examples of this fascinating aspect of melting and phase
>equilibrium. The latest is "Introduction to Phase Equilibrium" by Bergeron
>and Risbud. It is not easy going - at least for me - but I am determined.
>
>Perhaps an example will help make this a little clearer.
>
>The combination of CaO, Al2O3 and SiO2 which melts at the lowest
>temperature is as follows. You need to set you calculator on % to use these
>numbers. This mixture is supposed to melt at 1170C - there is no heating
>time given - at least I have not found it.
>
>CaO - 23.3
>Al2O3 - 14.7
>SiO2 - 62.2
>
>This is as close as I can get (given my limited time tonight) using raw
>materials - it contains other oxides as well so - in theory it should melt
>at an even lower temperature. I have tried this before and did not get a
>proper melt at cone 6 - I surmise I would have to do some ball milling to
>reduce the particle size and/or extend the firing.
>
>Wolastonite - 50.0
>EPK - 33.5
>Silica - 16.5
>Total - 100
>Ratio 7.31
>It should not craze on most cone 6 bodies - will not be a durable glaze (it
>would need some other oxides to balance the CaO.)
>
>All that CaO is going to lead to some of it coming out of the melt on
>cooling I suppose so fast cooling would be best.
>
>Any adjustment (up or down) in any of the three oxides will result in a
>higher melting temperature.
>Any different amounts will result in "extra" material being left over (out
>of the melt) at that temperature.
>
>It would seem to me that being near a eutectic would be the best way to get
>a clear glaze - I am also concerned that this would result in a glaze with
>a very short firing range - can anyone confirm those two statements?
>
>Most of our glazes have at least 5 oxides in them so the process of
>determining what is going on becomes very complicated - in most cases - for
>us - because we use less than pure materials - so complicated we can never
>really find out what is really going on. Never the less - we can still use
>this information. Keep in mind that moving away from a eutectic is how we
>get many of the effects we want. It would be most helpful - I think - to at
>least know when we are moving away from or towards the eutectic point of a
>glaze.
>
>I do welcome any comments on what I have said - in the interest of finding
>ways to use this type of information.
>
>
>>----------------------------Original message----------------------------
>>In message , Autumn Downey writes
>>>----------------------------Original message----------------------------
>>>I have been reading a bit about eutectics and that's left me somewhat
>>>confused. If some of the glaze theory people have time to comment, I'd
>>>really appreciate their thoughts.
>
>
>
>Ron Roy
>93 Pegasus Trail
>Scarborough, Ontario
>Canada M1G 3N8
>Tel: 416-439-2621
>Fax: 416-438-7849
>
>Web page: http://digitalfire.com/education/people/ronroy.htm
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