Richard Burkett on mon 7 apr 97
Tom Buck says:
>matches a given eutectic (ie, melts at a predictable cone), and in this
>sense it becomes moot whether the B2O3 is acting as a "flux" or as a
>"glass-former" or perhaps as both.
It would be good to keep the meaning of the word "eutectic" clear. Glazes
are best made NOT to match an exact eutectic point (the lowest possible
melting temperature for a given mixture of materials), as that is not a
very stable point for glaze melting. Generally the most stable, widest
firing range glazes are made a some point well away from actual eutectic
mixtures. Yes, eutectic effects are part of the melting of glazes, but
"melts at a predictable cone" is not a good definition of the word
eutectic.
Specifically, from a dictionary:
1. Of, relating to, or formed at the lowest possible temperature of
solidification for any mixture of specified constituents. Used
especially of an alloy whose melting point is lower than that of
any other alloy composed of the same constituents in different
proportions.
2. Exhibiting the constitution or properties of such a solid.
Richard
Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
Ric Swenson on wed 9 apr 97
>----------------------------Original message----------------------------
>
>Tom Buck says:
>>matches a given eutectic (ie, melts at a predictable cone), and in this
>>sense it becomes moot whether the B2O3 is acting as a "flux" or as a
>>"glass-former" or perhaps as both.
>
>It would be good to keep the meaning of the word "eutectic" clear. Glazes
>are best made NOT to match an exact eutectic point (the lowest possible
>melting temperature for a given mixture of materials), as that is not a
>very stable point for glaze melting. Generally the most stable, widest
>firing range glazes are made a some point well away from actual eutectic
>mixtures. Yes, eutectic effects are part of the melting of glazes, but
>"melts at a predictable cone" is not a good definition of the word
>eutectic.
>
SNIP-----------------
>
>Richard Burkett - School of Art, SDSU, San Diego, CA 92182-4805
>E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
>Home Page: http://rohan.sdsu.edu/dept/rburkett/www/burkett.html
>CeramicsWeb: http://apple.sdsu.edu/ceramicsweb/index.html
---reply----------
Richard,
I agree completely.
I remember being taught that a EUTECTIC is more of an "anomaly" than
anything else. A "happy accident" of lowest melting point of an admixture
of materials....certainly NOT the basis for good glaze formulation ....IMHO.
Ric Swenson, Bennington, Vermont
rswenson@bennington.edu
Gavin Stairs on wed 9 apr 97
At 12:30 PM 07/04/97 EDT, Richard Burkett wrote:
>----------------------------Original message----------------------------
>
>Tom Buck says:
>>matches a given eutectic (ie, melts at a predictable cone), and in this
>>sense it becomes moot whether the B2O3 is acting as a "flux" or as a
>>"glass-former" or perhaps as both.
>
>It would be good to keep the meaning of the word "eutectic" clear. Glazes
>are best made NOT to match an exact eutectic point (the lowest possible
>melting temperature for a given mixture of materials), as that is not a
>very stable point for glaze melting. Generally the most stable, widest
>firing range glazes are made a some point well away from actual eutectic
>mixtures. Yes, eutectic effects are part of the melting of glazes, but
>"melts at a predictable cone" is not a good definition of the word
>eutectic.
....
Hi All,
I assume that Tom's use of eutectic in this case is in relation to it
existence as the liquid part of a slurry, which is what glazes generally
are. The eutectic point identifies that point at which the only phase
present is the eutectic, so the mixture melts at a well defined point (but
only after the glaze constituents have blended together - see Max Richens'
post concerning the time taken to homogenize a glaze in the fire). In a
line blend, on either side of the eutectic point the eutectic will be
present with different solid phases in degree of their excess in the
mixture. The liquidus line above the eutectic identifies the temperature
at which all of the solid phase dissolves in the eutectic. So the eutectic
only really exists in "pure" form at the eutectic temperature. Above that
temperature, the liquid phase is some solution of solids in the eutectic.
On cooling, if the liquid is sufficiently low viscosity, and if the cooling
is sufficiently slow, the excess solids may crystallize out, which gives
rise to crystal glazes, among other phenomena.
So it is true that the eutectic point is an unstable place to design a
glaze. Such a glaze will liquify rapidly at a single temperature, if you
can hit it, and will probably be quite runny above that temperature. So
you may get a runny glaze one batch, and an immature matte the next. But
the eutectic is the temperature and mixture which drives all the glaze
mixtures in its region, and is therefore very important, even far away from
the eutectic point. When I use the word in relation to a glaze mixture off
the eutectic point, I generally do so in the understanding that this is a
somewhat loose application of the term, but useful to refer to the melt
phase, and useful also to remind myself at what temperature the glaze may
begin to melt on the way up, and finish solidification on the way down.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
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