I.Lewis on tue 18 may 99
To: clayart=40lsv.uky.edu
From Ivor Lewis ivorredhill=40yahoo.com.au
Monday, 17 May 1999 Australian Central Standard Time
Subject: Eutectic Quandary
From time to time people dealing with ceramic problems use the term =
=22Eutectic=22.
This was done recently in a posting to Clayart and a recipe was given in
molecular oxide terms with the statement that =22 this Seger forms a =
eutectic at
cone 03=22.
I accept the intent of the person who posted this information. Mr. Seger=92s
recipes have always been a good starting point for a glaze composition, look=
at
the popularity of the Seger Cone 8 recipe. It underscores almost eighty =
percent
of all published cone 8 glaze recipes.
Recently I became interested in the confusion which surrounds the use of the
term =22Eutectic=22. I use the definitions found in the American Ceramics =
Society
book of Phase Diagrams for Ceramists. The large diagrams which are on the =
wall
in front of me convey information which shows that a eutectic is a =
relationship
between precise mineral compositions. For example, there is a point at 1100
degrees C in the CaO-SiO2-Al2O3 system which seems pretty useful to stone =
ware
glazers. Fancy having a glaze which melts at such a low temperature, think =
of
the power and fuel savings. Well chums, I formulated it out of whiting, =
flint
and alumina hydrate which were the colosest I could get to the pure oxides =
and
fired it to cone 8 flat 9 over. Not the prettiest glaze I have ever made. =
No,
just a friable white powder. Now had I made it out of the mineral =
compositions
shown in the chart it would have melted. What are those minerals. On the =
chart
is says Tridymite, Anorthite and Pseudowollastonite. You ever heard of =
these?
Can you buy them? In fact I ran a whole series of tests combining minerals =
which
you can buy at the ceramic supply house. The best was from Wollastonite, =
Kaolin
and Whiting. This fused and gave an indication that with the addition of a
supplementary flux (I would use Soda Felspar) it would be an excellent =
glaze.
Now methinks there is some confusion about what is intended and what can be
interpreted.
My confusion arises because there is another term, not often seen, which is
=22DEFORMATION EUTECTIC=22 (sorry, not shouting) I emphasise because it is =
hard to
find anything about the concept and most books ignore it. At this point I =
refuse
to give a reference. A Deformation Eutectic is that point at which any =
mixture
of minerals or compounds will start to change shape under influence of its =
own
weight as it is heated. It could be assigned a temperature if the hot =
junction
of the thermocouple were embedded in the specimen.
So every time we use a cone or a bar which bends or droops we are employing =
the
Deformation Eutectic. We are not using the Thermodynamic Eutectic, defined =
as
that
=22=85.invariant (unique temperature, pressure, composition) point for a =
system at
which the phase reaction on the addition or removal of heat results in an
increase or decrease respectively of the proportion of liquid to solid =
phases,
without change of temperature......=22
Ernest M. Levin et al, =22Phase Diagrams for Ceramists=22 . P 6, note (17). =
Compiled
by the National Bureau of Standards, 1964 . American Ceramics Society. =
Columbus,
Ohio.
Sadly, though people talk and lecture about Glaze Chemistry the allied =
subject
of Glaze Physics seems a forgotten topic. Perhaps it should be revived.
Best wishes to all. Now I must go and load the kiln for another firing.
Ivor Lewis.
Ray Aldridge on thu 20 may 99
At 01:11 PM 5/18/99 EDT, you wrote:
>----------------------------Original message----------------------------
>To: clayart@lsv.uky.edu
>
>>From Ivor Lewis ivorredhill@yahoo.com.au
>
>
>between precise mineral compositions. For example, there is a point at 1100
>degrees C in the CaO-SiO2-Al2O3 system which seems pretty useful to stone
ware
>glazers. Fancy having a glaze which melts at such a low temperature, think of
>the power and fuel savings. Well chums, I formulated it out of whiting, flint
>and alumina hydrate which were the colosest I could get to the pure oxides
and
>fired it to cone 8 flat 9 over. Not the prettiest glaze I have ever made. No,
>just a friable white powder.
This is a fascinating subject. I know nothing about it, but it seems to me
that somewhere I read that these oxides must already have been melted
together and cooled for the "eutectic" to cause a remelt at the calculated
temperature. This would seem to be supported by the more-succesful
experiment of using wollastonite to fulfill some of the silica and whiting
requirement.
I know even less about metallurgy than glaze physics, but I seem to
remember that there are alloys that melt at a lower temperature than any of
the component pure metals-- but the initial melt has to proceed to a
temperature that causes one of these components to begin melting.
Could easily be wrong.
Ray
David Hewitt on thu 20 may 99
I am glad to see someone else interested in Eutectics. Perhaps you might
like to look at my web site:-
http://www.dhpot.demon.co.uk
Click on Pottery Techniques / Eutectics and Phase Equilibrium Diagrams.
Also you may be interested in the Matrix Glaze program which allows you
to plot your recipes onto a phase equilibrium diagram and relate this to
the eutectic points of the main oxides.
In message , I.Lewis writes
>----------------------------Original message----------------------------
>To: clayart=40lsv.uky.edu
>
>From Ivor Lewis ivorredhill=40yahoo.com.au
>
>Monday, 17 May 1999 Australian Central Standard Time
>
>Subject: Eutectic Quandary
>
>From time to time people dealing with ceramic problems use the term =
>=22Eutectic=22.
>This was done recently in a posting to Clayart and a recipe was given in
>molecular oxide terms with the statement that =22 this Seger forms a =
>eutectic at
>cone 03=22.
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP18 3DQ, UK. Tel:- +44 (0) 1633 420647
FAX:- +44 (0) 870 1617274
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Evan Dresel on fri 21 may 99
------------------
The trouble with trying to formulate a glaze at a eutectic composition
is that very slight changes in composition will move you away from the
eutectic and the melting temperature will increase rapidly. In addition
glazes on one side of a eutectic will tend to be very different from
ones on the other sides so it is hard to control your results.
Trydimite is a form of SiO2 like quartz and crystobalite. Anorthite is
a calcium feldspar and pseudowollastonite is, I believe another calcium
silicate. If you did your calculations correctly you should have been
able to come close. Make sure you convert everything to moles properly
-- don't forget the silica in the anorthite and wollastonite.
Don't know about the deformation eutectic term. It kind of makes me
cringe when someone uses a thermodynamic term for a process which
involves kinetic variables such as grain size, heat-work, etc.
Sounds like you are learning a lot.
-- Evan in W.Richland WA where the weather is pleasant
pedresel=403-cities.com
I.Lewis wrote:
=3E
=3E ----------------------------Original message----------------------------
=3E To: clayart=40lsv.uky.edu
=3E
=3E =3EFrom Ivor Lewis ivorredhill=40yahoo.com.au
=3E
=3E Monday, 17 May 1999 Australian Central Standard Time
=3E
=3E Subject: Eutectic Quandary
=3E
=3E =3EFrom time to time people dealing with ceramic problems use the term
=22Eutectic=22.
=3E This was done recently in a posting to Clayart and a recipe was given in
=3E molecular oxide terms with the statement that =22 this Seger forms a =
eutectic at
=3E cone 03=22.
=3E
=3E I accept the intent of the person who posted this information. Mr. =
Seger=92s
=3E recipes have always been a good starting point for a glaze composition, =
look
at
=3E the popularity of the Seger Cone 8 recipe. It underscores almost eighty
percent
=3E of all published cone 8 glaze recipes.
=3E
=3E Recently I became interested in the confusion which surrounds the use of=
the
=3E term =22Eutectic=22. I use the definitions found in the American =
Ceramics Society
=3E book of Phase Diagrams for Ceramists. The large diagrams which are on =
the wall
=3E in front of me convey information which shows that a eutectic is a
relationship
=3E between precise mineral compositions. For example, there is a point at =
1100
=3E degrees C in the CaO-SiO2-Al2O3 system which seems pretty useful to =
stone ware
=3E glazers. Fancy having a glaze which melts at such a low temperature, =
think of
=3E the power and fuel savings. Well chums, I formulated it out of whiting, =
flint
=3E and alumina hydrate which were the colosest I could get to the pure =
oxides
and
=3E fired it to cone 8 flat 9 over. Not the prettiest glaze I have ever =
made. No,
=3E just a friable white powder. Now had I made it out of the mineral =
compositions
=3E shown in the chart it would have melted. What are those minerals. On the=
chart
=3E is says Tridymite, Anorthite and Pseudowollastonite. You ever heard of =
these?
=3E Can you buy them? In fact I ran a whole series of tests combining =
minerals
which
=3E you can buy at the ceramic supply house. The best was from Wollastonite,
Kaolin
=3E and Whiting. This fused and gave an indication that with the addition of=
a
=3E supplementary flux (I would use Soda Felspar) it would be an excellent =
glaze.
=3E
=3E Now methinks there is some confusion about what is intended and what can=
be
=3E interpreted.
=3E
=3E My confusion arises because there is another term, not often seen, which=
is
=3E =22DEFORMATION EUTECTIC=22 (sorry, not shouting) I emphasise because it =
is hard to
=3E find anything about the concept and most books ignore it. At this point =
I
refuse
=3E to give a reference. A Deformation Eutectic is that point at which any =
mixture
=3E of minerals or compounds will start to change shape under influence of =
its own
=3E weight as it is heated. It could be assigned a temperature if the hot =
junction
=3E of the thermocouple were embedded in the specimen.
=3E
=3E So every time we use a cone or a bar which bends or droops we are =
employing
the
=3E Deformation Eutectic. We are not using the Thermodynamic Eutectic, =
defined as
=3E that
=3E
=3E =22=85.invariant (unique temperature, pressure, composition) point for a=
system at
=3E which the phase reaction on the addition or removal of heat results in =
an
=3E increase or decrease respectively of the proportion of liquid to solid =
phases,
=3E without change of temperature......=22
=3E
=3E Ernest M. Levin et al, =22Phase Diagrams for Ceramists=22 . P 6, note =
(17).
Compiled
=3E by the National Bureau of Standards, 1964 . American Ceramics Society.
Columbus,
=3E Ohio.
=3E
=3E Sadly, though people talk and lecture about Glaze Chemistry the allied =
subject
=3E of Glaze Physics seems a forgotten topic. Perhaps it should be revived.
=3E
=3E Best wishes to all. Now I must go and load the kiln for another firing.
=3E
=3E Ivor Lewis.
Evan Dresel on sun 23 may 99
Ray is mostly correct -- you have to get the solids to melt which will
be at a higher temperature than the eutectic point. I believe it is a
little more complicated than getting up to the melting point of the
endmember solids though. Otherwise we would all have to fire to the
point where flint melts. I'm not up on the details but the particle
size, density of packing, etc. have an influence. For example,
microscopically, the point of an angular particle squeezed up against
another will start to react first, then you get a mixture of a different
composition with a lower melting point which reacts with the rest of the
grains, so you have more melt, and so on.
-- Evan
Ray Aldridge wrote:
>
> I know even less about metallurgy than glaze physics, but I seem to
> remember that there are alloys that melt at a lower temperature than any of
> the component pure metals-- but the initial melt has to proceed to a
> temperature that causes one of these components to begin melting.
>
> Could easily be wrong.
>
> Ray
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