Ivor and Olive Lewis on thu 30 aug 07
Well Friends, I have read that article by Dave Finkelnberg, "How Glazes =
Melt".
I had mixed feelings when I read it.
First, I am very pleased that he gives the ingredients of the 1170 =
Eutectic in the System CaO-Al2O3-SiO2 as being Wollastonite, Anorthite =
and Tridymite. This is not done by other authors in the glazing field.
But I think he lets his readers down with his panel "A Eutectic is your =
Friend" by failing to give analyses of eutectic points for other Alkali =
and Alkali Earths elements. Which ingredients should we select, using =
his example to guide us, if we wished to exploit that information ?
Then there is the analogy Dave uses to explain the nature of a Eutectic. =
His example is the depression of the freezing point of water when it is =
combined with common salt, Sodium chloride. A solution containing 23.3% =
of Sodium chloride in water freezes at minus 21 degrees Celsius. But the =
resulting substance is not a mixture Ice and Sodium Chloride. It is =
frozen water and Sodium chloride di-hydrate, NaCl.2H2O. The complete =
phase diagram NaCl-H2O can be found in P.J Durrant, "General and =
Inorganic Chemistry" p162. 1956.=20
A mixture of sub-zero water and common salt at the same temperature =
better represents the behaviour of a glaze recipe. Melting events in =
such systems are best described by Kingery and his co-authors in =
"Introduction to Ceramics", Ch 10.=20
Without these two panels Dave Finkelnberg presents a good simplification =
of complex events, an excellent piece of work.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
John Britt on sun 2 sep 07
Ivor,
You posted this a while back but no one has responded. I was happy to see
that David took the time to write his article and that CM published. He
did a great job writing on this difficult subject and I did not want to
see it pass without some discussion.
But I had some problems with the artile too, in that I am a practical
application freak.
I am with you on your point: "Which ingredients should we select, using
his example to guide us, if we wished to exploit that information?=94
When does a potter ever use Alumina as a glaze ingredient? When are there
ever only three oxides in a glaze?
Glazes are very complex especially with the ingredients potters use. As
soon as you throw feldspar or clay in to a glaze recipe (which is almost
every time) you have about nine oxides present, which takes you out of the
available charts. So knowing that eutectics exist is a lot like flying to
London. It is interesting to know about the Bernoulli principle or
Newton=92s third law but does it help you get to London? No.
What helps you get to London is a ticket and directions to the airport.
The only time I have ever read of a practical use of eutectics and glazes
is the reference to lithium and a cone 1 eutectic with shinos. But I have
never tested this hypothesis. All my cone 10 shinos (feldspar and clay)
melt with or without the eutectic with lithium feldspars.
I have heard the term used when a glaze runs and someone says that =93it
must have formed a eutectic=94, or =93the two glazes form a eutectic=94, but=
there is never any specific practical way to use this information.
My understanding is that here are often multiple eutectics and so knowing
when adding some ingredient will push you up or down the trough is
unknown. It just seems that without specific charts for all the
ingredients we use the concept is more academic than practical.
Just my opinion,
John Britt
www.johnbrittpottery.com
Edouard Bastarache Inc. on sun 2 sep 07
John,
Smart told me 2 days ago on the webcam he does not
use "eutectics" in his daily job, he does not find
use in
formulating glazes for the Gien factory, he has so
far
been responsible for the glazing/firing of 30
millions
pieces at the factory.
This corroborates your opinion on their use.
Gis la revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
http://www.ceramique.com/librairie/
http://www.flickr.com/photos/30058682@N00/
http://myblogsmesblogs.blogspot.com/
Edouard Bastarache Inc. on sun 2 sep 07
"When does a potter ever use Alumina as a glaze
ingredient? When are there
ever only three oxides in a glaze?" (John Britt)
I use alumina hydrate to make white shinos.
Gis la revido,
(A la revoyure)
Edouard Bastarache
Spertesperantisto
Sorel-Tracy
Quebec
http://perso.orange.fr/smart2000/livres.htm
http://www.pshcanada.com/Toxicology.htm
http://www.ceramique.com/librairie/
http://www.flickr.com/photos/30058682@N00/
http://myblogsmesblogs.blogspot.com/
Ivor and Olive Lewis on mon 3 sep 07
Right John, I am with you right down the line. These are very complex
issues. Dave has made a valuable contribution.
It would be a rare event for there to be a eutectic represented in any
mixture of individual materials we use. I have only been able to find
one natural example of a popular glaze ingredient, that of Nepheline
syenite. It is a Rock, a mixture of minerals , that has a mixed
crystalline (note"crystalline" not "vitreous") structure that leads me
to that conclusion.
You may recall my suggestion, in a thread which ran about six months
or so ago, that people should try to melt the composition of Clay,
Silica and Whiting suggested by Michael Cardew . This is supposed to
represent the CaO-Al2O3-SiO2 eutectic at 1170 deg C. I asked cone six
people to try it out. Almost no response. I am able to get this to
melt at Cone 8 and I can explain why.
When you go to four ingredients names change. The lowest temperature
at which the crystalline phases can coexist with the melt (which
defines the term "Eutectic") now becomes a "Quaternary Piercing Point"
In many simple systems there are two or more Eutectic reactions. This
is because it is possible for chemical reactions to create several
compounds.
Sodium oxide and Silicon dioxide can combined to make two kinds of
Sodium silicate, one which melts at 1089 deg C, NaO2.SiO2 and one
melting at 874 deg C Na2O.2SiO2. These together can form a eutectic
with a melting point of 837 deg C. There is another Eutectic between
the lower melting point silicate and Quartz with a melting point of
789 deg C.
It is important to know the names of the minerals or compounds that
must be there, in the slop recipe, because without their presence
melting will just not happen. A mix of Whiting, Quartz and pure
Alumina of the correct proportions for the 1170 deg Eutectic melting
event refuses to melt even at cone 9. This is where Dave F made a most
valuable public contribution, by naming the correct ingredients,
Anorthite, Wollastonite and Silica.
If you want the finest description of the processes that bring those
compounds of a glaze recipe to a molten vitreous state there is none
better than that written by Prof David Kingery and his co-authors in
"Introduction to Ceramics", Ch 10, pp 448-513. As I recall, the term
"Eutectic" never enters into the discussion.
My interest in Eutectics arises out of the work I did with Dr Owen J.
Dunmore more than fifty years ago. We were working on the phase
changes that happen in steel when it is heat treated. My task was to
measure the temperature at which those phase changes took place and
then do the measurements of magnetic susceptibility to estimate the
proportions of the phases present . Someone else did the
microstructure studies. It took about six months to collect data and
compile a Phase Diagram for each Steel composition.
Given that experience and a willingness to study the topic of
Eutectics I regard most claims as nonsense. To induce a eutectic by
introducing lithium oxide would only work if two ingredients were
added, provided silica was there in the form of Tridymite. You could
try it with Lithium Silicate, Li2O.2SiO2 and Spodumene. This eutectic
composition melts at 980 deg C. If Lithium Carbonate were used that
would melt at 723 deg C.
Thanks for getting in touch
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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