Jorge Nabel on sat 29 jul 06
Hola Clayarters,its me again from this south of the map asking something
clay related. A couple of years ago a potter gave me "a tip" of an eutectic
being formed with 2/3 potash feldspar and 1/3 sodium feldspar for a ^6 glaze.
Im trying to make some Currie grids, and in another book they say
the eutectic
is being formed exactly the opposite, 1/3 potash 2/3 sodium feldspar.
Anyone has an idea of this? I made a button test of both and dont see clearly
an eutectic appearing there.
Im trying to make some Currie grids ^6 glazes with no frits ,no boron
and no Neph Sye,
so this eutectics could be very important .
Hope someone has this clear.
Haya paz.Jorge en Buenos Aires
Lynne and Bruce Girrell on sun 30 jul 06
Jorge in Buenos Aires wrote:
> A couple of years ago a potter gave me "a tip" of an eutectic
> being formed with 2/3 potash feldspar and 1/3 sodium feldspar for a ^6
glaze.
> Im trying to make some Currie grids, and in another book they say
> the eutectic is being formed exactly the opposite, 1/3 potash 2/3 sodium
feldspar.
A eutectic mixture is one that provides the _lowest_ melting point possible
with the given constituents.
So, with soda spar and potash spar, there will be some combination of them
that will produce the lowest melting point. A simple one axis line blend
will produce that answer if that's really what you want to know.
But I don't think that's what you're really looking for. What you want is a
good ^6 glaze, not the lowest melting point glaze, right? And I think that
your choice of whether to use a soda spar or potash spar will more likely be
driven by other considerations. Maybe not. I'm not sure what is driving your
decisions.
A Currie tile will reveal the eutectic combination of the fluxes that you
choose along with silica and alumina in various proportions. It shows up as
a line of glazes that are glossier than the others. Toward the A corner the
glazes will become more matte and toward the D corner the glazes will become
more matte. The glossier glazes are the ones with the lower melting points.
Having just mixed a set of glazes for a Currie test yesterday, I understand
why you might be reluctant to mix up an additional 35 glazes, but I feel
that you would be best served if you make the effort to make tiles based on
both combinations, one with corner C as 1/3 Na, 2/3 K and the other with
corner C as 2/3 Na, 1/3 K. While you're at it, I suggest that, since you
will have plenty of glaze available, you should consider making more than
one tile for each glaze set. The little squares sometimes don't represent
larger areas well and seeing more than one sample of each combination is
instructive. Also, multiple tiles allows you to see if result vary depending
on placement in the kiln.
Bruce "hope I haven't confused things further" Girrell
Ivor and Olive Lewis on mon 31 jul 06
Dear Jorge Nabel,
Interesting question.
<formed with 2/3 potash felspar and 1/3 sodium felspar for a ^6 =
glaze.......and in another book they say the eutectic
is being formed exactly the opposite, 1/3 potash 2/3 sodium felspar. =
Anyone has an idea of this? I made a button test of both and don't see =
clearly an eutectic appearing there.>>
Now Bruce Girrell says that <the _lowest_ melting point possible with the given constituents.>> which =
is more or less the definition of Eutectic so both suggested mixtures =
cannot be Eutectic in nature.
I think some confusion arises from the fact that it is difficult to say =
which specimen is the one with the lowest melting point when all have =
solidified when you do line or matrix blends. Just what information is =
contained in a series of fired specimen tiles??? I do not accept that =
Glossy glazes are an indication of, to coin a word, "Eutecticism" They =
tell us that total melting has happened and cooling has been =
sufficiently fast to prevent crystal growth so a Glass has formed. The =
Matt samples tell us that the cooling rate was sufficiently slow to =
allow some samples to throw out crystals or that some of the samples =
are immature. To "Know" what the melting point was needs a facility =
called a "Differential Thermal Analyser" to determine melting and =
freezing points.
That being said, there are sources of information called "Phase Diagrams =
for Ceramists" which give us useful and reliable information. J. F. =
Schairer compiled a diagram which combined Silicon Dioxide with two =
minerals, one an orthorhombic Potassium Alumino Silicate and the other, =
Carnegieite, a Silica deficient Felspar. His blends yielded crystalline =
product that included Soda Felspar and Potash Felspar. There is a =
region where these minerals occupy neighbouring fields and the division =
between them, called a "Join" has a temperature range from 1150 +/- 20 =
deg C to 1020 +/- 5 deg C. I think this is what Ian Currie calls a =
"Eutectic Valley" .
Using Schairer's Phase Diagram I might suggest the unique ( there can, =
by definition, be only one) Eutectic mixture between your Soda Felspar =
and Potash Felspar would contain 45% Soda Felspar and 55% Potash Felspar =
with a predicted melting point of 1075 Deg Celsius.
Now I find this a most interesting result for the following reason. It =
is almost identical in composition to one of the popular Rocks (Rocks =
note, not Minerals) in the studio potters list of ingredients. One which =
I have pointed out in the past to clayart readers as the only substance =
we use that would introduce a Eutectic Melting Event into a recipe. It =
is an ideal ingredient for people firing to cone 6. Were I thinking of =
going along that path I would use it to the exclusion of Borate =
compounds or Frits. This favoured ingredient.....Nepheline Syenite.
Jorge, I hope you are successful.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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