Autumn Downey on fri 3 jul 98
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.
Are most working glazes near the eutectic or the eutectic plus more silica?
Someone said (I think) that the best clear glazes were at the eutectic point?
Is the main importance of knowing about eutectics the understanding that
melting points aren't linear? And that there is the possibility of mixing
or overlapping two seemingly stable glazes and having them run madly because
of this?
Is there any predictability of the eutectic point or is it a matter of doing
blends to find it?
Are eutectics roughly a part of limit formulas or do they need to be
considered separately?
Thanks,
Autumn Downey
downeya@internorth.com
Yellowknife, NWT, Canada
Craig Martell on sat 4 jul 98
At 01:30 PM 7/3/98 EDT, you wrote:
>----------------------------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.
>Are most working glazes near the eutectic or the eutectic plus more silica?
>Someone said (I think) that the best clear glazes were at the eutectic point?
>Is the main importance of knowing about eutectics the understanding that
>melting points aren't linear? And that there is the possibility of mixing
>or overlapping two seemingly stable glazes and having them run madly because
Hi:
I hope my wrist holds out so I can voice an opinion about this!
At present, I'm working on some new cone 6 glazes for my wife and also a
supplier who wants to buy some new bases if I can come up with anything
interesting. That's always debatable isn't it???
I'm looking for all kinds of eutectic points at cone 6 so I can add more
silica and alumina to make real durable, functional glazes for this temp.
The first approach is to line blend stuff that is known to form eutectics.
Feldspar and calcium will form a eutectic as will boron and barium for
example. I'm doing flux triaxials and line blends of everything I can think
of, including frits and slip clays. My first considerations are visual
examination of the melted blends to see what proportion of materials will
give the most active melt. Then, I can start blending in the alumina and
silica with the idea that the more aggressive melts can take more Al and Si,
making harder glazes. The other big consideration is color response and
texture which will be influenced by the fluxes present in the glaze. Once I
have glazes that look promising, the next step is to put them into the Seger
formula to see where they are in terms of the suggested limits for that
temp. Anyway, this is sort of a round about answer to your question Autumn.
It may well be true that a lot of good clear glazes are based on a eutectic
point. One would certainly stand a better chance of clarity and good fit
when the proper amounts of Al and Si were blended in. I don't think all
glazes are based on eutectics. Perhaps ash glazes and calcium matts are an
example of this. The calcium in the glaze is much more than needed for a
eutectic melt with feldspar and the main consideration in the matts is
devitrification and crystal development to make the glazes matt. Ash glazes
have high amounts of calcium to promote stringing and separation. The
calcium level in ash glaze Seger formulas is usually about .8 to .9, which
is beyond the magic amount for a spar-calcium eutectic. There are lots of
other examples of eutectic and non eutectic glazes. Shall I go on?? OK, I
can hear you guys snoring so I'll quit.
Gee, my wrist feels pretty good. Maybe typing is good therapy?
later, Craig Martell-Oregon
David Hewitt on sun 5 jul 98
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.
>
For me an understanding of eutectics and phase equilibrium diagrams has
been fundamental to my understanding of glazes.
>Are most working glazes near the eutectic or the eutectic plus more silica?
>Someone said (I think) that the best clear glazes were at the eutectic point?
To try and answer these points may I start by first considering a simple
line blend of two substances, A and B (which could be Al2O3 and SiO2),
and lpot the temperature at which the different proportions of A nd B
melt on a graph. The vertical axis is temperature and horizontal axis
has A 100% on the left decreasing to 0% on the right, and B 100% on the
right decreasing to 0% on the left. The graph will show a temperature
for each mixture at which the mix becomes all liquid. Typically this
line decreases to a point and then increases. This point is the eutectic
temperature for that mix and all components have gone into the liquid
phase. For a clear shiny glaze with this mix we need to fire just above
this temperature.
In practice we will have decided what temperature or cone we wish to
fire at and will have selected a recipe which will give us this clear
shiny glaze. If we then increase or decrease the silica in this recipe,
we are going to continue to fire it to the same temperature or cone.
Let us then draw a horizontal line at this eutectic temperature on our
graph. If we then consider a mix to the left of this point, and firing
to the same temperature, not all components of the mix will have melted.
You will have A + Liquid. If the mix is to the right of the eutectic
point you will have B + Liquid. The A + Liquid will look different to
the B + Liquid when cooled. So according to what effect you want with
your glaze, eg alumina matt you will want a mixture that that will have
alumina which has solidified out before the whole mixturehas solidified.
This is a very much simplified explanation of what I think happens and
it only considered two components. A much better way is to consider your
glaze as having three components, basic oxides, amphoteric oxides and
acidic oxides. If you have an analysis of your glaze grouped in this way
you can plot its position onto a triaxial phase equilbrium diagram. If
you were to create a series of recipes using the same materials but
varying the proportions so that the resultant points on the triaxial
graph covered an good area of the graph (60% to 80% acidic, 10% to 30%
basic and 5% to 15% amphoteric) you would clearly find the area where
you would get your clear transparent glaze and, surrounding this, the
areas where you would find your various matt and crystalline glazes.
Mike Bailey and I wrote a series of articles on this for Ceramic Review
some years ago now. Copies of the texts and graphs etc. are available in
an A4 booklet form. Details of this availability are included in my web
page given below.
>Is the main importance of knowing about eutectics the understanding that
>melting points aren't linear? And that there is the possibility of mixing
>or overlapping two seemingly stable glazes and having them run madly because
>of this?
>Is there any predictability of the eutectic point or is it a matter of doing
>blends to find it?
>Are eutectics roughly a part of limit formulas or do they need to be
>considered separately?
I consider them to be complementary to each other. Limit formulae are
based on a lot of practical experience for which an understanding of
eutectics can provide some basic explanation.
>
>Thanks,
>
>Autumn Downey
>downeya@internorth.com
>Yellowknife, NWT, Canada
>
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
Mark Mondloch on sun 5 jul 98
Craig,
I'm not snoring at all. Are the known eutectic points, such as the spar-calciu
that you mention published somewhere?
Sylvia
Craig Martell wrote:
> ----------------------------Original message----------------------------
> At 01:30 PM 7/3/98 EDT, you wrote:
> >----------------------------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.
>
> >Are most working glazes near the eutectic or the eutectic plus more silica?
> >Someone said (I think) that the best clear glazes were at the eutectic point?
> >Is the main importance of knowing about eutectics the understanding that
> >melting points aren't linear? And that there is the possibility of mixing
> >or overlapping two seemingly stable glazes and having them run madly because
>
> Hi:
>
> I hope my wrist holds out so I can voice an opinion about this!
>
> At present, I'm working on some new cone 6 glazes for my wife and also a
> supplier who wants to buy some new bases if I can come up with anything
> interesting. That's always debatable isn't it???
>
> I'm looking for all kinds of eutectic points at cone 6 so I can add more
> silica and alumina to make real durable, functional glazes for this temp.
> The first approach is to line blend stuff that is known to form eutectics.
> Feldspar and calcium will form a eutectic as will boron and barium for
> example. I'm doing flux triaxials and line blends of everything I can think
> of, including frits and slip clays. My first considerations are visual
> examination of the melted blends to see what proportion of materials will
> give the most active melt. Then, I can start blending in the alumina and
> silica with the idea that the more aggressive melts can take more Al and Si,
> making harder glazes. The other big consideration is color response and
> texture which will be influenced by the fluxes present in the glaze. Once I
> have glazes that look promising, the next step is to put them into the Seger
> formula to see where they are in terms of the suggested limits for that
> temp. Anyway, this is sort of a round about answer to your question Autumn.
>
> It may well be true that a lot of good clear glazes are based on a eutectic
> point. One would certainly stand a better chance of clarity and good fit
> when the proper amounts of Al and Si were blended in. I don't think all
> glazes are based on eutectics. Perhaps ash glazes and calcium matts are an
> example of this. The calcium in the glaze is much more than needed for a
> eutectic melt with feldspar and the main consideration in the matts is
> devitrification and crystal development to make the glazes matt. Ash glazes
> have high amounts of calcium to promote stringing and separation. The
> calcium level in ash glaze Seger formulas is usually about .8 to .9, which
> is beyond the magic amount for a spar-calcium eutectic. There are lots of
> other examples of eutectic and non eutectic glazes. Shall I go on?? OK, I
> can hear you guys snoring so I'll quit.
>
> Gee, my wrist feels pretty good. Maybe typing is good therapy?
>
> later, Craig Martell-Oregon
--
Mark & Sylvia Mondloch
Silver Creek Pottery & Forge
W6725 Hwy 144
Random Lake, WI 53075
mondfam@execpc.com
http://www.execpc.com/~mondfam
Craig Martell on mon 6 jul 98
>----------------------------Original message----------------------------
>Craig,
>I'm not snoring at all. Are the known eutectic points, such as the
spar-calciu
>that you mention published somewhere?
Hi:
Yes, some of them are published in Hamer's dictionary and I think there is
some info on this in Ceramic Glazes by Parmelee. You have to kind of dig
around in the Hamer book to find some of this stuff though. There is a lot
of info in Hamer's book on eutectic mixtures and theory plus some
significance for potter's etc.
If you wanna know, the eutectic mix for calcium and potash spar is:
potash spar 70%
calcium carb. 30%
This is an approximate proportion and you have to dink around with fusion
tests and line blends to fine tune this stuff. Speaking of that, I usually
do fusion button line blends of spars and whiting in 1/2 inch porcelain lab
crucibles. You mix the two ingredients dry and pack them tightly into the
crucible. Then, you tap them out onto a tile for firing. There are
pictures of fusion button tests in the Ceramic Spectrum by Robin Hopper, and
some info on eutectics too. Anyway, the fusion buttons will give a much
clearer picture of fusion rate than simply applying the mix flat, as one
would a glaze.
gotta go and chase my dog, Craig Martell-Oregon
David Hewitt on mon 6 jul 98
In message , Mark Mondloch writes
>----------------------------Original message----------------------------
>Craig,
>I'm not snoring at all. Are the known eutectic points, such as the spar-calci
>that you mention published somewhere?
>Sylvia
Yes. The American Ceramiuc Society has published a mass of data on this
subject. Look out for 'Phase Diagrams for Ceramists'. I have had access
to a Second Edition dated 1969. I do not know if there have been later
editions.
--
David Hewitt
David Hewitt Pottery ,
7 Fairfield Road, Caerleon, Newport,
South Wales, NP6 1DQ, UK. Tel:- +44 (0) 1633 420647
Own Web site http://www.dhpot.demon.co.uk
IMC Web site http://digitalfire.com/education/people/hewitt.htm
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