Tom Buck on fri 9 feb 07
Greetinsgs, Lili:
You write as spritely as ever, glad you are keeping healthy, etc.
You wonder if the notion of eutectic is of use to potters. Here is my
take, arising from my efforts to design new glazes and fix old ones.
Firstly, a bit of background: the word eutectic comes to us from
metallurgists who used the term to tell each other the temperature at
which two different metals would form an alloy via going from solid state
to liquid state on heating, and then staying as an alloy when cooled.
Some time ago ceramic scientists/engineers began using eutectic to
describe both the temperature and the composition of glasses formed from
two or three components, generally Groups 1 & 2 metal oxides combining
with the glass-formers, silica, alumina, and boria (SiO2, Al2O3, B2O3).
These specialists developed a graphical way of presenting eutectic
data; their diagrams became known as phase diagrams. Over the years many
researchers reported on glaze mixes that produced glasses at temperatures
below the melting points of the components, and so some detailed ceramic
phase diagrams emerged.
W.G. Lawrence, in his book, "Ceramic Science for The Potter",
cited some eutectic mixtures and their corresponding melting temperatures.
For example, potassium oxide, K2O when combine with boron oxide, B2O3,
formed a eutectic at 770 oC with this makeup: 67% by weight B2O3 and 37%w
K2O. and K2O & silica (SiO2) formed a eutectic at 750 oC with 33%w K2O and
67%w SiO2. Lawrence also cited three-components systems, eg, 69%
SiO2 + 23%K2O + 8% Na2O (sodium oxide) melted at 540 oC.
Ok, we have details of eutectic mixes and their corresponding melt
temperatures, So, how do we use this information.
Ivor Lewis gave one example: he cited the eutectic temperature,
1170 oC, and mixture for CaO-Al2O3-SiO2 (percentage by weight of each) and
then quoted four different recipes that should produce this eutectic. Ivor
wanted to show that a simple three-part mix of standard materials would
yield good glass at Orton Pyrometric Cone 6. With today's widespread use
of computers and glaze calculation programs for them, it is fairly easy to
design such recipes from scratch.
Also, the phase diagram data (similar to that quoted by Lawrence
in his book) offers a good guide to designing a new glaze for a specific
temperature/cone. This is especially valuable for designers of low-fire
glazes, although mid-fire and high-fire are not ignored. eg, what is the
eutectic for 38%CaO + 62%SiO2? Off-hand I'd guess 1150 oC. Well, I'd be
wrong: it is 1436 oC, well above Cone 10/12. The eutectic data will steer
me away from combinations that would not work at the Cone level at which
my claybody reaches maturity.
Friend Jon Singer has been doing some very interesting glaze
designs of late, and he has been forthcoming with some background
details. Yet, I suspect he has made use of eutectic information for his
successful glazes. perhaps he will comment.
so there you go, Lili...ceramic eutectics is allowing us to find
new mixes by glzcalc and a few tests.
til later. be well. peace. Tom
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
Marcia Selsor on fri 9 feb 07
Tom,
Were you a teacher? The clarity of your explanantions are brilliant.
I always enjoy reading them.
I hope to see you in Loiusville.
Take care
Marcia Selsor
http://marciaselsor.com
Hank Murrow on fri 9 feb 07
On Feb 8, 2007, at 10:06 PM, Tom Buck wrote:
> eg, what is the
> eutectic for 38%CaO + 62%SiO2? Off-hand I'd guess 1150 oC. Well, I'd be
> wrong: it is 1436 oC, well above Cone 10/12. The eutectic data will
> steer
> me away from combinations that would not work at the Cone level at
> which
> my claybody reaches maturity.
Dear Tom et al;
Very nice post of which the above is a small but important sample. I
have found that most potters do not know that Calcium has a higher
melting point than Silica! So the useful germ contained in eutectic
theory is that one can arrange for a specific combination of network
former (Si, B, P) and network modifier (Li, Na, K, Ca, Mg, Ba, etc.) in
which both materials will melt at a lower temperature than either would
alone. That is the 'secret' in the theory and practice of eutectics,
imho. A very useful one that should be better understood. Thanks for
your very complete post this morning.
Cheers, Hank
www.murrow.biz/hank
Ivor and Olive Lewis on sat 10 feb 07
Dear Hank Murrow,
In order to prove that is correct I would be most pleased if you would =
test the Cardew Recipe of Silica, Whiting and Kaolin that I provided, =
firing it to no higher than Orton Cone 6, which is about thirty degrees =
below the predicted fusion point.
Sincere regards.
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on sat 10 feb 07
Well done Tom,
You obviously accessed Lawrence's first edition of "Ceramic Science for =
the Potter". I am wondering why the information you speak of was deleted =
from the second edition ?
Best regards,
Ivor
Hank Murrow on sun 11 feb 07
On Feb 9, 2007, at 11:21 PM, Ivor and Olive Lewis wrote:
> Dear Hank Murrow,
>
> In order to prove that is correct I would be most pleased if you would
> test the Cardew Recipe of Silica, Whiting and Kaolin that I provided,
> firing it to no higher than Orton Cone 6, which is about thirty
> degrees below the predicted fusion point.
Dear Ivor;
I am probably not the person you meant this directive for, as i do not
fire to cone 6......... only cone 10_11.
Cheers, Hank
www.murrow.biz/hank
John Baymore on sun 11 feb 07
Nice post, Tom (as usual )
Another little thought about eutectics:
In formulating working glazes based on eutectics information, it is very
unusual that you would use the specific eutectic point for the basis of a
working formulation. To use this "bottom of the valley" point can result
in a glaze that is very finnicky.
When looking at phase diagrams to evaluate glaze possibilities, you might
want to carefully look at the "steepness" of the temperature gradients
surrounding the eutectic point in question. (Those are the lines that
look like a topographical map.) The steeper the gradient at a particular
mixture relationship, the more "fussy" the resultant mixture will be to
variations in firing temperature or to errors in materials measuerment in
the batch.
The best areas to start to work with are areas NEAR the eutectic point and
that have a somewhat gradual "slope". This helps give you both a
reasonable firing range and latitude in weighing errors.
And as soon as you add other materials into a composition....... you also
alter the exact characteristics of that carefully studied phase diagram.
You probably want to use them as a general guide only for most uses.
best,
................john
John Baymore
River Bend Pottery
Wilton, NH
http://www.JohnBaymore.com
JBaymore@compuserve.com
Ivor and Olive Lewis on tue 13 feb 07
Dear John Baymore,=20
"Finniky" ? An unusual term to apply to a glaze formulation.
Perhaps you could give me an objective description based on your =
observations of fired samples of known eutectic compositions.
For example, have you fired any of the recipes I have provided ? Your =
results would provide welcome information to our knowledge base.
By the way, based on your analysis, it might be unwise to include a clay =
as an ingredient in any glaze formulation, especially if other =
ingredients contain Sodium or Potassium. On the other hand, temperature =
gradients around the 1170 point in the CaO-Al2O3-SiO2 system are quite =
shallow. So the prognosis for the formulations I have given seems quite =
promising.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Ivor and Olive Lewis on tue 13 feb 07
Dear Hank Murrow,=20
Be adventurous, buck the trend and fire a sample anyway. You might get a =
pleasant surprise.
You could also test the version made with Free Silica, Free Alumina and =
Whiting. The Chemistry is different. I wonder what can be said about the =
effect of this on the result?
Best regards,
Ivor
Hank Murrow on tue 13 feb 07
On Feb 12, 2007, at 5:08 PM, Ivor and Olive Lewis wrote:
> Dear Hank Murrow,
>
> Be adventurous, buck the trend and fire a sample anyway. You might get
> a pleasant surprise.
Dear Ivor;
Perhaps if I get my gas test kiln up and running again I might, but for
now a 28 cuft load to cone six for one test would be too extravagant.
Cheers, Hank
www.murrow.biz/hank
Ivor and Olive Lewis on thu 15 feb 07
Dear Hank Murrow,=20
I appreciate that Hank. Have a similar problem. Which is why my first =
request was to those who fire to cone 6. No reason not to fire a sample =
to you usual Cone#. Ron is going to do that for me.
You could get the same oxide proportions by blending and testing CaCO3 =
33.01%, Alumna Hydrate 17.72%, Silica 49.72%. Which would be useful as a =
comparison.
Have a good weekend.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
=20
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