Jim Murphy on thu 10 apr 03
Hi Ivor,
"I'm listening" (ala Frasier).
Sure, Wollastonite, in glazes, as a substitute for Whiting (CaCO3) and a
portion of free silica has many benefits, such as: more-bang-for-your-buck
(about 1/2 of Whiting, by weight, goes out the kiln vent as CO2 gas); less
energy may be required since the SiO2 and CaO in Wollastonite react more
readily to form silicates, etc.
Come on though, glazes are getting too easy. We need a bigger challenge here
at Clayart (grin,chuckle,grin).
Let's discuss how Wollastonite, or more specifically CaO, along with Potash
Feldspar (high K2O) may be used in ceramic bodies to reduce maturation
temperature and improve vitrification.
Why ? Because I read a little note about Feldspar in Ceramic Industry
Magazine's 2003 Materials Handbook regarding feldspar use in bodies which
says, "The softening range of the body decreases, and refractoriness
increases with increased K2O, at the expense of NaO and CaO, in the feldspar
used. With semivitreous bodies which do not contain any CaO, soda feldspar
compositions mature slightly sooner than potash feldspar. When lime is added
the situation is reversed. The reason: lime causes a sharp increase in the
fusibility of potash feldspar but not of soda spar."
I'm interested in that last part about lime causing a sharp increase in the
fusibility of potash feldspar. Now, I'm no expert here, so maybe you and
some others can help piece things together with me. Hopefully, I'm not
oversimplifying - here it goes.
You mentioned the CaO-Al2O3-SiO2 Phase Equilibrium Diagram. My sources tell
me the CaO-Al2O3-SiO2 eutectic composition by weight (62%w SiO2, 14%w Al2O3,
24%w CaO) melts somewhere around 1160 C.
I believe there is also a eutectic for K2O-CaO-SiO2 (60%w SiO2, 30%w K2O,
10%w CaO) melting somewhere around 950 C.
I believe a fair share of glaze chemistry and material selection is based,
in part, on these eutectics.
In ceramic bodies - I think - eutectics may still apply somewhat. Eutectic
points are defined by composition - not by temperature, however, other
physical parameters may takeover, such as particle-size diversity, whether
materials are crystalline or not. These physical parameters have an impact
on the melting temperature/behavior although the eutectic point does not
change.
I've read lime-rich clays can help strengthen earthenware bodies, presumably
due to the fusing action of CaO. I've read a collection by Seger of European
high-fire porcelain body formulas used at the end of the nineteenth century
had Limestone added - presumably for translucency and better vitrification.
(Serves tableware 6.5% limestone, Limoges Fine 1.6%,and Limoges Ordinary
1.3%)
I found this "Cone 8 Hotel China" recipe below in Robert Tichane's book,
Clay Bodies.
Hotel China (Cone 8)
Kaolin 35
Ball Clay 9
Feldspar 20
Flint 35
Whiting 1
It appears the Whiting may be used to source CaO to act along with the
feldspar and decrease the maturation temperature - I'm only guessing. My
goodness though - 35% Flint in a Cone 8 body - that seems like alot to me.
When I think of hotel china, I think of strong, fully vitrified durable
pitchers, plates, etc. The high amounts of Kaolin and Flint sure do make for
a refractory body. Does all that Flint in the body create its strength ? Is
it the extra free-silica that allows CaO and K2O to perform their magic
trick during firing ?
How about using CaO, via Wollastonite, to help vitrify and strengthen
midfire (Cone 4-6) ceramic bodies though? My conscience tells me 35% Flint
in a midfire (Cone 6) body may not be such a wise thing without resorting to
Na spar which opens another can of worms. I work with casting slip, so
greenware/fired strength and no warpage are key rather than body plasticity.
Is there a magic CaO-K2O ceramic body "eutectic" and/or CaO/(free Silica)
ratio that needs to be satisfied to allow CaO to cause that "sharp increase
in the fusibility of potash feldspar" in a midfire ceramic body ?
All theories, speculation or any midfire body recipes employing
Wollastonite, Whiting (CaCO3), etc. are welcome.
TIA,
Jim Murphy
iandol on sat 12 apr 03
Dear Jim Murphy,
To understand the behaviour of the ceramic compounds we use, you need to =
be aware that Equilibrium Eutectics are not composed of Molecular oxides =
in the was they are described in our text books about glazing. Kingery =
and his co-authors fully describe Sintering and Vitrification I cannot =
recall that they use the term "Eutectic" in that section. The also do a =
good number on the Microstructure of Ceramics so the origins of the =
strength is well known.
It is relatively simple to disprove the claim that the mixture of oxides =
you suggest (62%w SiO2, 14%w Al2O3, 24%w CaO) will melt at the =
temperature you give. Make up a sample from quicklime, quartz and =
alumina and fire it to Cone 6. Rapidly cooled, your sample should =
exhibit all the qualities of a glass if it is true that eutectic =
mixtures melt readily. My experience is this does not happen. I took the =
temperature beyond Cone 8 and I still have a friable powder.
The example from the K2O-CaO-SiO2 system is interesting but the same =
restrictions apply which would prevent this being used. Where do you buy =
Potassium Monoxide and what are its chemical properties? Heated in air =
it will react with both water and carbon dioxide to form Caustic Potash =
and Pot carbonate. The caustic will further react with carbon dioxide. =
Potassium Carbonate has a lower melting point than the Eutectic.
Your belief that "a fair share of glaze chemistry and material selection =
is based, in part, on these eutectics" is common throughout popular =
glaze literature. But it is an incongruous and erroneous concept if the =
Thermodynamic description of the term Eutectic is implied. The =
Scientific Definition (see ACS Literature) states that the mixture of =
substances in the system you are heating changes from solid to liquid at =
a single temperature with the addition of heat energy. If this were the =
case every kiln firing would be a disaster.
As I have said before, "Introduction to Ceramics" is a good place for =
all ceramic artists to start their technical and scientific education if =
they are to understand clays and glazes. Not necessary if you are just =
into the art side of it all, but essential if you wish to understand =
firing processes or you have to teach Glaze Chemistry.
CaO "Bang for Buck". "Down Under" Whiting is the cheaper alternative. =
Paid Au$32 this week for 5 kg of Wollastonite. Whiting is about Au$15 =
for 20 kg bag, Silica about the same. But I will use Wollastonite.
Must get back into the studio. Impatient to get the results from more =
than three hundred glaze samples. Load today, fire tomorrow, view on =
Tuesday.
Best regards,
Ivor=20
Jim Murphy on sun 13 apr 03
Ivor,
Thanks for the book reference (Introduction to Ceramics, 2nd Edition
W. David Kingery, H. K. Bowen, D. R. Uhlmann).
I'll check it out.
Best wishes,
Jim Murphy
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