iandol on sat 31 may 03

Dear Ron Roy,=20

The CaO-Al2O3-SiO2 1170 deg Celsius Eutectic can only be constructed if =
you have the following minerals. Recalculate the Oxide content using =
Wollastonite and Anorthite, with Silica to make up for not having =
Tridymite available.=20

There are several alternative recipes which can be constructed if you =
have all the potential ingredients to hand. Only one will melt at or =
near the precise temperature of the Eutectic Point.

The trick may be to use such things as Eutectics to add to the Cone 6 =
collection. This would seem to be an ideal candidate. But as you say =
<-interesting it would melt at 9-10 - expansion is right in the ball =
park.>> from which I infer you would not expect melting to occur.

I thank Michael Banks for enabling me to do my tests on this =
composition. He provided me with a sample of Anorthosite, a rock which =
contains a high proportion of Anorthite.

Best regards,

Ivor Lewis

Jim Murphy on sat 31 may 03

OK, OK, OK,

What if we approach this in a slightly different manner ?

Manuals for glaze calc programs seem to focus alot on just Flux Unity and
Si:Al ratios, especially for glaze durability and surface finish (matt,
glossy, etc.).

If the unity formula for this CaO-Al2O3-SiO2 eutectic @1170 C is

CaO 1.00*
Al2O3 0.32
SiO2 2.41

what can be said about the role played in fusion and/or melting power by the
apparent HIGH alkali-to-alumina ratios (CaO:Al2O3) with respect to
relatively low SiO2 ?

Are there any expectations or generalizations - with respect to melting
power, surface finish and durability - suggested by past research,
literature, and/or experience - if one uses glaze calc software to
formulate, looking at the ratio in THIS way - [(CaO:Al2O3):SiO2] ?

In other words, the focus is shifted, to capitalize on the strength of the
CaO-Al2O3-SiO2 system using the (alkali-to-Alumina ratio)-to-Silica ratio to
develop the desired glaze effect.

In general, is there any merit in using HIGH alkali-to-alumina ratios to
"create" better fusion in glazes and/or clay bodies at Cone 6 or Cone 10 ?

Best wishes,

Jim Murphy

Louis Katz on sun 1 jun 03

Hi Ivor, Ron,
I understand this principle, that you need the right minerals to form
the eutectic, but often I think in ceramics we come at eutectics from
the top, that is from above the liquidus down. If there is just a
little of a material that does make a liquid below the liquidus, in
this case 1170C and can dissolve or melt some of the CaO, Kaolin and
Silica, then sooner or later (perhaps much later) the whole mess will
be melted. I think this is why in studio pottery we often find glazes
melting above the temperature the mixture is expected to melt at, but
also often why it melts at all.

Also included in my understanding is that if you have some, say,
Soda/Alumina/Silica glass and some of the glass composed of the
eutectic composition we are discussing all mixtures of the two will
melt at a lower temperature than 1170. I think because of this the
question often becomes not just what temperature do you fire to, but,
"at that temperature how long does it take to reach or sufficiently
approach equilibrium to make all the stuff melt".

I have several times been amazed at the difference 1% of a frit will
make to the maturity of a claybody around cone 04. My assumption has
been that it allows many normally inactive (at 04) fluxes to enter
into the melt. Once in the melt they are become part of a much more
complicated system Alumina/Silica/Calcia/ Magnesia/Zinc Oxide/Soda
/Pottasia. Outside the melt, the talc probably has no impact on
vitrification.

I can't claim to know much about this subject matter, but have slogged
about half way through Introduction to phase Equilibria in Ceramics by
Bergeron and Risbud.

Because we don't fire to or near "equilibrium", we don't use pure
materials, we have marginal temperature measurement, and for the most
part don't run any tests to determine if part of our glazes remain
crystalline, these eutectic compositions serve only as guides for what
might happen. But they do seem to be useful information. I would never,
had I not seen it on paper first, assumed that I could get an
exclusively calcia fluxed glaze.

Louis Katz
lkatz@falcon.tamucc.edu


> Dear Ron Roy,
>
> The CaO-Al2O3-SiO2 1170 deg Celsius Eutectic can only be constructed
> if you have the following minerals. Recalculate the Oxide content
> using Wollastonite and Anorthite, with Silica to make up for not
> having Tridymite available.
>

iandol on mon 2 jun 03

Dear Louis Katz,=20

Some interesting points in your 1st June Post (2003-153). I like "Coming =
from the top" . Given that a mixture of glaze ingredients is totally =
liquid at the point at which we cease to heat the stuff, and my glazes =
look very wet, you will get a Eutectic forming from your recipe if you =
cool so that everything has a chance to Crystallise. That which =
crystallises at the Solidus, the point at which the mixture ceases to be =
a liquid may have the Composition and the Microstructure of a Eutectic. =
But it must be crystalline. If it is Glassy, Vitreous, then you have =
arrested crystallisation before the reactions were completed and there =
can be no "Eutectic" structure present.

In para 2. Are you speaking about the process of heating or of cooling. =
Being clear on this point is essential.

I can see the benefits of heating two Glasses formed from Eutectic =
Mixtures. This would be similar to heating two frits. But if it is =
glasses you are heating, the principle of a Eutectic reaction cannot be =
invoked as the melting mechanism because there is no crystalline =
structure. All you do is pass through the Glass Transition Temperature. =
The glass or Frit which melts first then becomes a solvent for things =
which have higher melting points. This is a Fact which is beyond =
question.

If you are speaking of Cooling, a slow cooling rate will allow for =
complete crystallisation. Then the material is not a "Glaze" because =
there will be no vitreous phase. A surface coating but not a glaze!!

Certainly, adding a low melting point frit will enhance the rate of =
dissolution of higher melting point compounds. But in a clay body which =
is not intended to be fully vitrified the frit is accelerating the =
reaction to maturity by enhancing the process of Liquid Phase Sintering. =
Care must be taken to distinguish what is happening and why.

By the way, I am not sure the 1170 mixture would make a glaze. Fast =
cooling might ensure retention of the vitreous phase at room =
temperature. My sample gave me an opaque green vitreous material. But =
more tests would be needed to guarantee that it was a Glaze.

There are times when it would be better to ignore statements about the =
presence of "Eutectics' and their behaviour. I repeat, Eutectic =
compositions are rare in the field of Ceramic Arts. For the majority of =
the time we neither create them nor use them nor do they appear in our =
work.

Best regards,

Ivor Lewis.

Ron Roy on tue 3 jun 03

Hi Ivor,

Yes - I know how small amounts of contaminants lower melting points - those
are the materials we use and so we count on them being there.

Can you explain why your mixture does not work - the diagrams say it will
at 1170C - what is the trick - how did they get it to melt? Perhaps it must
be melted first - and will melt then?

If I were going to a eutectic - say for making cone 6 glazes - I would try
to keep the CaO/Al2O3/SiO2 in the right proportions and add other fluxes to
get the proper melting.

We also have to keep in mind - about eutectics - all the material is
involved in the melt - nothing left over - and many of our glazes rely on
the left overs to produce the glazes we want. Perhaps that is why eutectic
mixtures are so rare in our glazes - that and the short melting ranges we
detest so much.

I am away for a week Ivor so I will not be able to take part in this
discussion until I return - if it is still going on that is.

RR.


>The CaO-Al2O3-SiO2 1170 deg Celsius Eutectic can only be constructed if
>you have the following minerals. Recalculate the Oxide content using
>Wollastonite and Anorthite, with Silica to make up for not having
>Tridymite available.
>
>There are several alternative recipes which can be constructed if you have
>all the potential ingredients to hand. Only one will melt at or near the
>precise temperature of the Eutectic Point.
>
>The trick may be to use such things as Eutectics to add to the Cone 6
>collection. This would seem to be an ideal candidate. But as you say
><>-interesting it would melt at 9-10 - expansion is right in the ball
>park.>> from which I infer you would not expect melting to occur.
>
>I thank Michael Banks for enabling me to do my tests on this composition.
>He provided me with a sample of Anorthosite, a rock which contains a high
>proportion of Anorthite.
>
>Best regards,
>
>Ivor Lewis

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513

iandol on wed 4 jun 03

Dear Ron Roy,=20

I am pleased you have given the quotations from Taylor and Buy they =
illustrate clearly what other commentators omit when explaining the =
behaviour of the Alkali Earth Elements.

These authors do not say that the group 2 elemental oxides will cause =
more melting. The say that the effect is to cause a greater degree of =
fluidity. This is not the same as changing the temperature of fusion.=20

I am prepared to say that raw CaO, BaO, SrO, ZnO, MgO, all well known =
refractory compounds, do not change the temperature of fusion. They =
change fluidity (...reducing viscosity...)when accepted into solution =
in a solvent silicate melt. Increasing the pace or rate of flow is a =
change in Flux. These oxides also have an effect on Surface Tension, but =
until problems happen this seems to be ignored as well. Those teaching =
"Glaze Preparation and Use" should be aware of this and mention it in =
their presentations.

How great it would be if the a segment, similar to the calculation of C =
of E, could be included in glaze calc programs to estimate the degree of =
fluidity and measure of surface tension.

Enjoy your travels.

Best regards,

Ivor Lewis=20