iandol on sun 1 jun 03
Dear Jim Murphy,
A good point you make, to refocus on the relationships we project onto =
the three fundamental groupings.
As I say in another posting, the generosity of Michael Banks enabled me =
to investigate the commonly held claim that Eutectics operate to cause =
melting and this phenomenon is attributable to Oxides in the =
formulation. This is a common concept delivered to us from way back and =
can be found in literature of many of the popular writers. Perhaps I am =
the only one who considers it to be an erroneous concept.
The sample I assembled from Whiting, Silica and Aluminium Hydrate failed =
to melt heated beyond Cone 8. I agree with the observation of Ron Roy =
that substituting Wollastonite and introducing Kaolin changes the =
situation and that some degree of fusion will take place, but at a =
higher temperature and in the compass of a longer time frame. But it is =
doubtful that this change is instantaneous and complete at the stated =
temperature which it would have to do and be to comply with the =
Thermodynamicists definition of "Eutectic". One glance at the Phase =
diagram for this System tells us that we must use Anorthite, =
Wollastonite and Tridymite to engage the Eutectic.
People may need to reconsider the roles of each of the elemental oxides =
included in the R2O column of the Unity Formula and accept that the =
Group 2 Alkali Earth Metal Group Oxides (RO) are Refractory but Soluble. =
So they make no contribution to the Melting Process. Conversely, Group 1 =
Alkali Metal Group Oxides (R2O) have a high degree of Fusibility and =
once molten they become strong solvents.
Now if I wanted to get a refractory brick which would resist the ravages =
of Molten Iron but remained neutral in its behaviour to Iron Oxides at =
around 1600 deg Celsius I would employ a mixture of Lime and Clay or =
Dolomite and Clay. Which might lead to a generalisation, which is what =
you request, that increasing the proportion RO oxide and R2O3 oxide =
against the Silica fraction makes a glaze less fusible.
Thanks for giving me a chance to rethink some of the concepts we employ.
Ron Roy on tue 3 jun 03
There is no doubt about the fluxing action of the alkaline earths - as an
example - Louis's example - almost all fluxed with CaO and melting to a
glaze at cone 9.
This from Taylor and Bull again.
Page 25. ... The fluxing ability of of calcium oxide is good and its effect
in reducing the viscosity is very marked.
Page 28. .... about MgO - Like calcium it has an active fluxing action at
high temperatures and also reduces the melt viscosity, being most effective
in the region of 1200C.
To say the alkali earths are not fluxes must be confusing to potters -
especially those who are just getting into the study of glazes. I can only
reiterate - if you add CaO to a cone 6 glaze you will get more melting. If
you add BaO, SrO, MgO or ZnO to the eutectic mixture being discussed - you
will get more melting.
>People may need to reconsider the roles of each of the elemental oxides
>included in the R2O column of the Unity Formula and accept that the Group
>2 Alkali Earth Metal Group Oxides (RO) are Refractory but Soluble. So they
>make no contribution to the Melting Process. Conversely, Group 1 Alkali
>Metal Group Oxides (R2O) have a high degree of Fusibility and once molten
>they become strong solvents.
15084 Little Lake Road
Ababi on fri 6 jun 03
I want to add here
Many people consider me as a glaze guru. I know the truth I know I will never arrive to
the foothill of the knowledge of Ivor Ron or Lawrence.
This is the reason why I add glaze addict to my signature.
By the way to differ glaze addiction from the well known ones - as the time passes
your brain is clearer.
I tried to read your letters in this thread - I failed and slightly disappointed time after
The truth about software is like driving a car being a good or even an excellent driver
without perhaps knowing the the structure of the metal plastics and the glasses ( not
glazes) which the car is made from.
I use the Eutectic to explain my students how a glaze is created.
Never in my short pottery life I tried to build a glaze based on Metallic
CaO-Al2O3-SiO2 Eutectic, still I hardly fail with my glazes!
Am I a genius? My mother said so!
It does not matter!
I talk about the good glazes not lichen crater ETC.
The softwares as well as the good books
As well as your knowledge - the older folks that do it in your head have the limit tables.
When you create the glaze according to the limits it is closely certainly to be a good
one. with the proper Eutectic!
I know Lawrence, in Matrix there is a way to see the Eutectics, it is beyond my
understanding at this point of my addiction.
Since using Insight and later Matrix - following their limits I hardly fail in glazes.
About the use of the Surface tension I shall write in a separate letter with some
examples in my site.
Kibbutz Shoval Israel
---------- Original Message ----------
>>How great it would be if the a segment, similar to the calculation of C of
>>could be included in glaze calc programs to estimate the degree of fluidity
>>and measure of surface tension.
>Matrix calculates surface tension using Deitzel coefficients in dynes/cm -
>can't remember where I got these.
>As with the use of C of E I guess this value for each glaze is indicicative
>and subject to a lot of variables other than the chemistry of the glaze.
>School of Art
Lawrence Ewing on fri 6 jun 03
>How great it would be if the a segment, similar to the calculation of C of
>could be included in glaze calc programs to estimate the degree of fluidity
>and measure of surface tension.
Matrix calculates surface tension using Deitzel coefficients in dynes/cm -
can't remember where I got these.
As with the use of C of E I guess this value for each glaze is indicicative
and subject to a lot of variables other than the chemistry of the glaze.
School of Art
Matrix Glaze Software:
Glaze Chemistry Instruction:
New Zealand Society of Potters website:
----- Original Message -----
Sent: Wednesday, June 04, 2003 8:30 PM
Subject: : Metallic CaO-Al2O3-SiO2 Eutectic
Dear Ron Roy,
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.
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.
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