iandol on sun 29 sep 02
Kaye and Laby, "Tables of Physical and Chemical Constants" give the =
following melting point values.
Cu2O =3D 1235=B0, CuO =3D 1326=B0C. Hence, neither of the oxides of =
copper will be fluid at Orton cone Six. I have confirmation from two =
other sources. One of these is given as a reference by F. Hamer.=20
Perhaps you are right to agree with Frank Hamer on this one. It seems as =
thought the only meaning given to this term in the ceramic sense is that =
it conveys the idea that it causes melting or fusion. But you are =
limited by Frank Hamer's understanding to the situation.
Do you know where this notion that mixtures of materials will melt or =
fuse at a lower temperature than they do as individual materials =
originates. Who started the ball rolling? It seems to disregard the =
teachings of Kingery et al.
Now, think about it, what does the term "Flux" conveys other than =
I have these ideas in my mind.
One is that a flux can change fluidity without affecting the degree of =
melting or the temperature at which melting commences. Another is that =
it can also change surface tension once it is dissolved. Add to this; =
that it can mean activating a surface so that other materials adhere. =
Then there is the very basic meaning of "Flow", moving in response to =
Gravity. In addition there is the dissipation, dissolution or discharge =
of unwanted artefacts, elements or potential defects, a cleansing =
action. Perhaps a flux may also increase the power of a melting agent to =
dissolve even more refractory substances once it has, itself, dissolved.
All are applicable to the situation of a glaze on a pot. But we never =
seem to talk about these activities. When do we discus them?
It is necessary that the surface of the clay on which the glaze sits is =
eroded to give adhesion that surface. Additional fluidity is needed to =
expel seeds or bubbles so that scars heal over. Materials which are =
often stiff when they are in a molten condition need to be made to flow =
more freely. Since we put a lot of very refractory oxides in our glazes =
we need to be certain that these will dissolve.
Do the copper oxides do these things. I do not know. but if I wished to =
find out I would not search books written for potters by Potters. I =
would set up the practicals and make the observations.
Ivor Lewis. Redhill, South Australia
Gavin Stairs on mon 30 sep 02
At 04:49 AM 29/09/2002 +0930, you wrote:
>...Do you know where this notion that mixtures of materials will melt or =
>fuse at a lower temperature than they do as individual materials =
>originates. Who started the ball rolling? It seems to disregard the =
>teachings of Kingery et al....
This is one of those generalizations that must have a few exceptions. But
a cursory examination of a few phase diagrams will lead to the idea. I
think the theoretical explanation would have to do with lower energy states
owing to increased degrees of freedom.
As for somewhat refractory materials being classified as fluxes, I think it
comes from the same idea. If there is a eutectic mixture of two somewhat
refractory materials with a lower melting point that the pure material,
which is the general case, then one can be considered a flux for the other,
or vice versa, regardless of the temperatures involved. Flux just means
flow, implying liquid.
Ron Roy on wed 2 oct 02
Seger defines a flux as anything in the RO column (CaO, BaO etc.) CaO fits
We cannot read melting points as any indication of an oxides ability to
help melting - for instance the melting point of CaO by itself is 2570C,
MgO is 2800C and Strontium oxide is 2430C - all are higher than CuO and all
are fluxes at cone 6.
Even combinations of refractories like silica and alumina melt at a lower
temperature than the two alone.
We don't have to understand this from a chemical point of view - all we
have to do is observe the results from our Kilns.
I think a lot of the Hamer book - time after time I have found his
information helpful - of all the books written by potters I find his the
most useful for general information.
I am reading in Ceramic glaze Technology by Taylor and Bull - "copper oxide
is an active flux in glaze and it will reduce the firing range of any
composition to which it is added.
It is interesting that they also note that Nickel oxide (NiO) is
refractory, has a melting point of 1990C - when added to glazes it will
increase the maturing range of a glaze.
As you can see - melting points are not any kind of indicators of fluxing
power. In the end - I am assured that melting is unpredictable and there
are volumes of data on this all based on experiments.
I do hope Gavin will explain to us just why NiO is so reluctant to join in.
>Kaye and Laby, "Tables of Physical and Chemical Constants" give the
>following melting point values.
>Cu2O =3D 1235=B0, CuO =3D 1326=B0C. Hence, neither of the oxides of copper=
>fluid at Orton cone Six. I have confirmation from two other sources. One
>of these is given as a reference by F. Hamer.
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