iandol on sun 15 jun 03
Dear Ron,
Ref your posts of 3rd June and 13th June.
First of all, the 1170 Eutectic in the CaO-Al2O3-SiO2 system will only =
work if it is compounded from the materials named in the Primary Phase =
Fields of the Phase Equilibrium Diagram. This is made clear in =
literature relating to the preparation and utilisation of Phase =
Equilibrium Diagrams.
As I said in my post on this subject, in this system those compounds are =
Anorthite, Pseudo Wollastonite and Tridymite.
So the only starting point is Anorthite, Wollastonite and Quartz.
This combination of compounds, in correct proportions, will fuse to a =
glass when heated to Cone 8. This is a fact I found by testing. My =
sample mixture made form Alumina, Quartz and Whiting did not even =
sinter. My sample mixture compounded from Whiting, Kaolin and Quartz did =
not sinter. My sample made from Wollastonite, Kaolin and Quartz did =
sinter. Nor were these postage stamp patches, they were fifty gram =
samples contained in bisque cups.
Now I know quite well that if each of these were to be enhanced with =
between ten and twenty percent Boron Frit I would get full fusion. But =
that is not the point. As an experiment it would prove nothing which is =
not already known and frequently used.
One of the most telling facts leading to my belief that there are no =
Eutectic melting points in our glaze mixtures are those Eutectic melting =
temperatures in the Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 systems, at 740C =
and 695 C respectively. If Eutectics were prevalent in glaze recipes, =
Fusion Inducing Fluxes such as GB and Boron Frits would not be necessary =
to get middle fire stoneware glazes to melt. We would know that any =
materials containing Sodium or Potassium would accomplish the task for =
us.
RR <melting points are not germane to the argument - even the mixture of =
alumina and silica has a lower melting point than either material =
separately.>>
I suppose most of the authors I have read will say or agree with your =
statement. But, regardless of the number of times that statement is =
published, a mixture of Alumina and Silica will not melt at a =
temperature lower than the melting point of either. Just try fusing a =
mixture of ground Quartz and powdered Corundum. A brief glance at the =
Phase Diagram indicates why. The Eutectic you speak of at about 1590 deg =
C. is not formed from those pure compounds or the two oxides, but from a =
mixture of Mullite, a compound of Aluminium, Silicon and Oxygen and =
Cristobalite which are the substances in the primary phase fields. The =
Phase diagram for this system can be found in Kingery et al, =
"Introduction to Ceramics", p 305 who give a most useful and =
enlightening commentary.
RR<range fluxes - do you think the glaze will melt properly?>>
By eliminating Frits 3134, 3269, 3195 and 3124 from the recipes in =
"Mastering Cone 6 Glazes" I would predict that the glaze mixtures =
remaining would not mature at or below cone 6. The Frits you include =
proved the initial liquid phase in which the other ingredients can =
dissolve. I would predict this outcome for any glaze which relied on a =
boron frit to induce fusion except, perhaps, those containing Nepheline =
Syenite.
There might be less confusion if, when reading or selecting ingredients =
for a glaze, people clearly understood information given in many text =
books on the subject.
The need is to clearly define the purpose for which an ingredient is =
included, be it as a compound or described as an abstract oxide. Ask the =
question "Why do I need to add Calcium Oxide". Hamer and others clearly =
state that Calcium Oxide will "Lower the Viscosity of a Molten Glaze". =
This can only happen after the material which carries the Calcium Oxide =
has dissolved in a liquid which already exists. He does not say it will =
lower the viscosity by "Causing Melting". He gives us a material phase =
which is already in a fluid state.=20
It so happens that one of the meanings of this term "Flux" is to cause =
"Flow". And to decrease viscosity means increasing an ability to flow. =
So in that sense I have no argument about describing the Group 2 Alkali =
Earth Oxides as fluxes. But please, let us not say they cause fusion, or =
in simplistic terms "Melting".
Again Kingery et al give, in Chapter 10, an excellent commentary on the =
Ceramic Processes which change recipes from material batches into =
Ceramics and Glazes.
Best regards,
Ivor Lewis. Redhill, South Australia
Ron Roy on thu 19 jun 03
Hi Ivor,
You must tell us - are there any authors who say CaO is not a flux at cone 10?
You mis read my suggestion of removing the mid range fluxes for a cone 6
glaze. I am suggesting you remove the whiting from the following cone 6
glaze - it will not melt properly - because of the fluxing action of the
CaO.
Untitled Recipe 2
-----------------
F3134............... 20.00
WHITING............. 15.00
G 200 SPAR.......... 20.00
EPK................. 15.00
SILICA.............. 30.00 30.00%
----------
100.00
FORMULA & ANALYSIS
------------------
*CaO........ .77 --- 13.68%
*MgO........ .00 --- .06%
*K2O........ .08 --- 2.41%
*Na2O....... .15 --- 2.93%
Fe2O3...... .00 --- .17%
TIO2....... .00 --- .05%
B2O3....... .23 --- 5.07%
AL2O3...... .32 --- 10.25%
SiO2....... 3.42 --- 65.36%
P2O5....... .00 --- .02%
RATIO 10.84
EXPAN 455.41
WEIGHT 314.28
>RR <>melting points are not germane to the argument - even the mixture of
>alumina and silica has a lower melting point than either material
>separately.>>
>I suppose most of the authors I have read will say or agree with your
>statement. But, regardless of the number of times that statement is
>published, a mixture of Alumina and Silica will not melt at a temperature
>lower than the melting point of either. Just try fusing a mixture of
>ground Quartz and powdered Corundum. A brief glance at the Phase Diagram
>indicates why. The Eutectic you speak of at about 1590 deg C. is not
>formed from those pure compounds or the two oxides, but from a mixture of
>Mullite, a compound of Aluminium, Silicon and Oxygen and Cristobalite
>which are the substances in the primary phase fields. The Phase diagram
>for this system can be found in Kingery et al, "Introduction to Ceramics",
>p 305 who give a most useful and enlightening commentary.
>RR<>range fluxes - do you think the glaze will melt properly?>>
>By eliminating Frits 3134, 3269, 3195 and 3124 from the recipes in
>"Mastering Cone 6 Glazes" I would predict that the glaze mixtures
>remaining would not mature at or below cone 6. The Frits you include
>proved the initial liquid phase in which the other ingredients can
>dissolve. I would predict this outcome for any glaze which relied on a
>boron frit to induce fusion except, perhaps, those containing Nepheline
>Syenite.
>There might be less confusion if, when reading or selecting ingredients
>for a glaze, people clearly understood information given in many text
>books on the subject.
>The need is to clearly define the purpose for which an ingredient is
>included, be it as a compound or described as an abstract oxide. Ask the
>question "Why do I need to add Calcium Oxide". Hamer and others clearly
>state that Calcium Oxide will "Lower the Viscosity of a Molten Glaze".
>This can only happen after the material which carries the Calcium Oxide
>has dissolved in a liquid which already exists. He does not say it will
>lower the viscosity by "Causing Melting". He gives us a material phase
>which is already in a fluid state.
>It so happens that one of the meanings of this term "Flux" is to cause
>"Flow". And to decrease viscosity means increasing an ability to flow. So
>in that sense I have no argument about describing the Group 2 Alkali Earth
>Oxides as fluxes. But please, let us not say they cause fusion, or in
>simplistic terms "Melting".
>Again Kingery et al give, in Chapter 10, an excellent commentary on the
>Ceramic Processes which change recipes from material batches into Ceramics
>and Glazes.
>Best regards,
>Ivor Lewis. Redhill, South Australia
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
iandol on sat 21 jun 03
Dear Ron,
If I were to rely on Authors of books for the Ceramic Artist for this =
information I would in all probability not find a direct reference =
simply because of the way things are written and the way things are =
thought about. Kingery et Al provide a coherent account of the =
Vitrification process. I was writing about my experiences investigating =
the CaO-Al2O3-SiO2 system. My results indicated quite clearly that the =
mixture compounded to give pure oxides would not melt at the indicated =
temperature.
Right, let's look at it from your point of view. Remove the Whiting from =
your hypothetical recipe and it fails to meet an undeclared =
specification, apart from maturing at cone 6. But it appears as though =
it might be a smooth transparent glaze if cooled quickly. You say that =
with the Whiting omitted it fails to flow, to smooth out.=20
Are you saying there is no evidence of Fusion? Or are you saying there =
is evidence of melting but that the degree of fusion is not to your =
expectations? If there is no evidence of melting having happened, so =
that the sample is still a friable powder, then you are correct, whiting =
causes melting. But if there is evidence of fusion at Cone 6 then =
"Melting" cannot be attributed to Whiting.
I continue to maintain that Frits, as in your example and Felspars in =
higher fired glazes, provide a molten environment which acts as a =
solvent for refractory constituents, in your example the Kaolin, Quartz =
and Whiting. Lime derived from whiting dissolves in this melt and alters =
the fluidity and surface tension making the liquid less viscous. Silica =
dissolves and increases the bulk of the Glassy Phase, Kaolin gives both =
Silica, adding more vitreosity and Aluminium Ions which prevent the =
re-establishment of oxygen bridges preventing long range regrowth of the =
tetrahedral Silica crystal lattice.
Now if you insist I am wrong, then my contention that removing the =
melting agents, that is the Frit and Felspar will prove this. Your =
recipe, which has a major proportion of Calcium Oxide (0.77 Mol)in its =
constitution will melt and fuse proving that this material is a flux in =
the melting sense.
There is an important distinction in the way each of us thinks. As you =
see, it is semantics. I say Calcium oxide is a flux in the sense that =
"in solution" it will make a fluid less viscous, allowing it to flow =
freely. You say Calcium Oxides is a flux because it makes things melt, =
that it can initiate fusion.
Beyond this there is no other way I can clarify the situation.
Best regards,
Ivor Lewis. Redhill, South Australia
Ron Roy on mon 23 jun 03
Hi Ivor,
Yes there will be some evidence of melting but it will look quite
underfired compared to the to the glaze with the whiting included.
My point is - if CaO is a refractory then how do you explain the increase
in melting when it is included?
If a material included in a glaze increases the melt why not call it a
flux? If we take it out and there is not the same degree of melting how can
we call it a refractory?
At what temperature do you think your mixture of CaO, SiO2 and Al2O3 will
melt in a standard type pottery Kiln? Will that temperature be below all of
the melting points of all the oxides present? If so how could this happen
if one was not a strong enough flux to make it happen?
We need to understand that Calcium oxide does not start it's fluxing action
till around 1100C - that is only 70C below your expected melting
temperature. We know that - in order for a flux to work under these
circumstances - time is going to be a factor. That mixture will melt at
1170C if given enough time. If it were otherwise the experimenters would
not have said so.
If you say take out the KNaO and it would not melt as well - you would be
right - but it is the same thing - reduce the fluxes and it will not melt
as well. It seems to me that all the fluxes will naturally have different
effects on melting - ignoring the fact that some do not start their fluxing
till higher temperatures will of course lead to failure.
Perhaps it would be useful to talk about ZnO which has a melting point of
1975C but is the main flux for Bristol glazes - do you consider that oxide
a flux or not?
It is Seger who called any oxide that ends in a singe O a flux - it is also
true that some fluxes do not start their work until certain temperatures
are reached.
RR
>Right, let's look at it from your point of view. Remove the Whiting from
>your hypothetical recipe and it fails to meet an undeclared specification,
>apart from maturing at cone 6. But it appears as though it might be a
>smooth transparent glaze if cooled quickly. You say that with the Whiting
>omitted it fails to flow, to smooth out.
>Are you saying there is no evidence of Fusion? Or are you saying there is
>evidence of melting but that the degree of fusion is not to your
>expectations? If there is no evidence of melting having happened, so that
>the sample is still a friable powder, then you are correct, whiting causes
>melting. But if there is evidence of fusion at Cone 6 then "Melting"
>cannot be attributed to Whiting.
>
>I continue to maintain that Frits, as in your example and Felspars in
>higher fired glazes, provide a molten environment which acts as a solvent
>for refractory constituents, in your example the Kaolin, Quartz and
>Whiting. Lime derived from whiting dissolves in this melt and alters the
>fluidity and surface tension making the liquid less viscous. Silica
>dissolves and increases the bulk of the Glassy Phase, Kaolin gives both
>Silica, adding more vitreosity and Aluminium Ions which prevent the
>re-establishment of oxygen bridges preventing long range regrowth of the
>tetrahedral Silica crystal lattice.
>
>Now if you insist I am wrong, then my contention that removing the melting
>agents, that is the Frit and Felspar will prove this. Your recipe, which
>has a major proportion of Calcium Oxide (0.77 Mol)in its constitution will
>melt and fuse proving that this material is a flux in the melting sense.
>
>There is an important distinction in the way each of us thinks. As you
>see, it is semantics. I say Calcium oxide is a flux in the sense that "in
>solution" it will make a fluid less viscous, allowing it to flow freely.
>You say Calcium Oxides is a flux because it makes things melt, that it can
>initiate fusion.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
iandol on tue 24 jun 03
Dear Ron Roy,
I think my reference to the behaviour of Lime and Silica will be to hand =
by the time you get this.
I have a problem with transliterating your comment <included in a glaze increases the melt....>> Are you saying there is a =
greater volume of liquid. Or are you saying the rate at which the volume =
of liquid increases accelerates. Or do both of these things happen. I =
accept that both can and most probably will happen
I think the key to understanding is acceptance that once some liquid , =
and in the case of a glaze batch containing a Frit this will be derived =
from the Frit, then the things dissolving in that first liquid will =
change its character. Calcium oxide, if it is that and not =
Pseudo-Wollastonite from that solid state reaction, will lower the =
viscosity of the Frit fluid as well as its surface tension. If it is the =
Pseudo-Wollastonite it would also increase the bulk of the fluid because =
of the addition of the silica.=20
If it can be said that Whiting provides Calcia or Lime, "This dissolves =
into the melting batch where, as a Flux, it alters viscosity and surface =
tension. These attributes make the melting glaze flow and increase the =
volume of the Vitreous Melt" I can accept Lime as a FLUX. But I will =
resist the opinion that Lime makes a glaze melt in the sense that it =
creates fusion at a lower temperature.
Relating to your comments <mixture of CaO, SiO2 and Al2O3 will melt in a standard type pottery =
Kiln?>> Working with pure oxides that are free from KNaO contaminants, =
I anticipate that sintering would start via Silica at just under 1000 C =
but would not really get under way until the temperature reached the =
Tamman Temperature of Alumina which is in the region of 1460. This =
opinion carries the caveat that there are no solid state reactions which =
create Anorthite, Pseudo-Wollastonite or Gehlenite. If that were not the =
case, then the temperature would be determined by composition, rates of =
solid state reactions and particle sizes. There are records of Lime rich =
Shales being fired where these solid state reactions have taken place =
but the time scale is not that of a potters kiln firing.
Regarding Zinc Oxide as a Flux. For me to comment you would have to =
write out your definition of Flux ! As I said in my previous post, we =
are in the realms of Semantics not Pottery. But that said, such Bristol =
Glaze recipes as come to hand have Colemanite, Nepheline Syenite and/or =
Felspar as constituents of the batch. Zinc Oxide is a refractory oxide =
which, in a vitreous solution, lowers viscosity. Perhaps a better =
question to ask is "Does Zinc oxide enter into a solid state reaction =
with free Silica or Mullite ?".=20
Great discussion.
Best regards,
Ivor Lewis. Redhill, South Australia
iandol on sun 29 jun 03
Dear Ron,
Relating to the behaviour of materials in a glaze I cannot make any
further comment which would add to what I have already said. To go
further I need you to simply, clearly, unambiguously and unequivocally
define "Melt" and "Melting", "Flux", "Fluxing" and "Fusion". You might
also prime yourself for the discussion by writing down similar
definitions for "Dissolve", "Solvent", "Solute", "Solution" and
"Solvation" which are words expressing concepts I find necessary when
I talk about the behaviour of materials as they change phase and
interact with one another.
I can be a little more helpful with the notion of a "Solid State
Reaction". This is the Chemistry which happens when two or more solid
materials in intimate contact retain their solidity but, activated by
an increase in temperature diffuse into each other and combine as a
new chemical compound. I became familiar with this process when I was
employed in the Powder Metallurgy Department of a Company which made
Magnets from Iron, Aluminium, Nickel and Cobalt, under the generic
names of Alnico and Alcomax. All the powders we prepared were minus
200 mesh and they were compacted under pressures of tens of tons per
square inch into tablets which were fired in a pure hydrogen
atmosphere. Melting was forbidden, it would have destroyed the precise
shapes and dimensions of the "tablets". We were, as it happens, making
storage cores for the very first generation of computer and that was
in 1955.
I think if you were to Google search for solid state reactions you
might turn up something interesting.
Looking forward to your comments.
Best regards,
Ivor.
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