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cream breaking red and strontium carbonate

updated sat 11 sep 04

 

Gaye Sekula on sun 29 aug 04


Is there a trick to mixing in strontium carbonate? Did I get a bad batch or
is it always so granulated? I did a test firing yesterday to ^6, which is
what the recipe states, and the strontium grains are still obvious. Maybe
I should fire it higher?
Thanks,
Gaye Sekula
San Antonio

John Hesselberth on mon 30 aug 04


Hi Gaye,

I would say you got a bad batch, a cheaper/coarser grade, or a
different material was accidentally substituted when your supplier was
weighing it out. My strontium carbonate has just a hint of graininess
when I rub it between my fingers, but it very quickly and easily
becomes a very fine powder. It disperses easily in glaze suspensions
and I have never encountered strontium grains in a fired glaze.

Regards,

John
On Sunday, August 29, 2004, at 01:05 PM, Gaye Sekula wrote:

> Is there a trick to mixing in strontium carbonate? Did I get a bad
> batch or
> is it always so granulated? I did a test firing yesterday to ^6, which
> is
> what the recipe states, and the strontium grains are still obvious.
> Maybe
> I should fire it higher?
> Thanks,
> Gaye Sekula
> San Antonio
>
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

Mary K on tue 31 aug 04


Dear John,

I would love to know what the graininess is in that recipe (Cream Breaking Red).....is it the RIO, or do you think maybe it was on an iron bearing clay? (baby steps, still learning) Mary K

John Hesselberth wrote:
Hi Gaye,

I would say you got a bad batch, a cheaper/coarser grade, or a
different material was accidentally substituted when your supplier was
weighing it out. My strontium carbonate has just a hint of graininess
when I rub it between my fingers, but it very quickly and easily
becomes a very fine powder. It disperses easily in glaze suspensions
and I have never encountered strontium grains in a fired glaze.

Regards,

John
On Sunday, August 29, 2004, at 01:05 PM, Gaye Sekula wrote:

> Is there a trick to mixing in strontium carbonate? Did I get a bad
> batch or
> is it always so granulated? I did a test firing yesterday to ^6, which
> is
> what the recipe states, and the strontium grains are still obvious.
> Maybe
> I should fire it higher?
> Thanks,
> Gaye Sekula
> San Antonio
>
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

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John Britt on tue 31 aug 04


Ivor,

Strontium Carbonate decomposes to Strontium Oxide at around 1100 C (2012
F).

JOhn Britt
www.johnbrittpottery.com

Ivor and Olive Lewis on tue 31 aug 04


Dear Gaye Sekula
I suggest you get a pestle and mortar and mull your material for a
while before adding it to your dry glaze batch.
Strontium Carbonate is tricky stuff. Unlike its cousins, Barium and
Calcium Carbonate, from the same family of elements, it does not
decompose within the temperature range in which we fire our ceramics.
It decomposed with heat at 1340=BA C, say around 2450=BA F, well below
your cone 6 maturity temperature.
So, to make it work in a glaze we would have to consider a new
chemistry. Even if you had access to Strontium Oxide which by
inference is necessary to make the glaze you would be faced with a
problem. Strontium Oxide melts only when you heat it to 2430=BA C (4400=BA
F).
So we are faced with a profound question......What is the Chemistry of
Strontium Carbonate in a glaze ? Please, would those who propose its
use as a substitute of Barium Carbonate enlighten all of us ?
Best regards,
Ivor Lewis.
Redhill,
S. Australia.



fire our goodies ----- Original Message -----
From: "Gaye Sekula"
To:
Sent: Monday, 30 August 2004 2:35
Subject: Cream Breaking Red and Strontium Carbonate


> Is there a trick to mixing in strontium carbonate? Did I get a bad
batch or
> is it always so granulated? I did a test firing yesterday to ^6,
which is
> what the recipe states, and the strontium grains are still obvious.
Maybe
> I should fire it higher?
> Thanks,
> > San Antonio
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.

John Hesselberth on tue 31 aug 04


On Monday, August 30, 2004, at 09:42 PM, Ivor and Olive Lewis wrote:

> Strontium Carbonate is tricky stuff. Unlike its cousins, Barium and
> Calcium Carbonate, from the same family of elements, it does not
> decompose within the temperature range in which we fire our ceramics.
> It decomposed with heat at 1340=BA C, say around 2450=BA F, well below
> your cone 6 maturity temperature.

Hi Ivor,

While I am sure this is true for the pure material, what happens when=20
it is dissolved or suspended in a glaze melt? I think using handbook=20
values for decomposition temperatures could be very misleading in this=20=

situation.

Regards,

John

John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

John Hesselberth on tue 31 aug 04


On Tuesday, August 31, 2004, at 10:34 AM, Mary K wrote:

> I would love to know what the graininess is in that recipe (Cream
> Breaking Red).....is it the RIO, or do you think maybe it was on an
> iron bearing clay? (baby steps, still learning) Mary K
Hi Mary,

I don't know the recipe so I can't answer your question. If you want to
send it to me off list I could take a guess. Most of our materials,
though, if bought from a high quality supplier will be finely enough
ground to avoid graininess. One exceptions is Wollastonite--it will
disperse but takes a little extra work. Cobalt oxide and red copper
oxide can be difficult to dispense uniformly but, again, it can be done
with some extra work and/or the help of a wetting agent. Zinc oxide
gives some people fits but, so far as I can tell, that only happens if
it has gotten wet or is in a very humid climate. I've never had a
problem with it.

Some materials are, of course, sold in less finely ground state so they
will produce some spots of color--or they are intended for use in clay
bodies and people use them in glazes. You usually want 200 mesh or
finer for glazes.

I always put my glazes through an 80 mesh screen at least twice
(usually 3 times, sometimes 4 if wollastonite is involved) and I don't
have problems with graininess.

Regards,

John
John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

John Britt on wed 1 sep 04


Ivor,

I used Hamer, and here are web links:

http://www.axxousa.com/srco3_m.html

Here is Digital fire:


Strontium Carbonate
Chemistry
SrO 70.200
Volatiles
CO2 29.800


No Mineralogy defined
Significant Temperatures
1100C: Melts
723C: Decomposition

Strontium Carbonate is a slightly soluble source of SrO used in glazes.
Some sources claim it decomposes at 1075C (others at 1340C) as follows:

SrCO3 -> SrO + CO2

However, the 'Ceramic Industry Materials Handbook' states that it starts
to disassociate as early as 800C in a purely oxidizing atmosphere, whereas
a CO2 atmosphere might delay break-down until around 1220C. This
information is supported by the fact that when the more stable calcium and
barium carbonate are added to bodies, pinholing and blistering are greater
than bodies containing strontium.

And here is another: from www.solvaychemicals.us

http://216.239.41.104/search?q=cache:tDXXjlexV-
YJ:www.solvaychemicals.us/pdf/Stroncarb-
1003.pdf+strontium+carbonate+decomposition&hl=en

Hope that is enough?

JOhn Britt
www.johnbrittpottery.com

Ivor and Olive Lewis on wed 1 sep 04


Dear John,
I also wonder about the same idea relating to the behaviour of
Strontium Carbonate <<, what happens when it is dissolved or
suspended in a glaze melt? I think using handbook values for
decomposition temperatures could be very misleading in this
situation. >>
I do not know !! I would say that no one knows, and I do mean that as
it relates to being part of a glaze formulation. This information may
never have been revealed by experimental exploration.
There are several scenarios which can be considered. You hit on two,
one is that the stuff does nothing, just sits there opacifying the
glaze, the second that it does dissolve into the glaze as a carbonate
without doing anything else. A third proposition would be the there is
a chemical reaction between the Strontium carbonate and free silica,
freeing Carbon dioxide and creating the potential for Strontium
Orthosilicate (Sr2SiO4, CAS No. 12138-28-2, mp.???) to crystallise on
cooling. If you could find the relevant thermodynamic constants,
calculation would show if this was feasible and the optimum
temperature at which the reaction would happen. Such behaviour would
mimic the other elements in Group 2, both Calcium and.Barium form
Orthosilicates.
By the way, I believe that Handbook values for the properties of the
materials we use are an important resource. Used with intelligence
they guard against making unqualified assumptions and drawing
unwarranted conclusions. I say this with the caveats that those who
choose to use them have some background in the Principles of Chemistry
and those who choose to ignore them refrain from inventing plausible
pseudo-scientific explanations for events or observations to mask
their ignorance.
If I say "I do not know" in spite of having information at my
fingertips I mislead no one.
Thanks for asking.
My best regards,
Ivor.

Ivor Lewis.
Redhill,
S. Australia.


----- Original Message -----
From: "John Hesselberth"
To:
Sent: Wednesday, 1 September 2004 1:02
Subject: Re: Cream Breaking Red and Strontium Carbonate


On Monday, August 30, 2004, at 09:42 PM, Ivor and Olive Lewis wrote:

> Strontium Carbonate is tricky stuff. Unlike its cousins, Barium and
> Calcium Carbonate, from the same family of elements, it does not
> decompose within the temperature range in which we fire our
ceramics.
> It decomposed with heat at 1340=BA C, say around 2450=BA F, well below
> your cone 6 maturity temperature.

Hi Ivor,

While I am sure this is true for the pure material, what happens when
it is dissolved or suspended in a glaze melt? I think using handbook
values for decomposition temperatures could be very misleading in this
situation.

Regards,

John

John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

______________________________________________________________________
________
Send postings to clayart@lsv.ceramics.org

You may look at the archives for the list or change your subscription
settings from http://www.ceramics.org/clayart/

Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.

John Hesselberth on wed 1 sep 04


On Wednesday, September 1, 2004, at 01:32 AM, Ivor and Olive Lewis
wrote:

> A third proposition would be the there is
> a chemical reaction between the Strontium carbonate and free silica,
> freeing Carbon dioxide and creating the potential for Strontium
> Orthosilicate (Sr2SiO4, CAS No. 12138-28-2, mp.???) to crystallise on
> cooling.

Hi Ivor,

There is some circumstantial evidence that something like this happens.
It is possible to formulate a glaze (Weathered Bronze Green) that has a
flux system of SrO 0.475, alkalis 0.49, and the rest being a bit of
calcium and magnesium. Silica is 2.8 and alumina 0.58--no boron.
Weathered Bronze Green melts at cone 6 and is quite stable to copper
extraction even with 5% copper carbonate loading. It is also quite
matte. It melts better and is more durable than I would have predicted
based on my work with glazes high in calcium. That limited data leads
me to speculate that strontium (in whatever form it is) is a more
active "flux" than calcium.

Remove the strontium carbonate and I don't believe the above glaze will
melt. I haven't tried it but will when I get a chance.

Regards,

John

John Hesselberth
http://www.frogpondpottery.com
http://www.masteringglazes.com

Ivor and Olive Lewis on thu 2 sep 04


Dear John Britt,
Thank you for those contacts. I will follow them through as best I can
with the resources available to me.
From those you give, which would you regard as Prime Sources and which
provide references to the information they give?
Yes, true, the partial pressure of a gas can influence the reactions
of compounds which yield that gas on decomposition.
Interesting that one of your quotes give a decomposition temperature
which is lower that the melting temperature provided. Could this be
correct ? Does your source explain this anomaly?
Does the 'Ceramic Industry Materials Handbook' cite the sources for
its information? The chemistry you give. Would this apply to the other
elements in the Alkali Earth Group? What are the concequences when
Whiting or Dolomite are being used in a glaze batch
You say " Hope that is enough?". I would say, "If you had to answer a
problem in an examination which could affect your future, whcih one
would you, or could you trust?"
Chemistry is such a tantalising subject.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.


.

Ron Roy on thu 2 sep 04


There is no doubt that strontium carb is a flux in middle and high
temperature ranges.

I am quoting from Taylor and Bulls "Ceramic Glaze Technology"

"The use of Strontium in conjunction with Barium and Calcium gives benefits
beyond what might be expected from the mixed base effect when there is need
to control crystallization in glazes. Indirect comparison with barium oxide
alone, strontium oxide produces brighter and more fusible glazes. Strontia
is known to improve the chemical resistance of glazes when introduced at
the expense of lead and zinc oxide, but it's primary role is to facilitate
the development of low temperature glazes. Using strontium oxide as the
flux, glazes with high gloss, firing at 1145C can be made with good crazing
resistance due in part to the highly developed glaze-body reaction layer."

Hamer reports disassociation of the carbonate (gives up CO2) at 1075C -
which makes sense with the above statement by Taylor and Bull - that it is
useful as a flux starting at 1145.

A more useful subject for a discussion of this kind - I would think - is
what effect does the overall mix of fluxes - in a glaze or clay - have on
dissolution temperatures.

Any body got any hard info on that?

RR


>Strontium Carbonate is tricky stuff. Unlike its cousins, Barium and
>Calcium Carbonate, from the same family of elements, it does not
>decompose within the temperature range in which we fire our ceramics.
>It decomposed with heat at 1340=BA C, say around 2450=BA F, well below
>your cone 6 maturity temperature.
>So, to make it work in a glaze we would have to consider a new
>chemistry. Even if you had access to Strontium Oxide which by
>inference is necessary to make the glaze you would be faced with a
>problem. Strontium Oxide melts only when you heat it to 2430=BA C (4400=BA
>F).
>So we are faced with a profound question......What is the Chemistry of
>Strontium Carbonate in a glaze ? Please, would those who propose its
>use as a substitute of Barium Carbonate enlighten all of us ?

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20

Ivor and Olive Lewis on thu 2 sep 04


Thanks for that note John.
I am suspicious of making claims about melting for any of the elements
which have refractory oxides. But I am certain that chemical activity
in the solid phase provides one potential pathway to generate unusual
material mixtures which, in a serendipitous way, do give us mixtures
which activate along "Join Lines" which are shown in Equilibrium Phase
Diagrams.
I have said this before about nascent Calcium Oxide derived from
Whiting or Dolomite reacting with Nascent Silicon Dioxide given up
when Kaolinite decomposes to silica and Mullite. Unfortunately I do
not have a complete phase diagram for SrO-Al2O3-SiO2 which might
reveal the potential for this to take place. In the Binary system
SrO-SiO2, melting is always above 1350=BA C.
You may well be right about the example glaze you describe. But how
would we explain the result without some serious analytical work.
There is certainly for at least one of the Alkali Earth Orthosilicates
to give mixtures with free silica which will melt at Cone 6 where
alumina is present in the recipe. So why not another.
Keep up the good work.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
.

Ivor and Olive Lewis on fri 3 sep 04


Dear Ron Roy,
Hamer's value differs by 325=BA C from those I have already given. I
distrust his value of 1075=BA C. Check his list of references. Where
does he get this knowledge from?
Statements about the maturation of a glaze only make sense if the
context in which the term "Flux" is being used is defined in an
unambiguous way and if it takes into account the substances which are
created when Strontium Carbonate dissociates.
If it is to Strontium Oxide and Carbon dioxide, the ACS give a melting
point of 2420=BA C for Strontium oxide (attributed to E.E. Schumacher,
1926) on the 1948 International Scale (Levin and others 1979).
Anyone who has access to the archives of the American Chemical Society
might like to consult the CAS data base and ascertain the melting
point of Strontium Ortho-Silicate, Sr2SiO4, (CAS No.13597-55-2). I am
unsure if Strontium forms a metasilicate which is analogous to
Wollastonite but there is information (ACS) to suggest that it does.
No doubt the empirical observations of Turner and Bull are, prima
facie, facts. But they tell us nothing about what is happening in
terms of Chemical Principles.
Dare I say that, at times, it would seem our community suffers from
myopic tunnel vision when we discuss the science and technology of our
craft. I often wonder how many glaze commentators and writers of
glazing books have really studied the teachings of Prof Kingery and
his Co-authors, especially Ch 10.
And yes, I agree with your suggestion <discussion of this kind - I would think - is what effect does the
overall mix of fluxes - in a glaze or clay - have on dissolution
temperatures. >> if you change one word; change "Fluxes" a term which
damns us to following the train tracks of history, to "Substances".
Best regards,
Ivor Lewis.
Redhill,
S. Australia.

Ron Roy on sat 4 sep 04


Hi Ivor,

When you say "myopic" are you referring to mine or yours?

I have to agree - there are flaws in the literature we have - yours and
mine - how can it possibly be that so many writers disagree on the
disassociation temperature of the carbonates we use? Could it be that there
is a general misunderstanding about this? Could it be that the system in
which disassociation takes place has a profound affect on the temperature
at which this happens and could it be that the process is so complicated
that the only way to find out is to construct the experiments and do the
melting - as is the case with phase diagram research.

Dragging into the discussion melting points is a red herring - we all
understand - or should - that when you combine any materials - the
inevitable happens - the melting temperature is lowered. To use the fact
that SrO melts at whatever temperature is to muddy the water. I again must
mention the fact that Seger calls the alkaline earths fluxes at the middle
range of our temperatures and up. There is no doubt about that - and we
also know that Strontium Carbonate must disassociate on order to be a flux
at those temperatures. Explain it what ever way you want but for the sake
of discussions with other potters - why not use the terms we at least are
familiar with?

RR


>Hamer's value differs by 325=BA C from those I have already given. I
>distrust his value of 1075=BA C. Check his list of references. Where
>does he get this knowledge from?
>Statements about the maturation of a glaze only make sense if the
>context in which the term "Flux" is being used is defined in an
>unambiguous way and if it takes into account the substances which are
>created when Strontium Carbonate dissociates.
>If it is to Strontium Oxide and Carbon dioxide, the ACS give a melting
>point of 2420=BA C for Strontium oxide (attributed to E.E. Schumacher,
>1926) on the 1948 International Scale (Levin and others 1979).
>Anyone who has access to the archives of the American Chemical Society
>might like to consult the CAS data base and ascertain the melting
>point of Strontium Ortho-Silicate, Sr2SiO4, (CAS No.13597-55-2). I am
>unsure if Strontium forms a metasilicate which is analogous to
>Wollastonite but there is information (ACS) to suggest that it does.
>No doubt the empirical observations of Turner and Bull are, prima
>facie, facts. But they tell us nothing about what is happening in
>terms of Chemical Principles.
>Dare I say that, at times, it would seem our community suffers from
>myopic tunnel vision when we discuss the science and technology of our
>craft. I often wonder how many glaze commentators and writers of
>glazing books have really studied the teachings of Prof Kingery and
>his Co-authors, especially Ch 10.
>And yes, I agree with your suggestion <
>discussion of this kind - I would think - is what effect does the
>overall mix of fluxes - in a glaze or clay - have on dissolution
>temperatures. >> if you change one word; change "Fluxes" a term which
>damns us to following the train tracks of history, to "Substances".
>Best regards,
>Ivor Lewis.
>Redhill,
>S. Australia.

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20

Ivor and Olive Lewis on mon 6 sep 04


Dear Ron,
Us? Myopic...Tunnel Vision? Neither ! I think you take the broad view
and are constantly searching for knowledge. MC6G is a testament to
that.
You are correct. The conditions under which physical properties are
determined will affect the outcome. The sources I use define pressure
and environment. Pressure is normally one standard atmosphere. Unless
stated, the environment is in Air.
There are chemical consequences to this if we are using Carbonates.
Given a gaseous atmosphere rich in Carbon dioxide it is my
understanding that decomposition will be delayed or suppressed. It is
also known that some oxides can enter into chemical combination with
this gas. For example, when Sodium Metal burns in air the final
product is Sodium Carbonate and Sodium Hydroxide. Burning sodium in
air prduces mostly Sodium Peroxide with some monoxide. These react
immediately with Water Vapour and Carbon Dioxide. High School
chemistry.
Yes, do the comparative experiments. I did when I noticed the
anomalies written in popular reference books for potters. A simple
test will show the error of claiming that mixtures of oxides have
lower melting points that the individual components. Take a mixture
of Calcium Oxide, Aluminium Oxide and Silicon Dioxide. Claim are made
that this mixture will melt at or about 1170=BACelsius. I have yet to
get such a mixture to melt in a firing to cone 8 or 9. It remains a
friable powder. I think this indicates that melting is not inevitable.
Others would tell me that if I keep it at that temperature for long
enough it will melt. I agree. But do they know, as I do, why?
You mention the ideas of Herman Seger. Perhaps you would tell us how
he defines "Flux". As I said, we must give an unambiguous definition
since the word has several meanings.
Is it important to bring melting points in to the arena? Yes! Because
if potters and ceramic artists insist on using the Unity expression
devised by Herman Seger and insist that we are dealing with actual
oxides and not oxides in an abstract mathematical sense, they have to
deal with the anomalies that arise from their position. They will find
that in "Phase Diagrams for Ceramists" no melting point values are
given for K2O or Na2O, nor are their values given in Equilibrium Phase
Diagrams (1979) Figs 407 & 501 or on Plates 4 and 5 of the Wall Chart
series. I wonder why?
Yes, if we heat mixtures they will eventually melt. As I point out,
David Kingery and his Co-authors discuss this process thoroughly in Ch
10 of "Introduction to Ceramics". Much of this information is
confirmed by David Barsoum, "Fundamentals of Ceramics" (1997). Perhaps
those potters who are unfamiliar with these works, and the terms they
use, should sit down, read and digest what is written.
Out of interest, what technical terms did I use that are not to be
found in Hamer's Dictionary?
Best regards,
Ivor Lewis.
Redhill,
S. Australia.

Louis Katz on tue 7 sep 04


I have had this problem with glaze tests using Strontium Carbonate.
Unless there is a problem material I never sieve small tests. Once I
mix up 1000 grams (2 pounds) or so I always sieve them. With the chunky
strontium carbonate the clumps seem to break up relatively easily in a
sieve. I would think 80 mesh would work as it does for me. I always run
the stuff through at least twice, once into another bucket and then
back. I use a rotary sieve on large buckets and a hand sieve with a
rubber spatula on smaller batches.

On the more technical side of this discussion just because something
does not melt by itself with out real high temperatures has never kept
alumina from entering into a glaze melt.
I too would like to know what happens to the carbonate, if it enters
the melt or not. Someone with a lot of knowlege about fining of glasses
might be able to answer our questions.

Louis
http://www.tamucc.edu/~lkatz
Down here in SOHO Texas

Ivor and Olive Lewis on wed 8 sep 04


Dear Louis Katz,
You say <<...just because something does not melt by itself with out
real high temperatures has never kept alumina from entering into a
glaze melt. >>
True! Nor does a high MP prevent MgO and CaO from dissolving in
silicate melts. Other materials which melt act as solvents for these
refractory substances to ensure that they "Melt" in the same way the
sugar "Melts" in hot water. Examples or these substances are the many
and varied frits which are on the market and substitutes for, as well
as actual, Gerstley Borate.
I am not sure that raw alumina is a common glaze ingredient except
where it is added as a matting and opacifying agent. Usual sources are
felspars and clays.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.

Ron Roy on thu 9 sep 04


Hi Ivor,

I did the experiment and did get some melting at cone 6 - but I was using
some materials already in combination - wolastonite, kaolin and silica.

I do think you are missing something though. I can easily see - when
looking at the CaO,Al2O3, SiO2 system phase diagrams - the melting point is
1170C. There is no doubt it is happening - what is missing is the method
they use to arrive at the data - why not find that out?

I have a cone 10 Celadon for instance - which is just melted at cone 10 in
a 12 hour firing - I also notice - the melting point of an already melted
sample - measured in my dilatometer - is about 800C. If I had just fired
that glaze to 800C it would have still been a friable powder.

Are you saying that the melting point of silica combined with alumina is
not lower than the melting points of either?

RR


>Yes, do the comparative experiments. I did when I noticed the
>anomalies written in popular reference books for potters. A simple
>test will show the error of claiming that mixtures of oxides have
>lower melting points that the individual components. Take a mixture
>of Calcium Oxide, Aluminium Oxide and Silicon Dioxide. Claim are made
>that this mixture will melt at or about 1170=BACelsius. I have yet to
>get such a mixture to melt in a firing to cone 8 or 9. It remains a
>friable powder. I think this indicates that melting is not inevitable.
>Others would tell me that if I keep it at that temperature for long
>enough it will melt. I agree. But do they know, as I do, why?

Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
=46ax: 613-475-3513=20

Louis Katz on thu 9 sep 04


Hi Ron, Ivor,

Wow, I have been off Clayart for a long time and this discussion is
still going on.

I am convinced that eutectic/liquidus diagrams are very useful, but
they are not the be all end all of information, just road signs, maybe
compass headings. One of the reasons these are just compass headings is
that we do not hold our glazes until they reach equilibrium. Others are
that there are always impurities, we dissolve some of the clay body. We
generally have some small amount of organic mater and reduction, our
glazes are not always totally homogenous.

It seems clear to me that feldspar will melt at a temperature different
than a simple mixture of its oxides. Beta - cristoblaite probably
melts at a different temperature than beta quartz, and while they are
probably equal in their ability to be dissolved by a melt, they may go
into solution at different rates.

Louis
http://falcon.tamucc.edu/~lkatz/b.mid my latest (opus 2) musical
composition.

Ivor and Olive Lewis on fri 10 sep 04


Dear Ron,
It is rather difficult to explain unless you hare in possession of the
relevant Phase Diagram so I will send a fragment direct to you.
Other folk might note when they look at that Fig 630 (ACS 1979 ) that
the substances named in the compound fields in contact with the three
join lines that meet to make the Point at which that 1170=BA C Eutectic
can be found are not Oxides. No prizes for the first to report the
names of these material to the list.
I challenge anyone and everyone to make the mixture for the Eutectic
using Pure, unadulterated (99.99% pure) oxides, Silicon Dioxide
(62.37%), Calcium Oxide (23.13%)and Aluminium Trioxide (14.50%) and
fire it to just above 1170=BA C or Cone 6 in an electric or gas kiln and
then tell me that they have formed a glass or a crystalline solid. To
ensure there is no contamination put this intimate mixture of oxide
powders in a stainless steel container and not in a ceramic container
if they want a "pure" result.
My recollection of gathering data for phase diagrams for Iron Alloys
is that all substances have to be brought into solution then cooled so
that the temperature at which crystallisation takes place can be found
by differential thermal analysis. Confirmation of the result is
obtained by microscopic examination of thin sections and
identification of the minerals which form during cooling.
Ron, you say <6 - but I was using some materials already in combination -
wolastonite, kaolin and silica.>> So did I. The melting you are
getting is "Fluid Phase Sintering". You did not create a solution.
Fluid phase sintering is one of the first important material reactions
in creating a glaze melt.
Then you ask <combined with alumina is
not lower than the melting points of either?>> It all depends on what
you mean by "combined". If it is combined chemically as in Kaolin
(Al2O3.2SiO2.2H2O) then my answer is "No", or Mullite (3Al2O3.2SiO2)
my answer is "Yes and No". But if you are saying "combined" as a
mixture of Corundum (Al2O3) and Quartz (SiO2)then I am saying "Yes".
Regarding your Celadon. When you reheat to do your dilatometer test,
are you reheating a mixture of the original ingredients, a glass, a
two phase mixture of crystals and glass or a total mixture of
crystals? I would say you have found the Glass Transition
Temperature.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.

Ivor and Olive Lewis on fri 10 sep 04


Dear Louis Katz,
Good message.
Had not some one, some where, at some time or other wrote that our
glazes melt at a "Eutectic point" because of the presence of a
"Eutectic Mixture" and that the components were "Oxides" I might never
bothered with the notion. But then I found the information repeated in
a way akin to that of a brass bound law that contradicted my previous
learning.
I disagree with that information. Why? Because what they claimed was
not in accord with what I was taught in tech college or with what I
learned when I was working in a Physical Metallurgy Lab making phase
change diagrams.
I wanted to know more. Who said it first ? Why ? When was this done ?
I read Singer and Singer . I also got off prints of articles from the
American Ceramics Society archives and studied those. Takes time,
costs money.
I keep on telling people to read Kingery et al, "Introduction to
Ceramics" Ch 10 to get good information about the processes we use
when we make and fire glazes.
Who cares any way. Mix a glaze recipe, put it on a pot, fire it. Enjoy
the aesthetic experience. What more is needed. Follow Mel. Make
thousands of bowls, scores of recipes, devise alternative firing
programs. I like the empirical way. Make hundreds of mixtures. Get
stupid ideas, test them.
But I do object to reading "Porkies" about the technology and science
of our craft and when if find them I will go hard to put the best
ideas available on the table.
Thanks for writing.

Best regards,
Ivor Lewis.
Redhill,
S. Australia.

Ps One of the most difficult things to do is to adequately define what
is meant by a "State of Equilibrium" as it relates to a
thermodynamically unstable material system. In the case of a Eutectic,
consider that all things must remain constant, that there is neither
addition nor removal of energy, there will be no change in temperature
or pressure and that all phases and materials are present
simultaneously.
.