Earl Krueger on thu 19 feb 04
On Tuesday, Feb 17, 2004, at 19:51 US/Pacific, Ivor and Olive Lewis=20
wrote:
> However, my humble experience is that glazes compounded
> from Soda Felspar, Potash Felspar or Nepheline Syenite
> struggle to mature at any temperature below 1200 =BAC
> ~2192=BAF even though they may show signs of melting.
> In theories given to us by "Glaze Masters", we should be
> able to mix Potash Felspar, Quartz and Kaolin and get a
> total melt at 985=BAC ~1805=BAF from a recipe containing
> 9.5%K2O, -10.5% Al2O3 - 80% SiO2.
> Similarly, and contrary to what we might expect, using
> Soda Felspar with Quartz and Kaolin and the following
> oxide values; 8% Na2O, 14% Al2O3, 78% SiO2 melting,
> in theory, should happen at 1050=BAC ~1922=BAF. but I would
> not lay bets on the event.
Ivor,
In all of these statements are you referring to the temperature
for a first melt of a mixture of ingredients or a re-melt of a fully
fused and cooled glaze?
It is my understanding that eutectics only describes the
freezing/melting point of a fully fused solution of materials
in which the various molecular species are uniformly dispersed
at the molecular level.
A mixture of macro sized particles of material, as we typically
have when we mix a glaze, does not fit this description. I would
expect melting observations of these mixtures to be dependent
on such factors as particle size, packing, etc. and most likely
quite different than what eutectic charts would predict.
In my opinion this is where the concept of heat-work comes
from. In general, a pure material or a previously melted eutectic
mixture will melt quickly over a narrow temperature range. Frits
are a good example. In a mixture of macro sized particles where
eutectic mixtures are created by thermal diffusion at boundaries it
takes time for the diffusion to occur. This time at a temperature
is what potters have come to call heat-work to explain their
observations.
Thanks.
Earl K...
Bothell, WA, USA
Ivor and Olive Lewis on fri 20 feb 04
Dear Earl ,
"Eutectic" is a term with several meanings, one of which is familiar
to most of us
You ask << In all of these statements are you referring to the
temperature
for a first melt of a mixture of ingredients or a re-melt of a fully
fused and cooled glaze? >>
You are right to wish to discriminate between the two. With regard to
standard glazing practice I consider all ingredients to be in a "First
Melting Event" situation.
With regards to a "Frit" within a glaze recipe I think this would hold
true as well, even though it is, if its structure is glassy a second
melt event.
But otherwise, as in the case of a mature glaze, then, this seems to
be second melting event.
You suggest <freezing/melting point of a fully fused solution of materials in which
the various molecular species are uniformly dispersed at the molecular
level.>>
I think this is a good and obvious logical response, Though I feel
that such a structure would be classed as a "Solid Solution" with each
compound being mutually soluble in the other since there is no
separation into separate phases. But check that out.
However each phase, or if you prefer, each substance, will nucleate,
form a "Crystallite" and grow according to its own predetermied
crystal lattice. So in a two compound system you end up with an
aggregate mixture which has an observable characteristic structure
when examined through the microscope. In the case of Nepheline Syenite
I understand that the structure is of interleaved sheet crystals. This
sort of reacton can happen in the solid state through diffusion and
gives a "Pearlite Structure" which is responsible for the hardening fo
steel. Though I do not understand the physical mechanisms which induce
a much lower melting point, they may involve factors of Surface Energy
or Latent Heat.
<from...>>
"Heat Work" is a tautology. Both are measured in the same units. A
much better viewpoint is to call this value "Time Energy" and relate
this to the mass which is being heated. If we know the temperature
intended to achive maturity, the mass of material being heated, its
Specific Heat, the rate at which heat is being supplied and the Latent
Heat of Fusion we can calculate an estimate of temperatue at which
maturity, as in the vitrification of a clay or the melting of a glaze
will happen. All of which is neatly summarised in the behaviour of
Orton's Cones without any mental hardship.
Enjoy your weekend.
Best regards,
Ivor Lewis. Redhill, South Australia
Earl Krueger on sat 21 feb 04
Ivor,
I've gotten a little confused due to there being multiple threads
going on that seem to be discussing the same or similar topics.
On Feb 19, 2004, at 20:22 US/Pacific, Ivor and Olive Lewis wrote:
> With regard to standard glazing practice I consider all
> ingredients to be in a "First Melting Event" situation.
> With regards to a "Frit" within a glaze recipe I think this would hold
> true as well, even though it is, if its structure is glassy a second
> melt event.
I agree. And, if you make the assumption that the frit has the lowest
melting point then this would provide the liquid that in turn
"dissolves" the remaining materials, as was your suggestion and
John H's question.
However, lacking a low melting point material I still believe the
most plausible explanation for how a glaze melts is by solid
state diffusion which results in localized eutectic compositions.
Once these small areas reach the melting point of the eutectic
I can see them drawing into the melt the remaining materials.
> Though I feel that such a structure would be classed as a "Solid
> Solution" with each compound being mutually soluble in the other
> since there is no separation into separate phases.
Agree. Solid Solution being a more academic term for my
more pedestrian words.
> So in a two compound system you end up with an
> aggregate mixture which has an observable characteristic structure
> when examined through the microscope. ... Though I do not
> understand the physical mechanisms which induce
> a much lower melting point, they may involve factors of
> Surface Energy
An interesting concept. I can see where a mixture, as you
described, might melt based on different principles than a
glass or a mixture of macro particles.
> If we know the temperature intended to achive maturity,
> the mass of material being heated, its Specific Heat,
> the rate at which heat is being supplied and the Latent
> Heat of Fusion we can calculate an estimate of temperatue
> at which maturity, as in the vitrification of a clay or the
> melting of a glaze will happen.
I think you need to add to this the kinetics of the chemical
reactions taking place. As you are aware, they are not
instantaneous and therefore time at a temperature must
be given for them to reach a state of equilibrium.
Later,
Earl K...
Bothell, WA, USA
Kind of fun dredging up data stored in the old brain
over 30 years ago and not thought about much since.
John Hesselberth on sat 21 feb 04
On Saturday, February 21, 2004, at 03:53 AM, Earl Krueger wrote:
> However, lacking a low melting point material I still believe the
> most plausible explanation for how a glaze melts is by solid
> state diffusion which results in localized eutectic compositions.
> Once these small areas reach the melting point of the eutectic
> I can see them drawing into the melt the remaining materials.
Hi Earl,
Part of the difficulty of communicating on this subject may be a rather
loose use of the word eutectic. The eutectic is, of course, the
composition of lowest melting point. But you don't have to have
exactly that composition to get melting substantially below the melting
temperature of the constituent materials. There is a whole range of
compositions that will result in melting below the temperatures to
which we fire.
That would seem to make the "dance" (aka solid state diffusion) you
describe a much more feasible thing to achieve in a real-world
situation. So, to me, the two most likely scenarios (where frits are
not involved) are 1) your "dance" which results in lower melting
materials being formed or 2) the presence of trace quantities of lower
melting materials embedded within the materials we think of as 'pure'.
But if 2) is operative what might some of those materials be?? We ought
to be able to locate some inorganic compounds which melt at about of
1000C, are stable, and likely to be found as impurities in the rocks we
crush up and call feldspars, whiting, etc. And, wouldn't they be likely
to also result in signs of melting in the individual piles of
ingredients which we know don't melt?
Hmm, I find myself leaning back toward the "dance" hypothesis.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on sun 22 feb 04
Dear Earl,
Seems we have a common understanding about how the melting process is
dependent on something which will dissolve the other ingredients
Now, about <believe the most plausible explanation for how a glaze melts is by
solid state diffusion >> This is what I get from reading Kingery et al
and if fits in with my experience of making alloys. But he also speaks
of lattice diffusion, surface diffusion, vapour transport and boundary
diffusion as being contributing mechanisms in the forces which drive
the sintering process.
But to you next comment <compositions.>> I have to say "Well, ahem! yes?", or "Perhaps" or
"Maybe" or "It All Depends" on what you mean by "Eutectic". If you
mean the common sense occurrence of anything which is capable of
melting at a lower than anticipated temperature you are most probably
have a very good and correct opinion.
But using the term "Eutectic" in the sense of those temperatures we
derive from Phase Equilibrium Diagrams you have to show that chemical
reactions take place which yield the compounds which are involved with
each specific temperature. If you analysis of a sample taken and
quenched then analysed yields that result then again you would be
correct in your assertion. For example, if you are choosing the 1170=BA
point in the CaO-Al2O3-SiO2 system making a glaze from kaolin, silica
and whiting you have to demonstrate the creation of Anorthite,
Tridymite and Pseudo-wollastonite.
Finally << I think you need to add to this the kinetics of the
chemical
reactions taking place >> Yes, this would have to be done. For
example, in the example I have given, or for that fact in any recipe
which includes Whiting or some form of calcium carbonate and free
silica, be this as an ingredient or as a decomposition product of clay
there will be a solid state reaction which yields Calcium
mono-silicate, a chemical some of us choose to exploit as
Wollastonite.
Equilibrium is another term which needs clarification and perhaps it
should not be invoked in the dynamic systems existing in a maturing
glaze.
I know how you feel << Kind of fun dredging up data stored in the old
brain over 30 years ago and not thought about much since. >>, but so
much has become know since those days, which is why I keep buying
books and why I think the Kingery "Introduction to ceramics",
Brownell's "Structural Ceramics" and modern Chemistry and Physics
texts are so important when trying to understand the Science and
Technology of our Art.
Best regards and have a good week.
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on sun 22 feb 04
Dear John,
True, there is untold confusion in the way the term "Eutectic" is
used.
The idea that << ...the eutectic is, of course, the composition of
lowest melting point.... >> has to be qualified by defining the
system to which reference is being made, the physical conditions and
the compositions of the contributing compounds. If those how know do
not provide that clarification they mislead their students.
By the way, the magic bullet ingredient you are speak of to induce
melting is present in almost every mineral and rock take from the
Earth, usually less than a few parts per million and it is generally
in the waters we use to mix our ingredients. Melting point is 805=BA C!
Best regards,
Ivor Lewis. Redhill, South Australia
David Hewitt on sun 22 feb 04
My preferred way of trying to understand what happens when we do a glaze
firing is to look at some sample equilibrium phase diagrams. I would
hazard a guess that most of our recipes are not precisely eutectic
compositions. As we heat up such a glaze and go above what could be the
temperature of the eutectic point, then some of the components start to
be liquid. If we go high enough we cross the liquidus line for the
particular recipe being used and all the components become liquid. Also,
interestingly, these phase diagrams show that it depends on which of the
eutectic composition our particular glaze composition falls, so the
composition of the phase between the eutectic point and the liquidus
line differs.
If this is of interest, examples of this are shown on my web site under
Pottery Techniques/Eutectics and Phase Equilibrium Diagrams
http://www.dhpot.demon.co.uk
David
In message , John Hesselberth writes
>On Saturday, February 21, 2004, at 03:53 AM, Earl Krueger wrote:
>
>> However, lacking a low melting point material I still believe the
>> most plausible explanation for how a glaze melts is by solid
>> state diffusion which results in localized eutectic compositions.
>> Once these small areas reach the melting point of the eutectic
>> I can see them drawing into the melt the remaining materials.
>
>Hi Earl,
>
>Part of the difficulty of communicating on this subject may be a rather
>loose use of the word eutectic. The eutectic is, of course, the
>composition of lowest melting point. But you don't have to have
>exactly that composition to get melting substantially below the melting
>temperature of the constituent materials. There is a whole range of
>compositions that will result in melting below the temperatures to
>which we fire.
>
>That would seem to make the "dance" (aka solid state diffusion) you
>describe a much more feasible thing to achieve in a real-world
>situation. So, to me, the two most likely scenarios (where frits are
>not involved) are 1) your "dance" which results in lower melting
>materials being formed or 2) the presence of trace quantities of lower
>melting materials embedded within the materials we think of as 'pure'.
>
>But if 2) is operative what might some of those materials be?? We ought
>to be able to locate some inorganic compounds which melt at about of
>1000C, are stable, and likely to be found as impurities in the rocks we
>crush up and call feldspars, whiting, etc. And, wouldn't they be likely
>to also result in signs of melting in the individual piles of
>ingredients which we know don't melt?
>
>Hmm, I find myself leaning back toward the "dance" hypothesis.
>
>Regards,
>
>John
>http://www.frogpondpottery.com
>http://www.masteringglazes.com
--
David Hewitt
David Hewitt Pottery
South Wales UK
Web:- http://www.dhpot.demon.co.uk
John Hesselberth on sun 22 feb 04
On Saturday, February 21, 2004, at 11:44 PM, Ivor and Olive Lewis=20
wrote:
> By the way, the magic bullet ingredient you are speak of to induce
> melting is present in almost every mineral and rock take from the
> Earth, usually less than a few parts per million and it is generally
> in the waters we use to mix our ingredients. Melting point is 805=BA =
C!
Hello Ivor,
Well, I'm going to speculate you are referring to sodium chloride,=20
although my handbook lists the melting point at 801C.
It would seem then, that a pile of finely ground feldspar, which as you=20=
suggest is contaminated with a bit of NaCl (or whatever), would show=20
signs of melting--maybe some fused particles-- well below its published=20=
melting point of about 1200C. I guess I will have to run that=20
experiment at various temperatures, get out my microscope and see where=20=
(at what fired temperature) I can begin to find fused particles vs. the=20=
unfired original. Or, if you have already done similar tests, I'll let=20=
it rest until I can read your published article on the subject and see=20=
if it all suddenly becomes perfectly obvious.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ivor and Olive Lewis on mon 23 feb 04
Spot on John,
Now you have set your self a second problem. Which Felspar will you
choose?
By the way, there are situations where the natural ingredients do not
contain common salt but will release Sodium Ions into solution. What
compound might form as your pottery of glazes dried if this were the
case and what would your predicted melting temperature be ?
Good contribution.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on mon 23 feb 04
Dear David Hewitt,
If glaze compositions containing K2O, Na2O, Al2O3 and SiO2 were
calculated to be close to any of the Eutectic Points given in Phase
Equilibrium Diagrams, then according to your writing we should get a
catastrophic melt down soon after a kiln's temperature passed though
the 700=BA barrier.
Perhaps you would enlighten us as to why we do not if we employ
felspars and other minerals?
Best regards,
Ivor Lewis. Redhill, South Australia
David Hewitt on tue 24 feb 04
Dear Ivor Lewis,
I think that we have been here before.
Firstly the point I was trying to make was that our recipes are rarely
such that they would be the recipe that would produce the eutectic for
the oxides in those particular materials. As such some of the glaze
materials would start to melt at some temperature, say your 700oC, and
as we progress above this the other materials start to melt until we
reach the liquidus level. When this is reached the glaze materials will
not just run off the pot because of such factors as surface tension and
viscosity. The glaze layer is usually very thin and microscopic
examination of the cross section of a piece of glazed pottery shows that
a buffer layer forms between body and glaze.
Some glazes do tend to run off the pot such a crystalline glazes. Over
fired and too thickly applied glazes also present this problem.
Does this help? But then I am sure that you know all this.
David
In message , Ivor and Olive Lewis writes
>Dear David Hewitt,
>If glaze compositions containing K2O, Na2O, Al2O3 and SiO2 were
>calculated to be close to any of the Eutectic Points given in Phase
>Equilibrium Diagrams, then according to your writing we should get a
>catastrophic melt down soon after a kiln's temperature passed though
>the 700º barrier.
>Perhaps you would enlighten us as to why we do not if we employ
>felspars and other minerals?
>Best regards,
>Ivor Lewis. Redhill, South Australia
--
David Hewitt
David Hewitt Pottery
South Wales UK
Web:- http://www.dhpot.demon.co.uk
Ivor and Olive Lewis on wed 25 feb 04
Dear David Hewitt,
In general I agree with your points.
But a Eutectic, as given in the Phase diagrams, is the simultaneous
achievement of Solidus and Liquidus since both have the same
temperature.
My question was about compounding recipes from readily available
materials such as a Felspar, Quartz and Clay to achieve the oxide
content of say, the 740=BA C Eutectic in the Na2O - Al2O3 -SiO2 system,
or the 695=BA C Eutectic in the K2O-Al2O3- SiO2 System.
If I am reading your teaching correctly, such mixtures will melt
entirely at the stated temperatures. It should not be hard to
calculate recipes from the following values :
Na2O 26% Al2O3 12% SiO2 62%
Theoretical Fusion Point 732 deg C.
Orton Cone Equivalent Cone 017>018
Or from
K2O 30.8% Al2O3 3.0% SiO2 66.2%
Theoretical Fusion Point 695 deg C
Orton Cone Equivalent (approximate near value) Cone 018>019
By the way, mixtures containing either Soda or Potash Felspar might
commence sintering round about the 600=BA C Mark.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "David Hewitt"
To:
Sent: Wednesday, 25 February 2004 3:25
Subject: Re: Fusion of glazes and heat-work
> Dear Ivor Lewis,
> I think that we have been here before.
> Firstly the point I was trying to make was that our recipes are
rarely
> such that they would be the recipe that would produce the eutectic
for
> the oxides in those particular materials. As such some of the glaze
> materials would start to melt at some temperature, say your 700oC,
and
> as we progress above this the other materials start to melt until we
> reach the liquidus level. When this is reached the glaze materials
will
> not just run off the pot because of such factors as surface tension
and
> viscosity. The glaze layer is usually very thin and microscopic
> examination of the cross section of a piece of glazed pottery shows
that
> a buffer layer forms between body and glaze.
> Some glazes do tend to run off the pot such a crystalline glazes.
Over
> fired and too thickly applied glazes also present this problem.
> Does this help? But then I am sure that you know all this.
> David
> In message , Ivor and Olive Lewis writes
> >Dear David Hewitt,
> >If glaze compositions containing K2O, Na2O, Al2O3 and SiO2 were
> >calculated to be close to any of the Eutectic Points given in Phase
> >Equilibrium Diagrams, then according to your writing we should get
a
> >catastrophic melt down soon after a kiln's temperature passed
though
> >the 700=BA barrier.
> >Perhaps you would enlighten us as to why we do not if we employ
> >felspars and other minerals?
> >Best regards,
> >Ivor Lewis. Redhill, South Australia
>
> --
> David Hewitt
> David Hewitt Pottery
> South Wales UK
> Web:- http://www.dhpot.demon.co.uk
>
>
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David Hewitt on thu 26 feb 04
Dear Ivor Lewis
In message , Ivor and Olive Lewis writes
>Dear David Hewitt,
>In general I agree with your points.
>But a Eutectic, as given in the Phase diagrams, is the simultaneous
>achievement of Solidus and Liquidus since both have the same
>temperature.
>My question was about compounding recipes from readily available
>materials such as a Felspar, Quartz and Clay to achieve the oxide
>content of say, the 740=BA C Eutectic in the Na2O - Al2O3 -SiO2 system=
,
>or the 695=BA C Eutectic in the K2O-Al2O3- SiO2 System.
>If I am reading your teaching correctly, such mixtures will melt
>entirely at the stated temperatures.=20
That is exactly how I read these phase equilibrium diagrams, but I thin=
k
that you may have practical difficulty in replicating this condition. I=
f
you do, then of course the other points that I made regarding viscosity=
,
surface tension, boundary layer between body and glaze and glaze
thickness apply to condition whether or not the glaze runs off the pot
at that temperature.
>It should not be hard to
>calculate recipes from the following values :
>Na2O 26% Al2O3 12% SiO2 62%
>Theoretical Fusion Point 732 deg C.
>Orton Cone Equivalent Cone 017>018
>Or from
>K2O 30.8% Al2O3 3.0% SiO2 66.2%
>Theoretical Fusion Point 695 deg C
I would be interested to know what recipes you find that will produce
exactly these oxide proportions and without other oxides being included=
=2E
>Orton Cone Equivalent (approximate near value) Cone 018>019
>By the way, mixtures containing either Soda or Potash Felspar might
>commence sintering round about the 600=BA C Mark.
>Best regards,
>Ivor Lewis. Redhill, South Australia
David
--=20
David Hewitt
David Hewitt Pottery
South Wales UK
Web:- http://www.dhpot.demon.co.uk
Ivor and Olive Lewis on fri 27 feb 04
Dear David
<think
that you may have practical difficulty in replicating this condition>>
Then, information derived from Equilbrium Phase diagrams is of no
value in designing new glazes or making predictions about old ones.
<viscosity, surface tension, boundary layer between body and glaze and
glaze thickness apply to condition whether or not the glaze runs off
the pot at that temperature.>>
I would agree that, regardless of the raw material in the glaze, once
it becomes molten these factors become paramount until the material
sets.
But surely, they have no influence on the melting process.
>Eutectic compositions expressed as Oxides:
>Na2O 26% Al2O3 12% SiO2 62% T=BA 740
>>K2O 30.8% Al2O3 3.0% SiO2 66.2% T=BA 695
>CaO 23.13% Al2O3 14.5% SiO2 62.37% T=BA 1170
<produce
exactly these oxide proportions and without other oxides being
included.>>
It is impossible to make either the K2O or Na2O examples as practical
recipes from the stated materials (Felspar, Kaolin and Silica) though
they could be achived by concocting high alkali frits. But since frits
are not featured in the phase diagrams mentioned they would be
inadmissible as components of a Eutectic composition.
A couple of years or so ago I did investigate the 1170=BA C point in the
CaO-Al2O3-SiO2 System, firing samples to Cone 8. Those made with
oxides and whiting or kaolin, silica and whiting did not fuse. That
made with Anorthosite, Wollastonite and Silica did. indicating to me
that it was the Minerals named in the diagrams which are necessaryand
not oxides of the Elements. Anorthosite for the occasion was kindly
supplied by Michael Banks. Such a pity it is not available from
Ceramic suppliers.
Best regards and thanks for the conversation
Ivor Lewis. Redhill, South Australia
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