John Hesselberth on sun 15 feb 04
Hi Everyone,
We have discussed this topic several times over the years and I tend to
buy into Ivor's hypothesis that the lowest melting material melts and
the other materials dissolve into that liquid. That hypothesis seems
particularly viable when there is a low melting frit involved in the
mix.
But I was cruising through the Appendices of Hamer and Hamer and ran
across the table of physical properties of many of the materials we
use. I began wondering how the zinc base glaze that is in MC6G melts.
The "fluxing" materials it contains and their reported melting points
per Hamer and Hamer are:
Neph Sy 1200C 2192F
Wollastonite 1545C 2813F
Zinc Oxide 1970C 3578F
Of course it also contains kaolin and silica.
So how does this glaze melt??? The lowest melting "flux" (Neph Sy)
melts at cone 6 yet this glaze is fluid significantly before that. I
don't have exact data, but certainly it makes a glaze a cone or 2 or 3
below 6--maybe not the best glaze, but one that has at least partially
melted.
Do impurities in "real" neph sy result in it melting at a much lower
temperature than the official one? Or is there really some substance to
this idea of eutectics where tiny particles of unmelted material can
rub up against each other and morph into a low melting eutectic at
which point the dissolving hypothesis can be operative? Or are the data
in Hamer and Hamer wrong? Or? Or? Or? Ideas? Thoughts? What I am I
missing here?
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Earl Krueger on mon 16 feb 04
John, Michael, et.al.
At temperatures above absolute 0 molecules dance.
The warmer it is, the more vigorous the dance.
As the vigor increases the dancers move farther apart.
With enough distance the partnerships fall apart.
The organization of a crystal transforms into
a milieu of unorganized writhing molecules.
A melt has occurred.
Imagine two crystals of different substances, A & B, in close proximity.
Think about the edges where they "touch".
As the vigor increases one dancer from A moves a little to one side.
This leaves a bit of space.
A dancer from B moves into the empty space.
As the dancer from A moves again, B intrudes even further.
And vice-versa.
The A-A and B-B partnership bonds are stretched.
Eventually they break.
The boundary becomes a mixture of A's and B's.
Which do not bind to each other as strongly as do A's to A's and B's to
B's.
A melt has occurred at a temperature lower than what would
be required to melt either A or B.
At least that's the model that works for me.
Earl K...
Bothell, WA, USA
Ivor and Olive Lewis on tue 17 feb 04
Dear Michael Wendt,
In support of what you write I would recommend sceptics should read
and digest the relevant chapter in "Introduction to Ceramics" By
Kingery, Bowen and Uhlmann; Ch 10, "Grain Growth, Sintering and
Vitrification".
But I would also caution that it is necessary to find Physical and
Chemical information about the materials we employ and their various
behaviours from alternative sources. In Hamer's Dictionary I find
notes about Nepheline and about Syenite with a reference back to
Nepheline which reveal nothing that could enable anyone to reasonably
answer John's question. There is nothing about Nepheline Syenite
Once any fluid forms it exercises its solvent qualities. But before
melting happens Atoms or Molecules most certainly migrate because of a
processes of Thermal diffusion, the "High Kinetic Energy" you talk
about. This effect is accelerated at pointed locations and when
particles are extremely small. The idea particle form will be a
Tetrahedron, the fragment sizes will be sub micron > 1/1000 mm and
particles will be in intimate contact.
But, as I wrote to John, Neph Sy is a separate agenda. Its low melting
point is readily explained when you are in possession of the facts.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on tue 17 feb 04
Dear John,
This is a good point and one which is misunderstood and, so it seems
to me, misrepresented through ignorance of viable facts.
It is one of the points I clarify in an article I have just sent away
for consideration by Janet Mansfield for publication in "Ceramics
Technical".
I do not wish to go into details at this moment but it would seem to
me firstly, that there is a distinction between a rock and a mineral
which is disregarded so there are material differences in behaviour.
Secondly, potters and ceramic artists and those who teach them have no
real understanding of this nebulous term "Eutectic".
The article is titled "Eutectics, Misunderstandings and
Clarifications" and it has been over three years in preparation.
To get back to your issue relating to that MC6G recipe ;Zinc
Semi-mat/Glossy which contains, among other things
> Neph Sy 1200C 2192F
> Wollastonite 1545C 2813F
> Zinc Oxide 1970C 3578F
Neph Sy is being treated as a mineral. It is a rock which contains a
Natural Equilibrium Eutectic. I leave it to interested parties to find
details for themselves.
Those who do will be enlightened
But if you want the whole story hope that Janet accepts my article
then buy the magazine.
Best regards,
Ivor Lewis. Redhill, South Australia
Michael Wendt on tue 17 feb 04
John,
The last time I mentioned this, I was thoroughly flamed. I too have seen
this effect with glazes composed of materials which do not melt at cone 10
when fired in separate heaps but do when mixed together.
I said before that any listed melting point is when the material is found to
be fully liquid without a solid phase present. This is not what glazes
require. If even a tiny portion of the material is starting to melt, the
contact points where the composition is correctly formulated to form the
lower melting mixture might account for the effect. It is well documented
that ball milling a glaze to decrease particle size lowers the melting
point, so very intimate contact is required.
If I say the temperature of fully molten Neph Sye is 2192 F, that
temperature measurement is the "average" kinetic energy of the atoms in the
material. Some of the atoms are at lower kinetic energy and some are at
higher energy in accordance with the black body radiation curve. These
higher energy atoms could well account for this effect. A good example of
this phenomenon is that a piece of red hot iron gives off radiation all the
way from infrared to visible light to UV and on up into the x-ray range.
I realize that this is not an adequate explanation, but it definitely does
happen.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
John wrote:
Hi Everyone,
We have discussed this topic several times over the years and I tend to
buy into Ivor's hypothesis that the lowest melting material melts and
the other materials dissolve into that liquid. That hypothesis seems
particularly viable when there is a low melting frit involved in the
mix.
But I was cruising through the Appendices of Hamer and Hamer and ran
across the table of physical properties of many of the materials we
use. I began wondering how the zinc base glaze that is in MC6G melts.
The "fluxing" materials it contains and their reported melting points
per Hamer and Hamer are:
Neph Sy 1200C 2192F
Wollastonite 1545C 2813F
Zinc Oxide 1970C 3578F
Of course it also contains kaolin and silica.
So how does this glaze melt??? The lowest melting "flux" (Neph Sy)
melts at cone 6 yet this glaze is fluid significantly before that. I
don't have exact data, but certainly it makes a glaze a cone or 2 or 3
below 6--maybe not the best glaze, but one that has at least partially
melted.
Do impurities in "real" neph sy result in it melting at a much lower
temperature than the official one? Or is there really some substance to
this idea of eutectics where tiny particles of unmelted material can
rub up against each other and morph into a low melting eutectic at
which point the dissolving hypothesis can be operative? Or are the data
in Hamer and Hamer wrong? Or? Or? Or? Ideas? Thoughts? What I am I
missing here?
Regards,
John
John Hesselberth on tue 17 feb 04
On Tuesday, February 17, 2004, at 02:11 AM, Earl Krueger wrote:
>
> A melt has occurred at a temperature lower than what would
> be required to melt either A or B.
>
>
> At least that's the model that works for me.
>
> Earl K...
> Bothell, WA, USA
Hi Earl,
That's a model that works for me too, but I had just about let Ivor
convince me that was all hogwash--at least that is what I think he was
trying to convince us of. His theory of the lowest melting material
melting and then acting as a solvent for the other materials seems to
make sense -- until this flash came to me when I was perusing Hamer and
Hamer and I realized that some glazes are made from mixtures of
materials that don't melt at all at glaze temperatures if, as Michael
says, they are in individual piles.
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
John Hesselberth on thu 19 feb 04
On Friday, February 20, 2004, at 01:35 PM, Michael Wendt wrote:
> Since Helmer Kaolin is very high in melting point, the small amount of
> low
> melting point material cannot overcome the clay portion when it is
> standing
> alone. The material appears to be mica. When I mix Helmer with
> dolomite
> and silica in the right proportions I get a solid mass that resembles a
> stony mat glaze while piles of each of the ingredients remains utterly
> friable even after a cone 10 firing. Amazing how significant those tiny
> amounts of impurity are!
Thank you Michael. Those are certainly important observations and help
a lot.
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Michael Wendt on fri 20 feb 04
Ivor mentioned to me in an earlier post that he has access to high purity
lab grade materials. His tests with these high purity materials support his
conclusion that it is the lower melting point impurities in commonly
available materials which initiate the melt by becoming fluid first and
bringing the other ingredients into the solution.
Part of the problem may lie in the way most lab analyses are done with
lower cost XRF equipment today.
XRF (x-ray fluorescence) can identify the amount of each material present by
concentration but does not identify the mineral species present.
XRD (x-ray diffraction) can identify the actual composition and mineral
species.
Since Helmer Kaolin is very high in melting point, the small amount of low
melting point material cannot overcome the clay portion when it is standing
alone. The material appears to be mica. When I mix Helmer with dolomite
and silica in the right proportions I get a solid mass that resembles a
stony mat glaze while piles of each of the ingredients remains utterly
friable even after a cone 10 firing. Amazing how significant those tiny
amounts of impurity are!
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
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