JJHerb@aol.com on thu 10 apr 97
To amplify slightly on Richard Burkett s definition of eutectic, take the
example of lead-tin solder. Lead melts at 327.3 degrees C and Tin melts at
231.8 degrees C. Mixtures (alloys) of these two materials melt at different
temperatures depending on the composition of the mix. The composition that
has the lowest melting temperature (180 degrees C (?)) is 67 percent lead and
33 percent tin. The reason 60-40 solder is so popular with stain glass
workers is because of this lowered (eutectic) melting point.
Non-eutectic mixtures have a melting range. The material melts (or freezes)
over a range of temperature. This means that for some temperatures the
material is a "slushy" mixture of crystals in liquid. This kind of material
is difficult to handle in soldering applications because it doesn t flow well
until it is all melted at a higher temperature. The Eutectic mixture melts
(and freezes) all at once.
Other mixtures, of metals, minerals, or cooking ingredients, have eutectic
compositions and melting points. One of the interesting results of early
experiments on igneous rocks was the discovery that the eutectic, Lowest
melting, composition of average earth materials is that of granite. This
common igneous rock is the first to melt and, more significantly, the last to
solidify. One of the things that the geologists have to consider is the
effect of water on the melting point of the mixtures. Water lowers the
melting points considerably and could prove a boon to potters if only one
could develop a reasonable hydrothermal pottery kiln. A few hundred psi
water vapor pressure in the kiln chamber might lower the effective firing
temperature, for the same amount of melting, several hundred degrees.
Joseph Herbert
JJHerb@aol.com
ivor & olive lewis on sat 21 nov 09
<
Monthly. I was looking forward to some lively and enlightening exchange on
this subject on Clayart. >>
Dear May Luk,
Have to say I missed both events, publication of David's article and the
ensuing discussion.
There is possibly more misinformation shared by potters about the nature of
Eutectic behaviour in material systems than solid scientific fact that can
be validated by experiment, observation and measurement.
If the temperatures of Eutectic Points are taken from diagrams in "Phase
Diagrams for Ceramists", they are only valid as representing the
solidification temperature of melts as they cool.
Any claims that a particular glaze recipe will melt because of the
intervention of a Eutectic Event must be qualified and validated by facts
relating to specific ingredients. For example, Michael Cardew give
information about the 1170 deg C point in the CaO-Al2O3-SiO2 system. A
recipe composed of those simple ingredients; Whiting, Aluminium hydrate and
Quartz will not melt at that temperature. It may begin to sinter above Cone
10 , 130 deg C higher than the eutectic point.
The only natural example of Eutectic melting that I have identified relates
to a natural mixture of minerals Soda Felspar and Potash Felspar. Potters
know this substance as Nepheline Syenite. This is a Rock. See Fig 786 in
"Phase Diagrams...."
To answer your signoff question, Depends on the ingredients !
Best wishes,
Ivor Lewis,
Redhill,
South Australia.
John Britt on sat 21 nov 09
Ivor,
No, you did not miss it. See below:
Here is the tread:
http://www.potters.org/subject102281.htm
"John Britt on sun 2 sep 07
Ivor,
You posted this a while back but no one has responded. I was happy to see=
=3D
that David took the time to write his article and that CM published. He
did a great job writing on this difficult subject and I did not want to
see it pass without some discussion.
But I had some problems with the artile too, in that I am a practical
application freak.
I am with you on your point: "Which ingredients should we select, using
his example to guide us, if we wished to exploit that information?=3D3D94
When does a potter ever use Alumina as a glaze ingredient? When are there=
=3D
ever only three oxides in a glaze?
Glazes are very complex especially with the ingredients potters use. As
soon as you throw feldspar or clay in to a glaze recipe (which is almost
every time) you have about nine oxides present, which takes you out of th=
=3D
e
available charts. So knowing that eutectics exist is a lot like flying to=
=3D
London. It is interesting to know about the Bernoulli principle or
Newton=3D3D92s third law but does it help you get to London? No.
What helps you get to London is a ticket and directions to the airport.
The only time I have ever read of a practical use of eutectics and glazes=
=3D
is the reference to lithium and a cone 1 eutectic with shinos. But I have=
=3D
never tested this hypothesis. All my cone 10 shinos (feldspar and clay)
melt with or without the eutectic with lithium feldspars.
I have heard the term used when a glaze runs and someone says that =3D3D93i=
=3D
t
must have formed a eutectic=3D3D94, or =3D3D93the two glazes form a eutecti=
c=3D3D=3D
94,
but=3D3D
there is never any specific practical way to use this information.
My understanding is that here are often multiple eutectics and so knowing=
=3D
when adding some ingredient will push you up or down the trough is
unknown. It just seems that without specific charts for all the
ingredients we use the concept is more academic than practical.
Just my opinion,
John Britt
www.johnbrittpottery.com "
Lee Love on sat 21 nov 09
On Sat, Nov 21, 2009 at 10:58 AM, John Britt w=
=3D
rote:
Seems like a pretty civil post to me.
>
> http://www.potters.org/subject102281.htm
>
>
> "John Britt on sun 2 sep 07
>
> Ivor,
>
> You posted this a while back but no one has responded. I was happy to see
> that David took the time to write his article and that CM published. He
> did a great job writing on this difficult subject and I did not want to
> see it pass without some discussion.
>
> But I had some problems with the artile too, in that I am a practical
> application freak.
>
> I am with you on your point: "Which ingredients should we select, using
> his example to guide us, if we wished to exploit that information?=3D3D94
>
> When does a potter ever use Alumina as a glaze ingredient? When are there
> ever only three oxides in a glaze?
>
> Glazes are very complex especially with the ingredients potters use. As
> soon as you throw feldspar or clay in to a glaze recipe (which is almost
> every time) you have about nine oxides present, which takes you out of th=
=3D
e
> available charts. So knowing that eutectics exist is a lot like flying to
> London. It is interesting to know about the Bernoulli principle or
> Newton=3D3D92s third law but does it help you get to London? No.
>
> What helps you get to London is a ticket and directions to the airport.
>
> The only time I have ever read of a practical use of eutectics and glazes
> is the reference to lithium and a cone 1 eutectic with shinos. But I have
> never tested this hypothesis. All my cone 10 shinos (feldspar and clay)
> melt with or without the eutectic with lithium feldspars.
>
> I have heard the term used when a glaze runs and someone says that =3D3D9=
3i=3D
t
> must have formed a eutectic=3D3D94, or =3D3D93the two glazes form a eutec=
tic=3D
=3D3D94,
> but=3D3D
>
> there is never any specific practical way to use this information.
>
> My understanding is that here are often multiple eutectics and so knowing
> when adding some ingredient will push you up or down the trough is
> unknown. It just seems that without specific charts for all the
> ingredients we use the concept is more academic than practical.
>
> Just my opinion,
>
> John Britt
> www.johnbrittpottery.com "
>
--=3D20
--
Lee, a Mashiko potter in Minneapolis
http://mashikopots.blogspot.com/
"Ta tIr na n-=3DF3g ar chul an tI=3D97tIr dlainn trina ch=3DE9ile"=3D97tha=
t is, "T=3D
he
land of eternal youth is behind the house, a beautiful land fluent
within itself." -- John O'Donohue
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