I.Lewis on thu 25 nov 99
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First, a heartfelt thank you to all those who have used their time and =
energy to
respond to statements which I have been making about the place of Eutectic =
in
our thinking. I appreciate and accept all comments in the spirit of good
companionship.
May I say that eutectic mixtures have very practical uses. In the days of =
lead
plumbing a material called Wiping Solder was used to repair joints in lead =
pipes
which had been split by expanding water. Wiping Solder is based on a mixture=
of
lead crystals in a fluid base. The fluid is the Eutectic of Tin and Lead.
Because of its presence the alloy stays mushy for a long time and the hot =
lead
paste can be shaped into a strong, physically robust, bulge of metal around =
the
break.
About three years ago I became concerned that those who write about glaze
chemistry may be miss-using the term Eutectic. I have the ACS Phase diagram
book, the thick one and also some of the wall charts. I also have a first
edition of Hamer=92s Dictionary so I am familiar with some of the writing =
which
has been mentioned. However, it was a statement in Glazes for the Studio =
Potter
by Cooper and Royle, a book I love and use often, well worth the money, =
which
had me worried.
These authors give common salt and ice as an example of a eutectic to =
illustrate
their text. I suppose anyone who lives in a snowy climate can relate to this=
or
in a hot climate where they make ice cream. It=92s a good method of getting =
a very
cold freezing mixture or melting packed snow on slippery roads. Mix rock =
salt
into crushed ice and stir. The mercury, or alcohol if you prefer drops like =
a
proverbial brick to a minimum temperature of minus21.1C (minus 5.98F) =
provided
the proportion of salt does not exceed sixty percent of the mixture.
I studied this for a long time. Yes, my common experience is that if I add =
salt
to ice the ice starts to melt and the temperature falls. I love to play
semantics, to ask, what do you mean, define your terms and other =
philosophical
games. But training as a technician and just the first year of a science =
degree
make me ask about assumptions, conjecture, anomaly. The Ice Salt thing is an
analogy. I am wary of analogies as ways of explaining natural phenomena. The
thing which counts is not the match with what we are seeking to explain. If =
the
statement about ice and salt is true it will occur at all temperatures down =
to
minus 21.1C. If it did this then it would be a good analogy So I devised the
following experiment and tested it. I took fine granulated salt and ice, a
spoon, a plastic container and a fridge thermometer which reads down to very=
low
temperatures to check my working temperatures. My freezer stood at minus =
14C. I
put salt and the crushed ice in the freezer and left them over night with a
plastic container and a spoon. The next day I mixed the two substances =
together
and put them back in the freezer at minus 14C(6.8F) and left them for twenty
four hours. Now our common knowledge predicts that I will have a liquid in =
the
pot because of the eutectic.
Guess what? No, don=92t guess. Repeat my experiment out if you are =
interested in
learning about glaze behaviour. Use about one fifth by weight of salt and =
four
fifths well crushed ice. Do not let the ice get wet. If necessary put it =
back
into the freezer to cool again before making your mixture. Recall, I have =
only
changed one factor. I have made everything start from the same temperature =
which
is not the same as shovelling salt from the pantry onto ice from the fridge =
in a
warm kitchen. How will your observations compare to those I made?
Best regards to all with inquisitive minds.
Ivor Lewis
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