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
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