michael wendt on mon 2 oct 00
There have been many excellent posts regarding fluxes. I can't add a lot =
except to say that it seems to go against intuition that two or more =
solid materials, all with very high melting points, when mixed together =
and heated, have a lower melting point than any of them alone. I can see =
why students are puzzled.
Perhaps , if they look at the atom in a different way, it might =
help. Most of the atom is empty space. Atoms are almost not here at all! =
Early nuclear models saw electrons as tiny planets with distinct orbits =
spinning about their tiny nucleus suns. Today's physics model envisions =
more of a cloud of electrons operating in shell configurations that vary =
in their ability to join with other atoms depending on the number of =
electrons in the outer shell.
Solid crystals appear rigid and unyielding at the visible scale, but =
at the atomic scale, they are very active. Metals can contact weld by =
touching under the right conditions. Why? At the atomic level, the atoms =
are bouncing around violently due to the heat they are experiencing even =
at room temperature (water freezes at 273.16 degrees Kelvin on the =
absolute scale).=20
As materials are heated, the "vibrations" become even more powerful. =
By the time we get close to the fluxing temperature, materials that seem =
solid to the naked eye are experiencing electron state changes that =
indicate the cloud of electrons that surround the nucleus are jumping =
from shell to shell and bonds that are stable at lower temperatures =
begin to break. We see part of this amazing world as visible light. =
Additional radiation is given off in the non visible spectrum all the =
way from infrared to ultraviolet and beyond.
So these crystals are still solid by definition but they are very =
receptive to new configurations. Not all materials work equally well. =
Some have very stable outer electron configurations and these are so =
stable they form few if any compounds. The noble gasses come to mind. =
Others, like oxygen and carbon, form incredible numbers of compounds.
From my physics classes I learned that objects at a given =
temperature have a bell shaped emission curve that indicates that while =
most of the molecules in a substance are at average velocity some are at =
higher energy levels and it is these higher energy molecules that may =
well account for the property we call fluxing action.
As they are heated, crystals lose their order and join with adjacent =
materials to become a complex liquid we call glass. As mentioned, =
cooling time affects whether the glass will recrystalize or stay a =
supercooled fluid, but the magic is that electrons become freer to =
wander at higher temperatures and thus allow two or more high melting =
point solids to join and form a lower melting point glass (or other =
liquid) without the need for a liquid phase material prior to the melt.
Metallic examples include solders which are formed from 25%lead, 25 =
% tin and 50% bismuth that melt at 130 deg Celsius even though the =
constituent metals melt at 327 deg c , 231 deg c and 271 deg c =
respectively. (source CRC Handbook of Physics and Chemistry). Crazy, =
huh?
best, Michael Wendt
iandol on wed 4 oct 00
Dear Michael Wendt,
Fluxes work because they melt fairly easily, they wet things, they clean =
surfaces, they flow and creep into narrow places and they make solutions =
by dissolving things which do not melt very easily. Now a five year old =
can understand those notions!
Your mention of the idea that two materials with high melting points =
will melt at a much lower temperature caught my eye. I believe you have =
a right to a healthy degree of scepticism. Trust your intuition.
There are four distinct meanings for the term Eutectic. Ceramic artists =
and potters have picked upon one of these, that of Equilibrium Eutectic.
People who believe equilibrium eutectics are formed when oxides are =
heated are in error. It does not happen. I know. I have done the =
experiments.=20
Information published in pottery books claiming the appearance of a =
glass on a line blend test tile which has been heated to say Cone 8 is =
an indication that there is an equilibrium eutectic which could occur as =
low as 690C is absolute nonsense.
A Eutectic mixture after cooling is always crystalline by nature and by =
definition. A glass is a super cooled solution of high melting point =
substances in a substance which has a low melting point.
If you wish to continue a private discussion on this topic I will be =
willing to answer your questions to the best of my abilities and suggest =
some practical experiments.
Best Regards,
Ivor Lewis
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