David Finkelnburg on mon 23 nov 09
Warning--this is technical, may provoke a sleep response!
Lest someone read something that wasn't there into my response to Ivor=
,
I have never suggested that a mixture of 1 CaO: .35 Al2O3 : 3.5 SiO2 is a
eutectic mixture. However, wide experience over time has proved it and
mixtures like it melt at far lower temperatures than the melting
temperatures of the individual ingredients. Why is this? What is
happening?
I propose that Lee's comment, in jest, about "angels on the head of a
pin," may not be so far off. Somewhere within our mix of powders of
whiting, alumina and silica it is likely a eutectic combination of
oxide atoms of calcium, aluminium and silicon in contact. As the eutectic
temperature for that combination is reached, those atoms form a liquid
(melt).
Once that first droplet of liquid forms we are no longer dependent on =
a
eutectic composition for glaze melting to proceed. Something called liquid
phase sintering can occur. For the truly diligent, I recommend reading
articles and books by W. David Kingery, among others, as references on this
subject, but I will warn you, the stuff is dry and dense enough to make eve=
n
my eyes glaze over! :-)
In low fire systems getting to liquid is a challenge. Thus lead was
used, because its compounds melt at a low temperatures. Now boron compound=
s
serve that purpose. Lithium, potassium and sodium also work at lower
temperatures to initiate melting. However, even typical higher temperature
fluxes can be drawn into the glass once a liquid phase is formed.
My point is, a glaze does *not* have to have a eutectic composition to
melt. However, in the absence of any liquid glass, if we ask, what starts
the melting in the first place, we must conclude that either a single
material melts, or, as I suspect is more often the case, a eutectic mixture
melts and then liquid phases form that continue the melting process.
Does any of this help you make better glazes, or better art? Only if
it can help you to think about why mixtures of oxides tend to melt at lower
temperatures than the individual oxides themselves.
Enough for this morning. The hump-thrown bowls are nearly ready to
trim!
Good potting,
Dave Finkelnburg
www.mattanddavesclays.com
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