Laura Conley on tue 14 jul 98
I understand the logic of your discussion. I have a few questions:
1) What is the "force" that keeps the glass in the melted state? In other words
why is this state more stable? Is energy required for crystallization?
2) You mentioned alumina - how do you think it affects this? Does it prevent th
atoms from rearranging easily into the crystal arrays?
3) Your comment regarding small changes in the ratios making a big difference in
the fired result has me thinking... At what rates do the different oxides
crystallize? Certainly some must be faster than others?
4) Does this perspective (that you present) suggest that with an infinite amount
of time, the amounts of all the oxides that did not contribute to the eutectic
ratio would crystalllize? If so, could this be used experimentally to determine
the eutectic ratio of any mixture?
5) Are there differences in where the crystals tend to form, surface or interior
and if so, how is this explained by your model?
> ----------------------------Original message----------------------------
> Ron provides a great lead-in to something I've been thinking about
> putting together, and since my last few posts have been kind of
> negative, I hope I can provide some positive input to this list.
> This is aimed more at those of you interested in more than the
> most useful part of the information for potters that a eutectic
> mixture is the one with the lowest melting temperature. That's
> all well and good but there are some neat other properties involved.
> I'm going to have to assume you can visualize the classic eutectic
> between at least two "endmember" phases as a graph of composition
> on the horizontal axis and temperature where crystalization (or
> solidification) begins on the vertical axis. The line curves
> down from each side to a eutectic at some composition between.
> Got it? Ok.
> To clip a bit from Ron's message (following his calculation of a
> eutectic mixture):
> >All that CaO is going to lead to some of it coming out of the melt on
> >cooling I suppose so fast cooling would be best.
> I learned about eutectics from the perspective of a geochemist --
> in other words with time on our side and an interest in what
> crystalizes out rather than in quenching the melt to a pure glass.
> A magma cooling in the earth's crust can have oodles of time to form
> crystals and the way that happens is really interesting. I think
> it has some relevance when you are talking about glazes since crystals
> can and do form even if you don't have millions of years to soak.
> I'll hit the punchline right away: at a eutectic point each of the
> stable phases will form at a rate that keeps the melt composition
> constant! You won't crystallize your calcium phase (I would assume
> a calcium silicate like wollastonite) without crytallizing the other
> phases (like silica, I suppose). If you think about it, it makes
> intuitive sense. Say you are at the minimum melting point composition
> and then cool it down until one phase starts to crystallize out.
> That would move the composition of the melt away from the eutectic
> -- it then would have a *higher* melting point and would have to
> solidify instantly. Can't happen. So all the crystal phases form
> together keeping the composition and the temperature(!) constant
> until either the whole melt is turned to crystals or the system
> is crash cooled forming glass instead of crystals.
> So what happens if you slow-cool a melt that isn't at the eutectic?
> Think of a composition half-way between one of the pure endmembers
> and the eutectic. You have melted the stuff so now your point on
> the graph is at a temperature up above the curve. As the melt
> cools the temperature first drops without changing the compostion of
> the melt. If you drop the temperature really fast -- boom you get
> a glass of the same composition with no crystals. But if you drop
> the temperature more slowly when you get to the curve (the liquidus
> for those of you who like fancy words) crystals start to form. But
> you only form crystals of the stuff on that side of the graph.
> If you are on the calcium rich side of the graph you will form those
> wollastonite (or whatever the phase really is) crystals. In this case
> however, taking out calcium makes the melt richer in silica, but
> that's ok because it moves the melt towards a lower melting
> composition. Going that direction keeps the melt liquid and life
> is good. So the temperature drops a little more and you remove
> some more calcium silicate and so on. What happens is that the
> composition and temperature slides down that liquidus curve
> towards the eutectic. If you were to quench the melt at any
> time until you reach the eutectic you would
> only have one kind of crystal (assuming only two phases here
> because it is somewhat more complicated for three or more). If you
> don't quench the melt then the composition will eventually shift
> until it reaches the eutectic where both phases will crystallize
> until there is no melt left.
> If you have three phases like you would show on a ternary diagram,
> first one phase crystallizes out shifting the composition until
> a second phase becomes stable. Then both of those crystallize
> and you slide down the boundary between the two until you
> reach the eutectic between all three. Then all three crystallize
> keeping the melt composition constant until it all solidifies.
> Ron then says:
> >It would seem to me that being near a eutectic would be the best way to get
> >a clear glaze - I am also concerned that this would result in a glaze with
> >a very short firing range - can anyone confirm those two statements?
> I guess the first statement is more or less correct. It seems to me the best
> way to get a clear glaze is to cool it quickly enough that no crystals form.
> I suppose that if you have composition where you are on the side of the
> eutectic of a phase where the crystals form easily then the further from
> the eutectic point the higher the temperature where the crystals first
> start. Then you have to cool a lot farther before the crystalization is
> quenched. If you start near the eutectic
> then the initial crystalization is at a lower temperature and you don't
> have as far to go before the melt gums up into a glass.
> I don't know all the factors that go into the firing range. I think it has
> a lot to do with viscosity which probably isn't strictly related to how close
> you are to the eutectic. But being near a eutectic will probably make the
> glaze fickle.
> Firstly, any tiny variation in composition will change the
> melting temperature a lot. Remember that those curves get steeper towards
> the eutectic. Also tiny composition changes would send you off to places
> where different phases would crystallize first so you may get unexpected
> Well, I'm tired and my dog needs her allergy shots before bed. Hope this
> was somewhat understandable. Good night.
> -- Evan in W. Richland WA -- just back from Seattle where I managed to
> restrain myself to buying only 4 pots.