Do You Need to Know About Eutectics to Make a Good Glaze?
Section: Glazes, Subsection: Chemistry
Description
This discussion was initiated by a question in July 1998 on Clayart as to whether a knowledge of eutectics would help produce a better glaze.
Article
This discussion was initiated by a question in July 1998 on Clayart as to whether a knowledge of eutectics would help produce a better glaze. David Hewitt provided some helpful information and Ron Roy made a thought provoking statement:
"It would be most helpful - I think - to at least know when we are moving away from or towards the eutectic point of a glaze".
Tom Buck Tom.Buck@hwcn.org
The word "eutectic" comes from chemists/metallurgists working with metal alloys many years back. The word was invented to indicate the LOWEST melting point achievable when two metals were mixed together (either as powders or as molten liquids), and resultant alloy tested for its melting point. Here is what a chemical dictionary says:
"eutectic. The lowest melting point of an alloy or solution of two or more substances (usually metals) that is comprised of the same components. Eutectic alloys are relatively few; they are particular alloys that have definite and minimum melting points compared with other alloys of the same metals."
People who were working in ceramic studies also found the term of use so they began applying it to the LOWEST melting point of mixtures that would form glass (i.e., a glaze mix) on a clay pot/vessel/form. Like a metal alloy, the ingredients would yield such a eutectic mixture when combined in a specific proportion. But, unlike metals (all pure substances chemically speaking, i.e., one molecular species only), glazes with the same fired mix of essential oxides could be derived from differing materials (not "substances" in chemical terms). As a result, the study of glass-forming materials is more complex than that of rather simple alloys.
To bring order to such complexities, ceramicists use "phase diagrams" and "triaxial diagrams" to represent the combinations/activities involved in various glaze mixtures. And to simplify things for these diagrams, all components in the mixture have to be rendered in molecular form, i.e., moles of specific oxides, e.g., CaO calcium oxide, K2O or Na2O potassium oxide or sodium oxide, SiO2 silicon oxide (silica), Al2O3 aluminum oxide (alumina) and other oxides. This is similar to a Seger (unity) Formula, which is itself a rendering of a glaze recipe into proportions of specific molecular species included in the components of the recipe.
As others have explained, one can make use of "eutectic" data when designing a glaze from scratch, or one can use "Limit Tables" which provide a guide to a Seger Formula (i.e., moles of essential oxides) needed for a successful glaze at a particular firing range.
In the end, however, all such knowledge won't guarantee a test glaze will succeed on a pot. But it will help point you in the right direction to retest and obtain a successful new glaze.
Gavin Stairs stairs@stairs.on.ca
A eutectic is, as the name sort of says, a mixture in which every thing melts at the same time. There are several tricks to it though. Every multiphase material (ok, ok, there are some weirdoes like borates) has at least one eutectic composition. Take Ron Roy's mixture, for example:
>CaO - 23.3
>Al2O3 - 14.7
>SiO2 - 62.2
He says this is supposed to melt at 1170C. And it probably does... the second time around. But the first time through, it will not melt until at least the first raw material has begun to melt. Actually, to split hairs, it may begin to melt at a somewhat lower temperature at which the atoms have time to diffuse across the distance of the particle size. This is our old friend heat work again. But lets stick with the temperature for a while.
If you made this mixture up out of pure materials, you'd have to go up to a rather high temperature (I don't have the references in front of me, so I can't say exactly what) to get the CaO to melt first. Then the silica and reluctantly the alumina would begin to dissolve into the flux. When that process was completed, you would have an equilibrium state (thermodynamics term meaning that no further changes will occur no matter how long you heat it) in which the liquid is homogenous and stays that way even when you cool it past the solidification temperature. That's a eutectic mixture. Now, if you heat it again, it will melt at the eutectic temperature, all at the same temperature, like a cube of ice. This is what frits are about.
Now, for a 3-phase mixture like Ron's, there are actually more than one eutectic. That is, more than one different composition of these materials that behave as I have described, having different melting points. The more materials you mix together, the more complex the picture, and the more eutectics you get, in general. Also, the lower the melting point of the lowest melting point eutectic. So, adding many materials is one of the means we have of making low melting point glazes. Just how this works is the next twist.
If you have a mixture which is just off the eutectic point (I'm ignoring a few subtleties here: wait for the advanced lessons for those), the mixture will begin to REmelt at the eutectic temperature, but it will not form a liquid: it will form a slurry of a solid (of the excess material) in a liquid WHICH HAS THE EUTECTIC COMPOSITION. So, the local governing eutectic sets the REmelting temperature of the glaze, a point we call the solidus. As the temperature rises, more of the excess material dissolves until finally it all dissolves and we have a liquid with no chunks. At this point there is no eutectic phase, and we have reached the temperature called the liquidus. The glaze has just now flowed off the pot.
The reason eutectics are not good glazes are several. First, they are touchy: one moment they're solid, and the next they're liquid. And, because of the little stuff that I omitted above, if you get the materials wrong on one side of the equation the temperature can be quite different than if you get them wrong on the other side. Next, when they go liquid, they are completely fluid, and may flow off the pots. Normal glazes have some degree of thixitropy, which is a big word to say that they tend to gel and resist flow to some degree. This is good.
A short expansion on melting points: When you start out with a granular mixture of components, they are initially separated one from the other, and therefore each one has its own melting characteristic according to the properties of that material alone. However, every grain contacts another grain, and where they touch something different can happen. Only two different materials can touch as solid granules at a single point (this is a statement of statistical probability, not topology). Where they do, atoms of the different grains can diffuse across the boundary, with the result that at a temperature not far from the eutectic temperature of those two materials together, they can melt together, forming a liquid droplet into which more material can dissolve and mix. By this means, we can predict the melting temperature of some (most?) powder mixtures: it will be the lowest biphase eutectic temperature of all the constituents. So, in Ron Roy's mixture of CaO, Al2O3 and SiO2, it will be the eutectic temperature of the CaO-SiO2 system. These temperatures are usually available from sources like the ACerS phase diagram series, but these are resources beyond the reach of most potters.
Note however that if we start not with the pure oxides but with the practical materials that Ron outlined, e.g., Wollastonite - 50.0, EPK - 33.5, Silica - 16.5, the lowest biphase eutectic will be that of perhaps Wolastonite - EPK, which is no doubt smaller than that of CaO -SiO2. Pure materials melt at higher temperatures, generally.
Evan Dresel pedresel@3-cities.com
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.
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 crystallization (or solidification) begins on the vertical axis. The line curves down from each side to a eutectic at some composition between.
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
crystallizes 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 crystallizing 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 composition 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 crystallization is quenched. If you start near the eutectic then the initial crystallization 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 results.
Authors
- Gavin Stairs (Owner)
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