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How Glazes Melt: In Search of the Elusive Eutectic
Posted By Dave Finkelnburg On December 9, 2009 @ 4:24 pm In Ceramics Monthly Master Class,Daily,Glaze Chemistry | 1 Comment
“Riddle me this,” commanded Arthur (Art) Clay as we labored diligently in the studio. “What do art professors lecture about, yet no human being has ever seen? Come to think of it,” he added, chewing thoughtfully on the handle of one of my best trimming tools, “no human ever will see it.”
“Do I get any clues here?” I asked, taking the bait.
“It’s genuinely important to anyone who works with anything ceramic,” he offered.
“I thought it might be.” I gave him my best can’t-you-be-more-helpful look.
“Well, although most artists don’t realize it, all ceramics make use of it.” My glare may have been slightly petulant. “Why, you could even say it’s-elemental!” he grinned.
“You aren’t,” I asked, with painfully measured words, “going off on another rant about a eutectic, are you?”
“The elusive eutectic!” Art positively shouted. I might have rolled my eyes, because he quickly added, “Listen Professor Lineblend, you mix glaze tests until I positively want to puke. None of that stuff would ever melt if it weren’t for a eutectic.”
“You know I’m visual,” I said honestly. “I need to see what happens when I fire something.”
“You can see it in a phase diagram,” he cried, throwing up his hands as if everyone should know what a phase diagram is.
“Right, and everyone should understand the cryptic chemical mumbo jumbo phase diagrams use,” I said, “and be able to interpret them, too. How can that gobbledygook tell me anything?”
“You-could try-reading.” The cadence of Art’s words revealed his impatience, but he almost smiled as he jammed a paper at me.
“In Search of the Elusive Eutectic-an Essay,” by Arthur Clay, read the title. “In all ceramic raw materials,” the paper began, “it is rumored there lurks a scientific phenomenon-a eutectic. Does it really exist, or is it just a figment of the imagination of wild-eyed ceramists? You decide. Eutectic; the term comes from the Greek eutektos, meaning ‘easily melted.’”
“Nice Wikipedia research, Art,” I allowed sarcastically.
Phases are specific forms of materials. The most familiar phases are solid, liquid and vapor. Any phase of a material is identical in composition and structure in all parts of that phase. For instance, a glass of water is the liquid phase of H2O, top to bottom; if it weren’t, we’d call it something else, like ice if it were solid (structural change), or lemonade if it had lemon and sugar dissolved in it (compositional change).
In ceramics, many phases come into play; The solid phase of raw materials, the liquid phase of a molten glaze matrix, the glass phase of fired, cooled glaze. Throughout the firing process, both chemical and structural changes take place, meaning the phase of the material changes. Quartz, for example, is a crystal phase of silica. All parts of quartz have the same chemical composition (SiO2) and the same hexagonal crystal structure. A glossy glaze, however, is made up of only a glass phase. It’s chemical composition is still SiO2, but it is no longer crystalline.
It continued, “The precise combination of two or more phases of any substances which has the lowest possible melting point of those phases is called the eutectic.”
“Okay, phase diagrams have to do with melting, but what is a phase and why do I care?”
Art frowned. “Do you think I should have credited Wikipedia?” Then he snapped back, “Uh, right, phases. Funny you should ask. Check out the handy sidebar ‘What is a Phase and Why do I Care?’”
Okay, I think I got it, or at least I knew where to look if I needed to get it later, so I read on. “For example, if, on a cold winter night in Yellowknife, you make a line blend of finely powdered ice (crystalline phase of H2O) and fine salt (crystalline phase of NaCl), and then warm up the blends, the blend that is 23.3% salt by weight will melt first. It will melt at a lower temperature than any other mixture of salt and ice which you might make. The eutectic composition is 23.3 weight percent salt in ice.”
“Hey Art, that actually makes sense!” I looked up, expecting to see Art beaming. Instead, he looked a bit-anxious.
“That is what a phase diagram can tell you” he said. “For two components, like salt and water, the diagram looks like that binary diagram over to the left. Up the side you read the temperature and across the bottom is the composition. You could think of the bottom line, the horizontal axis, as a line blend. 100 parts of ice on the left, 100 of salt on the right, and increasing amounts of salt, decreasing amounts of water as you go along from left to right. That white circle is where the eutectic point is. In fact, any two inorganic materials will have a eutectic composition. But there are a few little details you haven’t gotten to yet,” he said.
“For instance?” I growled.
“Well, to give you an actual ceramic example; there is the classic calcia-alumina-silica eutectic at about Cone 4. It melts at only 2138°F (1170°C). The composition of that eutectic is 23.25% calcium oxide (CaO), 14.75% alumina (Al2O3) and 62% silica (SiO2). That’s percent by weight,” he added, ever the stickler for details. “To illustrate three phases like this, it helps to have three sides on your diagram and then imagine you are looking straight down on it from above. This just looks more complicated, but it’s really not. It’s like the difference between a line blend and a triaxial blend.” Blends! Now he was beginning to speak my language.
“The three corners are labeled with the chemical components that make up the particular system represented by the diagram,” he added. “Since we’re working in two dimensions, we have to represent temperature with contour lines, like the lines on a topographical map. You can see it all here on the ternary diagram,” he said (see “Navigating the Land of Phases” on page 62).
In combination with alumina and silica, eutectics for alkalis occur at significantly lower temperatures than eutectics for alkaline earths. This is one reason alkalis are considered stronger fluxes.
Alkali Eutectic Points
Alkaline Earth Eutectic Points
“Now, I digress. We were going to address the…um…uh…details using the classic lime eutectic. It’s important, you see. Chinese potters were making celadon glazes with that composition 750 years ago. They did this not with the aid of chemical analysis, or phase diagrams, but by trial-and-error testing. However, the…er…detail is, if you just mix up whiting, alumina and quartz in the eutectic composition, the glaze won’t exactly melt at 2138ºF.”
“What are you telling me, Art?” I demanded.
“Well, the fact is, the mixture will melt in the vicinity of the eutectic temperature, you know, within a few cones.”
“A few cones? Vicinity? What? Why?” I spluttered.
“A phase diagram, regardless of the number of phases involved, is just that, a diagram of phases,” Art replied. The ‘phases’ that melt at this eutectic are a calcium feldspar called anorthite, a calcium silicate called wollastonite, and a high-temperature form of quartz called tridymite. If you mix…”
“Oh mix, shmix!” I interrupted. “What I want to know is, how does any of this help me make better art?”
“Now let me finish this part first,” said Art. “You know we can duplicate any glaze, at least chemically, by using glaze calculation. With a computer that’s easy. But that doesn’t tell us anything about how the specific materials used influence the fired results, even when glazes have precisely the same chemistry. The materials you use do make a difference, as you know.
“And regarding art,” he hurried on, “if an artist understands that eutectics exist, then the artist understands how glazes melt. Essentially, eutectics tell us that glazes melt because of what they are made of. I’m not just talking about their chemical makeup, but also about the materials used to arrive at that chemical makeup.
“What the eutectic tells us is that specific combinations of specific minerals will melt at a lower temperature than any other possible combination of those minerals. Knowing that, we artists just have to search, either using trial and error, or study, to find clues about what combinations may work for us.
“So, Art,” I interrupted again. “Are recipes that have already been tested such clues?”
“Of course,” he shot back. “And if you look at glaze recipes from both a chemical and a materials perspective, you will get a lot more out of them than if you just follow them cookbook fashion.”
“Art,” I laughed out loud, “if I can just use recipes that have already been proven, why should I study phase diagrams? If a chemist can tell me the composition of a glaze on a Chinese pot from the Sung Dynasty so I can even work out recipes that match it, why re-invent the wheel?”
“You may be right,” conceded Art. “But I truly believe the advantage of learning about eutectics is that the knowledge gained can help artists troubleshoot glazes that don’t work, understand glazes that do, and invent new glazes that don’t necessarily look exactly like someone else’s.
“Besides, you may not be willing to admit it,” Art observed a bit slyly, “but now you know about eutectics. There’s no going back.”
Phase diagrams, to studio artists, can seem like a different planet; unfamiliar, perhaps frightening and difficult to navigate. Just think of a phase diagram as a map of that planet-a map of melting points for different compositions. Each “map” is developed for a specific group of chemical elements. A complex phase diagram may represent the melting of three materials, which is called a ternary phase diagram. Each corner of the map represents the “home,” or 100%, of the components being mapped-in this case calcia (CaO), alumina (Al2O3) and silica (SiO2). Just as in line blends and triaxial blends, the further you move away from the 100% point, the lower the percentage of that component, and the higher the percentage of the other(s). The eutectic composition of 23.25% CaO, 24.75% Al2O3, 62% SiO2 is on our “map” of melting points, right inside the white circle, and the eutectic temperature of 2138°F (1170°C) is written right there (see A Eutectic Is Your Friend, on page 61).
You’ll recognize the “countries” of phase fields, like mullite and crystobalite and others (see “What Is a Phase and Why do I Care?” on page 60). The contour lines you see indicate temperature. Just as contours on a topographical map indicate elevation, these show the melting points of the various compositions (see the three-dimensional phase model below). The borders between phases are the minimum melting temperatures between those phases. Notice the arrows along the borders all point “downhill” to the eutectic.
This eutectic temperature is only valid if we melt the materials that are the adjacent phases. This “map” shows that, of all possible mixtures of minerals made only of calcium, aluminum, silicon and oxygen, only anorthite (calcium feldspar), quartz and wollastonite, will melt at precisely 1170°C. More importantly, it shows this is the lowest possible melting point achievable with these elements. With other minerals made from these elements, the minimum melting temperature will be higher.
Is this more than we need to know in order to formulate glazes that will melt well? Not at all; if we understand that a mixture of substances melts at a lower temperature than the substances themselves, and a eutectic shows the composition that melts at the lowest temperature, you can understand how any glaze melts. If you have a glaze that isn’t melting, you know it is either made of the wrong elements, or the combination of elements is just too far from a eutectic composition. Glazes melt because their composition is appropriate for the temperature the work is fired to.
No phase diagrams exist for most of the virtually infinite combinations of elements we could come up with. BUT, if we know eutectics exist, AND we know we are working with a relatively refractory material, we know we need to find a complex composition that will bring our refractory material into the melt.
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