What exactly is glaze made of? Why is it so hard to make a fluorescent fuschia colored Dinner Plate? What on earth is a flux? These are the pressing questions we hope to answer as we open up the Big Book of Ceramic Glaze Chemistry with our resident self-taught glaze chemist, Kyle Crowder.
Ceramic glazing is true Bill Nye territory—full of complexity and nuance. Formulating a glaze color is kinda like mixing paint, but with finicky variables that seem almost alive: metals, chemicals, heat, glass, and colorants. If this is sparking a fright-inducing reminder of a science class you flunked, let’s slow down and take it molecule by molecule. First, what is glaze and how does it work?
You can think of a ceramic glaze as being a combination of two halves—a base and its additions. The base is a clearish liquid full of “fluxes.” Fluxes are materials that catalyze the melting of silica and alumina, which are glass formers and stabilizers respectively. The second half of the glaze contains the colorants, which are typically transition metals like copper, cobalt or zinc. Our glazes are mostly base glaze, with anywhere from 1-10% colorants, but think of these aggregates as being inextricably linked in the way that a glaze’s color is determined by how they all interact.
When you are mixing paint, everything sort of turns out how you’d expect, and if you passed the first grade you can confidently deduce that when you mix red and blue you’re gonna get purple. Glazes are a little different. Once heat enters the equation, molecules begin to melt, meld, attach and change, affecting the glazes’ final color. The same goes for oxygen, or the lack thereof. In the glaze world, color can be thought of more as an effect of altering molecules, rather than pigments combining.
Our Glaze Chemist, Kyle Crowder explains, “Copper carbonate is a bright blue powder, but if you put it into a glaze and starve the kiln of oxygen, you get red. When silicon molecules and copper hang out together, coupled with a lack of oxygen, it changes the shape of the molecule and therefore presents a different color. If you take that same glaze and fire it in an oxidized atmosphere where there is enough oxygen to touch the glaze, it will come out green. The copper and silicon that was attaching a certain number of oxygen to its molecule, now attaches a different number of oxygen and reflects a different color.” Basically, it’s a real crapshoot if you don’t cross every t and dot every i.
Before anyone gets mixing, we get ideating. Our CMO, Connie Matisse, is the color brains and decision maker on what seasonal glazes will be. The decision involves a little bit of “because we love it” and a little bit of giving the people what they want. And maybe some creative divinity. Once we confirm that we must have, say, a Millennial Pink, we bring our creative heads to the lab where Kyle conducts a long winded series of tests, until the color is perfect as pie.
Kyle has been doing this for 4 years now and they’ve learned a thing or two. They say, “We have a collection of stains and raw oxide metals that I know work well with our base glaze and I have an idea of what colors they can create. It’s like mixing chemicals that I know will react in a certain color way.” Once the more granular parts of glaze composition are theorized and understood, they get the ball rolling on some quad blends.
“I’ll do a quad blend—four starting points in each corner—one might be a fully saturated blue, one a half saturated blue, one is white, and one is red. As you blend each one successively with the next one in the quadrant, you create these line blends going all the way around and corner to corner, and then if you point to one you like, it’s easy to calculate the recipe for the color. The quad blend looks like a gradient. We pick one, and start the process over again. Like, what does that look like if we add more opacity? We often add more opacity because Connie’s often drawn to saturated colors that are slightly subdued. Zircon or tin give it that look.”
Are you an abstract splatter painter devoted to whim? You might want to plug your ears. Because of the powerful influence of slight variables like temperature, oxygen levels, and the romance between molecules to completely alter a color, we have to engineer as much chance out of the process as possible. We do this so that when you order a Soapstone Bowl later tonight, get it, love it, and order another one months later, they match as closely as they can. The process of formulating glazes—while very creative— requires a solid understanding of chemistry, theory, and testing. And even if you think you have ALL your variables covered, things surprise you.
Let’s take Celery, for example. A good surprise! As mentioned before, we all know that adding blue paint into a glob of yellow paint and stirring it up will make green paint. But, in the world of ceramics, where molecules are changing and clinging and doing all kinds of funky stuff in the high temperatures of the kiln, that isn’t always the case. So for Kyle, our glaze chemist, it was quite a “breakthrough” (their words) when they were experimenting with Celery to find that if you combine a yellow stain and a blue stain, that it will actually turn green. As they describe it, “In pottery, 1+1 almost never equals 2—but this time it did.”
The reason that it worked is this: When you encapsulate a pigment, the actual pigment does not dissolve into the other part of the glaze, where it would be subject to molecular change, but it is suspended and locked within glass. If you were to look at a copper glaze under a microscope, you would see dots of copper, even though the materials that went into it were blue and red pigment. But, if you looked at your Celery plate under a microscope, you would see tiny blue and yellow dots sitting closely next to each other. The distance and scale causes your eye to see green. Science is magic.
While there are a lot of “Eureka!” moments in the glaze department, there are also a lot of flops. Stakes are a whole lot higher when we’re firing hundreds of pots at a time, and meeting revenue goals is dependent on those pots coming out “right”. Back when we were firing in the wood kiln, with licks of fire growing and softening in unpredictable patterns, chance results were fun and desired! Now, if things don’t come out as expected, it's a real pain in the butt. Our goal is to make consistent and uniform dinnerware, not funky art pottery.
“Glaze shrinkage” is something we’re always on the lookout for. When the glaze is applied to a bisque (pre-fired) pot, and thrown in the kiln, there is a possibility that the glaze won’t fit. If the glaze and the clay have different rates of contraction and expansion, we’ve got a “fit” issue on our hands. “Everything in the world has an expansion and contraction based on heat. So when you have a clay body that expands at a certain rate and a glaze, that does so at a different rate, the glaze might develop tiny cracks or cleave off in flakes.” says, Kyle.
“Proper thermal-expansion is one of the most important factors that makes a quality glaze and one that is most often overlooked by studio pottery. I spend much of my research making certain that our glazes match the clay body as best we can with the material limitations we have. Also, our materials we use at East Fork have gone through many transitions this year which have made this challenging to accomplish. As it stands right now, I estimate it could take 20 years before you would see craze lines from normal use. That being said, you could heat a plate up in the oven to 400 degrees F, drop it in water and you will see those craze lines immediately. It's all a function of time and heat.”
There are a lot of chemicals we steer clear of when mixing our glazes. We don’t use boron, a common self fluxing glass, which is costly both financially and to the earth. Same goes for chemicals like Uranium and Lead. Even though Uranium produces the most luminous orange, it’s extremely radioactive, so it’s a no go for us.
As for lead, it’s acutely toxic in its metallic form. “When you make a glaze with lead, the idea is that its stitching silicon molecules together in the kiln, and it forms a lead aluminum silicate, a structure in an of itself that does not dissolve under normal circumstances. We don’t use lead in our glazes at all. On our [production] end, it’s dangerous to handle before it’s formed into a glaze. But actually, once it’s fired in, it’s relatively safe, if formulated correctly.” says, Kyle.
Our materials are readily available in the environment, they don’t cost a lot to extract, and we don’t have a middle man doing any processing, except for the people classifying and bagging them. We buy the materials in as raw a state as you can. Fresh from the earth, crushed up, organized by particle size, with not a whole lot more interference. They are safe for us to work with, and safe for you to eat off.
Color is performed by light, and without reflectance there would be no light.” says, Kyle. There are stable materials we know that perform certain colors, and in this way, we can create other colors. And even though we do streamline the process to avoid idiosyncrasies, there are still some elements of chance that make our pottery special. We have no idea why some have iron spotting and some have none. We’d never thought it’d turn out this way, but a lot of companies try to replicate this with glaze speckling techniques— we just let the iron do it’s thing.
In closing, we feel confident saying that our pottery is beautiful and safe, and we work hard to make it that way. A glaze color can have a positive emotional impact, a thing we believe is important and worth giving the world. Still, if you threw a bowl in your backyard and let time and nature work it’s magic, it’d stay buried under the earth for some martian archeologist to dig up centuries later. Kyle echoes their concerns and hopes about this fact, saying, “I believe we have a responsibility to make this really beautiful because it will last forever. I really do want these pieces to last through generations like we say they will. I want our pottery to feel useful, useable and still be passed down.”