Ceramics Technical
Please be patient while I build out this section of the website. Face-to-face instruction is by far the best way to learn the physical handling of clays and glazes. But some of the technical information is much harder to "absorb," in particular if you only hear it once. While I hope eventually to have more information about how to make things and apply glazes, I'm starting with the kinds of information that are harder to retain, that my students (or anyone who is interested) might want to reference.
Clays and Glazes, the basic materials of ceramics
The more you understand the materials you are using, the more you can get out of them. Understanding clays and glazes can help you select (or even develop) what will best enable you to express yourself and reach your goals in ceramics. Even if you never formulate your own clay or glazes, the "show and tell" examples in this section of the website, and the testing procedures are probably far easier than straight theory for helping you grasp the basic concepts in how ceramic materials work. Understanding is key to troubleshooting when things go wrong, even if you are working with a commercial clay or glaze, or one that someone else formulated. If you use clay and glaze, it's worth having a basic understanding
Clay dug from my front yard, before processing
What is clay?
We all think we know what clay is. But do we really?
The chemical formula for clay is Al2•Si2•H2O but this is misleading! No clay is that pure. Clay is actually super fine pulverized/dissolved rock. The earth has made plenty of it for us-clay is plentiful throughout the world, but not all clays are created equal. There are various types of clay deposits. Clay that is simply dug from the ground, without any added ingredients is often called "wild clay." The photo above shows wild clay dug from the "wilds" of my front yard. Prospecting for clay is a topic I hope to cover in this section of the website, but for now just know that what plants grow in isn't clay. Clay is not generally at the surface where you can see it, you usually need to dig down. In fact, one my tricks for spotting clay that is for some reason close to the surface is that there won't be anything growing in it. A clay deposit has to be really huge to make it worth excavating at an industrial scale. But those like me who just want the experience of digging and using their own clay, even a modest amount can be harvested and tested. Historically, where there was really good clay-clay that could be used as-is, or with very few alterations, that is where you would find potteries, brickworks, or other local manufacture of ceramics products. However, most wild clay does not work anything like the clay you buy in bags and boxes from a commercial manufacturer. Store-bought clay almost certainly is not one single clay, dug from one location; it is almost certainly a recipe of ingredients, a "clay body." To use a cooking analogy-corn on the cob would be like wild clay; just by its self, just cooked with no other additives, you could eat it. A cake would be like a clay body; you can't make a cake with just flour, it wouldn't rise. You can't make a cake with just baking powder, it would taste awful; each ingredient adds something, and if the ingredients are in the wrong proportion you're not going to have a good cake.
The chemical formula for clay is Al2•Si2•H2O but this is misleading! No clay is that pure. Clay is actually super fine pulverized/dissolved rock. The earth has made plenty of it for us-clay is plentiful throughout the world, but not all clays are created equal. There are various types of clay deposits. Clay that is simply dug from the ground, without any added ingredients is often called "wild clay." The photo above shows wild clay dug from the "wilds" of my front yard. Prospecting for clay is a topic I hope to cover in this section of the website, but for now just know that what plants grow in isn't clay. Clay is not generally at the surface where you can see it, you usually need to dig down. In fact, one my tricks for spotting clay that is for some reason close to the surface is that there won't be anything growing in it. A clay deposit has to be really huge to make it worth excavating at an industrial scale. But those like me who just want the experience of digging and using their own clay, even a modest amount can be harvested and tested. Historically, where there was really good clay-clay that could be used as-is, or with very few alterations, that is where you would find potteries, brickworks, or other local manufacture of ceramics products. However, most wild clay does not work anything like the clay you buy in bags and boxes from a commercial manufacturer. Store-bought clay almost certainly is not one single clay, dug from one location; it is almost certainly a recipe of ingredients, a "clay body." To use a cooking analogy-corn on the cob would be like wild clay; just by its self, just cooked with no other additives, you could eat it. A cake would be like a clay body; you can't make a cake with just flour, it wouldn't rise. You can't make a cake with just baking powder, it would taste awful; each ingredient adds something, and if the ingredients are in the wrong proportion you're not going to have a good cake.
What is a Glaze?
Glaze is glass that melts onto-but does not run off-a piece of ceramic. That sentence is deceptively simple-in practice, a glaze is a balancing act between 3 major categories of ingredients.
Glass Former (generally silica)
Without a glass former, you can't get glass, so silica is necessary in all glazes. Nearly pure silica can be purchased from any ceramic supply store, but is also present in many of the ingredients used in glazes-in clay or in ground up rocks like the feldspars for example.
Stabilizer (generally Alumina)
If we all we did was melt the glaze on a pot, what we would get is a pot with no glaze on top sitting in a puddle of glass on the kiln shelf. Without a stabilizer, the glaze would just run off the pot like water running down hill.
Flux (aids in melting the silica & Alumina)
Both silica and alumina melt at temperatures higher than pottery kilns can reach. Fluxes help glazes to melt.
Eutectics
A eutectic is when you combine two materials and they melt at a lower temperature than either of them alone. Teaching in the Northeast, I have a great example of a eutectic that all my students understand; rock salt. Water generally becomes solid at 32 degrees F-but when you put rock salt on top of ice, you get water down to -20 degrees F! Silica and alumina are already a eutectic, then you add one or more fluxes, and that gets us down to a temperature where our glazes will melt onto our pots.
I did mention that all this is a balancing act, right? If any of these three basic parts of a glaze-glass former, stabilizer, flux, is out of proportion/balance, you are not going to have a good glaze.
Glaze is not paint
The first thing I tell my Intro to Ceramics students, is that glazes are NOT paints. Raw eggs are not an omelette-you need heat for the transformation. With glazes, what you see when you put it on the pot is NOT AT ALL what you are going to get when the work is fired. A glaze that looks green and chalky before firing could emerge from the kiln a glossy purple for example. Glaze tests, at the very least test tiles, are an absolute necessity in any ceramic studio to preview what glazes (might) look like.
Ground up rocks and clay and chemicals floating around in Water
Unlike paint, glazes don't stay mixed up. If you allow a glaze to settle, the heavy parts will of course settle faster. Some glazes settle faster than others-but using glaze that has not been properly mixed up-and sometimes a glaze will begin to settle in a matter of minutes-can ruin the glaze! Using a glaze that isn't well-mixed, you're going to use up some ingredients (the lighter ones) more than others, and essentially destroy the balance of the recipe. The first thing to understand about actually applying glazes is the batch needs to be well-stirred.
Glass Former (generally silica)
Without a glass former, you can't get glass, so silica is necessary in all glazes. Nearly pure silica can be purchased from any ceramic supply store, but is also present in many of the ingredients used in glazes-in clay or in ground up rocks like the feldspars for example.
Stabilizer (generally Alumina)
If we all we did was melt the glaze on a pot, what we would get is a pot with no glaze on top sitting in a puddle of glass on the kiln shelf. Without a stabilizer, the glaze would just run off the pot like water running down hill.
Flux (aids in melting the silica & Alumina)
Both silica and alumina melt at temperatures higher than pottery kilns can reach. Fluxes help glazes to melt.
Eutectics
A eutectic is when you combine two materials and they melt at a lower temperature than either of them alone. Teaching in the Northeast, I have a great example of a eutectic that all my students understand; rock salt. Water generally becomes solid at 32 degrees F-but when you put rock salt on top of ice, you get water down to -20 degrees F! Silica and alumina are already a eutectic, then you add one or more fluxes, and that gets us down to a temperature where our glazes will melt onto our pots.
I did mention that all this is a balancing act, right? If any of these three basic parts of a glaze-glass former, stabilizer, flux, is out of proportion/balance, you are not going to have a good glaze.
Glaze is not paint
The first thing I tell my Intro to Ceramics students, is that glazes are NOT paints. Raw eggs are not an omelette-you need heat for the transformation. With glazes, what you see when you put it on the pot is NOT AT ALL what you are going to get when the work is fired. A glaze that looks green and chalky before firing could emerge from the kiln a glossy purple for example. Glaze tests, at the very least test tiles, are an absolute necessity in any ceramic studio to preview what glazes (might) look like.
Ground up rocks and clay and chemicals floating around in Water
Unlike paint, glazes don't stay mixed up. If you allow a glaze to settle, the heavy parts will of course settle faster. Some glazes settle faster than others-but using glaze that has not been properly mixed up-and sometimes a glaze will begin to settle in a matter of minutes-can ruin the glaze! Using a glaze that isn't well-mixed, you're going to use up some ingredients (the lighter ones) more than others, and essentially destroy the balance of the recipe. The first thing to understand about actually applying glazes is the batch needs to be well-stirred.
Developing Clay Bodies and Glazes-Show and Tell
"Show and Tell" is probably the most interesting way to learn about clay and glaze chemistry. For most of human history, ceramics were done by trial and error and from master to apprentice. The ancient Greeks who made such sophisticated ceramics knew nothing of chemical analysis much less of the unity molecular formula developed by Herman Seeger in the nineteenth century. They didn't even use what we call glazes, they used colored slips and special firing methods. For me, grinding up rocks and seeing what they do when you heat them to over two thousand degrees and add other stuff is fun. I hope you agree. I hope to add a lot of clay and glaze formulation "show and tell" to this site! Here are a few steps in the process, with links to the pages that I have so far.
- Melt Test. The simplest test for a potential ceramic ingredient is the melt test. This just shows what happens with one ingredient when fired to a particular temperature
- Line Blends. Once you see what a material does by its self, you want to see what it does with something else. The simplest form of line blend is to start with one material and then add a little more of another material, then a little more and so on in a series of steps. You can see examples of this in my blog post making-glazes-with-east-40-clay-part-1.html
- Axial Blends. The basic idea is you have a bunch of tiles arranged in a triangle or square. On the "corners" you have different materials (or combinations of materials) and then combine them in fixed proportions with each other. This is definitely easier to show than tell!!! I have some 25 part axial tiles I made to illustrate this idea, now all I have to do is make another page and get them up on the website for you to see.
- Standing Tiles. The tests above are all done on flat surfaces, so you don't know if the glaze will melt, what the color would be etc. but not if it's going to run off a pot. Vertical or "standing" tiles show you if the glaze runs, and if so how much. In the blog post making-glazes-with-east-40-clay-part-2.html you can see examples of standing tiles.
- Testing on actual work. Obviously, the best test of all is to use it the way you're going to really use it, on actual pieces of your work. The blog post above also includes glaze testing on an actual vase. "Fit"-which is how the glaze sits on a particular clay body is important. A glaze could be fine on one clay body while the exact same glaze could have defects like pin holing, crazing or shivering on another clay body! Also, thickness and construction of the actual work interacts with the glaze. The work of two different artists, using the same clay body and glaze, could turn out differently. Your actual work is the final test.