The Firing Process for Ceramics

           The term “Firing” in ceramics means to transform clay from its soft, malleable state into a sturdy, durable state; fittingly named ceramic. The literal definition of ceramic is “made by clay and hardened by heat.” Which is why all the cups, bowls, and plates in your kitchen cabinet are called Ceramics. If you're interested in the science behind it, ceramic is a solid material comprised of an inorganic compound of metal, non-metal or metalloid atoms primarily held in ionic and covalent bonds. But enough of that, the fun part is the process of creating ceramics.

            So what is Firing? It is the process of bringing clay and its glazes up to a high temperature. The objective is to make the clay and glaze so hot that it "matures." Mature means they've reached their optimal level of melting. I don’t mean they melt like birthday candles on 14^th birthday. I mean the melting point on a molecular level. The firing process is usually completed in two steps: Bisque Firing and Glaze Firing. What? Two steps of fire? Isn't that hot? Yup. It sure is. 

 The first firing is called Bisque firing because they were baking clay into biscuits. I'm kidding, but really what happens is that clay, which is termed greenware, is put into a kiln that can reach outrageously high temperatures. Once clay becomes super dry, the term we potters like to use is "bone dry," it is ready to be fired in a kiln. It gets pretty sciencey once clay reaches certain temperatures. The goal is to get the clay to turn into its first stage of being ceramic. So once fired, when it reaches 660 °F, the chemically bonded water begins to evaporate out of the clay. By the time it reaches 930 °F, all of the water that was in the clay is completely evaporated and has become the first stage of ceramic. It then continues and reaches about 1730 °F. Once there, the clay has become sintered, meaning that it has transformed to a point where it is less fragile, yet porous enough for glaze application. Once the clay has reached this temperature the kiln is turned off, and everything needs to very slowly cool, so nothing cracks or shatters due to thermal shock. But after everything has cooled down, the greenware has now officially transformed into bisqueware.

            Once we have our bisqueware, we are ready to glaze. And no I don’t mean that delicious buttercream glaze frosting on a slice of spongey vanilla cake… mmm… Sorry, where was I? Oh yeah, glaze. Glaze for ceramics is what I call liquid glass. It's a combination of different materials, like silica, metal oxides, colorants, and fluxes. The combination of these materials are mixed with water and applied to bisqueware. There are thousands of types of glazes that do different things like adding color, adding decoration, or waterproofing your wares. There are tons of ways to apply glaze to bisqueware. You can paint it, dip it, spray it, dump it, splatter it, or even finger-paint it on. Glazes do all sorts of different things to ceramics. You can use it to create rough textures, use it to enhance underlying designs, or make pieces vitreous and shiny to hold liquids. There are so many things you can do with glazes, so much that I will make a post later specifically on glazes.

            Anyways, once we are all glazed up were ready for the second firing called the Glaze Firing. Again, this is an impervious layer or coating applied to our bisqueware to color, decorate, or to waterproof it. If you want something to hold liquid, it needs to be glazed. This firing is heated slowly to different temperatures, depending on the type of glaze firing, up to a state where the clay and glazes reach maturity and are then slowly cooled down again. Sounds simple enough, right? 

As I said, there are different types of glaze firing methods. Glazes and clay bodies all have different melting points. Take porcelain; for example, your typical porcelain is fired between 2381°F and 2455 °F. There are high fire clays, mid-range clays,  and low fire clays, and the same with glazes. The average low-firing temperature at which low-fire materials reach maturity is about 1940 °F. The average mid-firing temperature at which mid-fire materials reach maturity is between 2124 °F and 2264 °F. And the average high-firing temperature at which high-fire materials reach maturity is about 2381 °F.

Most ceramicists do not think of the temperatures in degrees; they think of it in cones. Cones are a measure of temperature for firing ceramics. It typically begins at cone 022, which is roughly 1090 °F. The temperature goes up it continues by increments of one, as in 022, 021, 020, 019, and so on until it reaches cone 01, which is roughly 2050 °F. Once it has reached past cone 01, it starts up in a normal count into cones 1, 2, 3, and so on to the standard cone 10, which is around 2345 °F. Cone temperatures can go higher than cone 10, but usually, ceramicist does not go any higher. A standard cone chart reaches from cone 022 to cone 10. So when greenware is fired in its first firing and turned into bisqueware, it's fired to about cone 06. The second firing is different depending on if it's a low-fire, mid-range fire, or high-fire. Low-fires can be around cone 04, mid-range can be around cone 4, and high-fires are typically cone 10.

Now that you know a little about the firing process, we can discuss some types of second firings. The main two are oxidation and reduction firings. Oxidation firing is usually done in an electric kiln, but it can also done in a gas kiln. Oxidation firing allows oxygen to interact with the glazes creating very bright and vibrant colors. This type of firing can use high temperatures or low temperatures. Reduction firing is done in a gas or other fuel-burning kiln since the corrosive effects damage the elements on an electric kiln. In a reduction firing, oxygen is restricted from interacting with the glazes during glaze maturation by regulating the fuel in a gas kiln, or by adding organic, combustible material, like in a Raku firing. As the organic material in the clay burns, it uses up all the oxygen in the kiln, creating an oxygen-less environment. Reduction firing is usually used to get mottled, rich, earthy colors that often come from the iron in clay, which shows through and gives a spotted look throughout the glaze. High temperatures are what is typically used in reduction firings.

Other types of firings are wood firing, salt or soda firings, and raku firings. Wood firings are precisely what it sounds like. Instead of gas or propane as the fuel, we use wood. It can take place in wood fire kilns or pit firings. In a pit firing, a large hole is dug in the ground; it's laced with wood and pots. Then the wood is burned and after it reaches temperature, or the fire goes out, the whole thing covered with dirt to smolder. Pots are then dug out once it has thoroughly cooled.

In salt and soda firings, either salt or sodium is introduced in the final stages of firing. Due to the salt or soda, the atmosphere in the kiln allows pieces to get glazed like finish without glazes. The inside of your pots usually needs to be glazed, because the salt or soda atmosphere often won't reach into them.

Finally, my favorite style is a Raku firing. Raku is a firing process where a pot is heated until it glows piping hot, it is then pulled out of the kiln with tongs and placed into a reduced atmosphere with some sort of combustible material. Such as a metal bucket full of pine needles, newspaper or sawdust and covering it with a lid. After a short cooling period, the piece is plunged in cold water, creating a crackle effect. In Raku, unglazed areas of clay turn black because of the carbon from smoke. When the carbon is cleaned off the glazed areas, they sometimes create bright metallic areas such as copper and bronze.

There are more types and many other alternative styles of firing, these were just a few ways. In future posts, I plan to go more in-depth on the specific styles. The firing process can be just as fun as the creation process of your ceramics. Almost every coffee mug in your kitchen cabinet has been through one of these firing processes. So if you’ve ever wondered about how they were made, now you know.

If you have anything you’d like to learn about or have something to share, let me know in the comments below! Until next time, see ya!