Three Main Types of Clay
The second installment of The Lowdown on Pottery involves an element that many outside the industry don’t know but definitely should if they are considering purchasing. There are three main types of clay and each have specific ways for making as well as unique characteristics in their finished state. These characteristics play an important role on function of the piece as well as durability and longevity.
There are Three Main Types of Clay…
Earthenware is a low-fire clay that is porous. It will always be porous regardless of how many firings it goes through. Earthenware must be bisque fired then painted with a food-safe glaze and fired a second time to 1830–1940℉ to be considered food safe. Since the clay body remains porous, the glaze acts like a shell around the piece and provides a barrier to the clay body itself. If the glaze shell of a pottery piece used for food or beverage is compromised, then the piece should not be used for anything but decorative purposes.
Earthenware has low tolerance to temperature changes, especially rapid temperature changes, for example, microwaves. Putting an earthenware cup of coffee in the microwave, for example, could result in the glaze crazing, or getting hairline cracks in it. Any food or liquid that seeps into the clay through crazing, cracks, or chips, can cause bacteria to grow in the pores of the clay body, the bisque piece. Bisque poisoning is a real thing it happens because of this.
Most paint-your-own pottery studios feature earthenware pieces for painting. Even prior to the first time using them, these pieces should be inspected for complete glaze coverage. Chips and cracks are easy to recognize, but crazing can be subtle and looks like hairline fractures in the glaze. Crazing can happen over time or even while cooling down in the kiln, especially if the glaze and clay body are not a match. There will be more on crazing in an installment dedicated to glazing.
To extend the longevity of earthenware pieces, do not put them in the dishwasher or microwave so that extreme temperature changes are avoided. Earthenware has been used to bake over fire for centuries and is still used in the oven today. It just needs to be heated with the oven and cooled down with it as well.
Examples of common earthenware include terra cotta planters and pots, earthenware souvenir and novelty sculptures, and Raku fired ceramics. Many ceramists prefer working with earthenware, especially if they are not producing food-safe ware. Wall decor pieces, mixed media, large installations, and other sculptural type work can be done in a more cost effective manner with earthenware pieces due to the lower firing temperature requirements which means less expense for energy and less ware and tare on kilns.
I owned Art Blooms Do-It-Yourself Pottery Studio in Boiling Springs and later Shelby, North Carolina. We started out as a Paint-Your-Own Pottery entertainment venue and then quickly expanded to include handbuilding and throwing with stoneware clay, kiln-formed glass, acrylic and watercolor painting, mixed media, recycled art, polymer clay and art journaling. Because of my commitment to my customers to paint pieces that have higher durability than earthenware, I offered stoneware pieces that I produced as an option for their painting pleasure.
With amazing help from my husband and children, we were in business for over four years before I got too cold and had to move back to Florida! Art Blooms was my pride and joy and remains one of my greatest professional accomplishments.💟
Stoneware is a mid-fire type of clay and is extremely durable, hardy, and food safe, even without glaze. When fired to the target temperatures of 2185–2232℉, the clay body vitrifies and all the pores are closed. This makes the body solid and prevents any food or beverage to penetrate through the edges. This means that even if the piece gets chipped, cracked, or has a glaze crazing issue, it is still food safe.
Stoneware truly attains stone-like qualities during this firing and can be used in the oven and microwave. It is also dishwasher safe. A great conductor of heat, stoneware baking stones have been popular for years.
Stoneware bakers are also very popular. Much like earthenware, it is recommended that stoneware pieces heat up with the oven, although they can handle being taken out of a hot oven and carried right to the table for serving.
Stoneware can also handle outdoor temperature changes. Although I recommend bringing plant pots in when freezing temperatures are expected, theoretically stoneware can handle the cold.
Stoneware is my choice for pottery. It offers the most versatility of clay bodies for throwing and handbuilding while providing durability and longevity to produce heirloom pieces. I use a variety of different clays depending on the intent for the pieces I am working on, but they are all in the stoneware category. It is the best of both worlds…durable and hardy for everyday function and the right level of moisture in its malleable form for sculpting, hand-building, and throwing on the pottery wheel.
Porcelain is high-fired pottery. Often we think of porcelain as delicate because of the porcelain sculptures we have seen like Lladro, Limoges, and Lenoz. However porcelain artifacts date back to 612 and the Tang dynasty. The fact is that they have lasted because of their durability.
Porcelain gets fired above 2305℉. It provides all the strengths that stoneware provides and then some, meaning that you can sculpt very fine forms that appear delicate but are actually quite durable.
Bone china is a more durable stronger version of porcelain. The clay is made by mixing ash from cattle bone, fine silica sand, and feldspar minerals. As a vegan, I do not use bone china clay. Historically, bone china has been considered top level pottery and highly desirable because of its crisp white clay body and its strength. Since bone china has over 30% white ash mixed in its clay body, the ceramic walls of a piece can be made extremely thin, even to the point of translucence, be even more resistant to breaks and chips than porcelain while still appearing delicate. Since it can be made so thin, the pieces are light in weight. Think dainty tea cups versus hearty coffee mugs.
Although I do like porcelain, I do not work with high fire clay bodies. I use a mid-fire porcelain that acts similar to stoneware. By using a mid-fire porcelain, I can fire my stoneware and porcelain pieces in the same way and use many of the same glazes.
A little about the cost differences between the types.
The cost of production increases with each level of clay as well. The mere fact of firing temperatures is a large part of that. A later installment will discuss alternative firing methods for finishing clay pieces in addition to electric kilns.
The expense of firing kilns goes beyond electrical costs as the elements in the kilns require more frequent replacement with higher temperature firings as do the thermocouple that regulate temperature during firings.
Each category of clay also requires its own types of glazes. Although some commercial underglazes can be used for all three types of clay, the majority of homemade glazes and commercial glazes can only be used on one type therefor cost increases for the business that produces more than one type of clay.
Visit RachelleEason.com for more information about pottery and to see Rachelle’s portfolio and her current collection available in her shop!
Engage With Your Own Pottery!
It’s a lot to take in, I know! When I taught this in the first week of my Ceramic 1 classes, they had the benefit of actually getting their hands in clay for the workshop part of the class after taking notes through the lecture segment. I have two suggestions for you after learning all this info.
- Take a stroll through your space and pick up pieces of ceramic you have around. Feel the weight, look closely at the glaze, and study the form looking for small delicate bits and heavier protrusions. Turn them over and check the bottoms. If a food-safe piece has an unglazed bottom than it should be stoneware or porcelain/bone china. If the bottom is glazed, it is most likely earthenware.
This is not a foolproof way to determine but it is the easiest for guessing. Non-food safe earthenware may not be glazed on the bottom and stoneware may be glazed on the bottom but finished with an unglazed foot ring that prevents it from attaching to the kiln shelf during firing. More on that later.
- Get your hands dirty! You may not have the opportunity to work with clay and have it fired, but you can engage with your clay pieces. Grab a ceramic plant pot and transfer a plant into it. While in process, engage with the pot and learn about her intimate characteristics. Notice I changed pronouns from it to her. Each pot has their own personality and carry the essence of their maker. Learning more about them engages you in the moment as well as sends an energy of protection to your plant since you are placing it in a ceramic sanctuary of sorts for its growth. Happy planting!
- I said two ways but I have actually have a third suggestion as well. You can pick up air dry clay at your local craft store. It is fairly inexpensive, which is good because there is a learning curve when using it successfully. It does not work like kiln-fired clay, but you can simulate a bit of the experience.
I used air dry clay in my college ceramic classes that had to move online due to Covid 19. Although it wasn’t ideal, it was the only way to simulate the class without needing a kiln for students to fire their pieces.
Air dry clay is never food safe regardless of the sealant you put on it, but it does make for an enjoyable adventure in making trinkets, beads, and whimsical fun!