Introduction to Ceramics: Kinds of Clay

Some notes taken from The Practical Potter: A Step-By-Step Handbook by Josie Washaw, and Marie at Potterycrafters.com

Clay is a wonderful material for additive and subtractive sculpture. You can take a large piece and carve away at it, or you can join pieces, adding them together. Clay is great at mimicking other materials. You can paint it in many different ways. You can press it against different surfaces to make it look like wood, canvas, a piece of fruit, etc. There are countless ways to press, cut, and stamp the surface for a wide variety of textures. And you can use it in mold making to create artwork from other materials such as plaster and bronze. It is hard to overstate the importance of clay ceramics in human history. We've been using it to make pottery since at least 14,000 BC. Our first written texts were on clay tablets. Clay is used in industry and in bricks - millions of people rely on clay for the homes they live in.

A receipt for the purchase of beer, from ancient Sumer, 3100-3000 BC

What exactly is clay? The chemistry explained:

Clay is made up of approximately 40% alumina, 46% silica, and 14% water. The basic mineral in clay is kaolinite (Al₂O₃·2SiO₂·2H₂O). There are many different types of clay, each with their own strengths and weaknesses. The three basic components of all clay are: The plastic (kaolinite), the flux (feldspar), and the filler (grog). You can altar the properties and color of your clay by adding and mixing these materials either in the dry stage as a powder, or in the plastic stage.

The Plastic:

Kaolin

The basic mineral in all clay is kaolinite (Al₂O₃·2SiO₂·2H₂O) (found in kaolin rocks), which is made up of approximately 40% alumina, 46% silica, and 14% water. 

Kaolinite under an electron microscope

When kaolinite is mixed with water it becomes plastic, meaning malleable. You can manipulate it without it cracking. As it dries, it hardens, becoming delicate and brittle. When heated in a kiln, the kaolinite particles melt and fuse together, the water evaporates, some gases are released, and the resulting structure hardens and becomes much stronger.

The Flux:

potash feldspar

Clay fluxes are mixed in to lower the melting point of the material, making it easier to melt and vitrify (to turn to a glass when heated). When cooled, flux helps strengthen the kaolin bonds that make the clay hard and resilient. The most common clay fluxes are potassium oxide and sodium oxide, both found in feldspars (some of the most common rocks in the Earth's crust). Calcium oxides are often used in ceramic glazes.

The Filler:

Grog (also called firesand and chamotte) is basically clay that's been fired at high temperature, and then ground up to a powder. It can be added to clay for two purposes. First, it reduces shrinkage (clay shrinks, as it dries). Secondly, shrinking too suddenly can result in "thermal shock" and cracking. Grog helps clay to dry evenly to reduce the chance of cracking, which is particularly important in raku firing, where the vessel changes temperature dramatically fast.

What are the different kinds of clay?

You can classify clay by the temperature necessary to melt it into a single, solid vessel (a process called vitrification). Clays are typically considered high-fire (2,350 °F, “cone 10”) or low fire (1,930 °F, “cone 04”). Another way to classify clay is by its source: primary or secondary (meaning sedimentary).

Primary 

Primary clays are very white, both before and after firing. They are still in the same place as their “mother rock”. They have large particles, and have little exposure to the atmosphere. These are extremely pure, fire at high temperatures, and are almost “non-plastic” so they are not good for hand modeling, but can be used with molds. Plasticity is what allows clay to hold its shape when soft. If a great deal of water is added, the clay loses its plasticity, turning into a sticky substance called “slip”. If the clay is too dry is also loses its plasticity and will be hard to work with. If your clay cracks as you bend it, add more water. Examples include:

Meissen plate from Swan Service for Count Brühl of Poland, one of the first true porcelain pieces in Europe, 1737-42

Porcelain (also called hard-paste, true porcelain, and fine china) is mostly pure kaolinite, or China clay, with some feldspar and quartz. It is the hardest clay to work with, the water always wants to go to the surface, even when you pat it down. You have to work quickly and purposefully, it’s like sculpting ice cream. Porcelain is very popular for dinnerware, tableware, teapots, cups, and figurines. It is also used in dentistry to make dental bridges and is used as insulation in electrical wiring. Heated between 1,200 and 1,400 °C (2,200 and 2,600 °F), it is prized for its strength and translucency.

Porcelain clays come in "long" and "short" varieties. Long clay is stickier and more plastic. Short clay is less so. In general, adding small amounts of water to porcelain clays greatly changes plasticity, making it harder to work with. When porcelain is bisque-fired it is already non-porous, so glazing is not required. Glazing is purely decorative.

Soft porcelain tureen, in the shape of a swan, Chelsea Porcelain, 1752-56

Red Waves, by Francoise Joris, made with paper porcelain

Paper Porcelain is a porcelain mixed with fine paper fibers (cellulose), to make it much more malleable, but also firmer and stronger, so it's easier to sculpt with. You can use it to make extremely thin and delicate decorative artwork. The paper fibers burn away when firing, leaving a strong, solid clay structure to the vessel - this also reduces cracking. All forms of clay can combine with paper to help achieve this effect. Some say the paper fibers make the porcelain so strong you can "raw glaze" it, meaning you paint the glaze while the clay is still "green", and fire it only once instead of having to bisque fire it. But this is still inadvisable, as it could cause blistering, etc. Try a test piece first.

Bone China is a premium, highly durable form of porcelain that contains real cattle bone ash (anywhere from 5-50% of the clay) as well as kaolin, feldspar, and phosphates. Bone china looks a lot like regular china, except that it is more translucent, and has a warmer, creamier color. Although it looks very delicate, it is actually the strongest and most chip-resistant of all ceramic dinnerware. It has mostly replaced true porcelain in modern times. There is no standard for how much bone ash should be present. The more bone ash present, the stronger and higher quality it is. Bone China is typically more expensive than true porcelain. 

 

Stoneware vessel, Han Dynasty, 206 BC - 220 AD

Stoneware is harder, nonporous, and less plastic than other clays - like stone. It fires higher, between 1,100 °C (2,010 °F) to 1,300 °C (2,370 °F), making it vitreous - it melts into a single, solid, non-porous vessel. It often contains impurities that make it look dirty, or grey. Although first used in Asia, the term "stoneware" is not used there. It is instead considered porcelain by local definitions, and so it is sometimes referred to as "near-porcelain". Stoneware is more opaque than porcelain. It consists of a mix of fire clay, 0-15% ball clay, feldspar, and grog. Stoneware is the global industrial standard for durable kitchenware. It’s dense and non-absorbent, making it safe in an oven, dishwasher, and microwave. It holds its shape beautifully, has a toothy feel, and is strong (hard as stone). It comes in different colors, but is most commonly grey when fresh, and fires to a bone white color.

Secondary 

Secondary clays have been transported away from their mother rock by wind and water erosion, and deposited according to the size of their particles, with the heaviest settling to the bottom. The finest particles settle in places where the water is stagnant. These clays are finer and more plastic, but contain impurities. These impurities change the clay’s color and lower the firing temperature. The more plastic a clay is, the more it absorbs water, increasing in volume. Clays with smaller particles will shrink more than clays with large particles. Examples include: 

Ball Clay is a highly plastic, fine-grained form of clay, normally used as an ingredient to mix with other clays. It’s rarely used on its own because it shrinks drastically and warps when fired. Instead it is blended with stoneware and porcelain to make them easier to shape and mold – it’s like a glue that holds clay particles together. commonly consists of 20–80% kaolinite, 10–25% mica and 6–65% quartz, along with small amounts of organic matter. Ball clay gets its name from the way it was harvested in 19^th century England, cut out of the side of hills and rolled downhill, onto transport carts. Unlike most secondary clays, ball clay is high-fire.

Creamware is a lightweight cream-colored earthenware from England. It looks like porcelain, it was cheap enough for the middle class. Wedgwood used creamware.

Earthenware is a term for many low-fire clays that come in a variety of colors, from white, cream, grey, buff, yellow, and red. Some earthenware can be fired as low as 600 °C.

Raku is a way of firing earthenware, developed in Japan. It’s a process, rather than a kind of clay, but you need specially treated earthenware for the vessels to survive the process. Vessels are pulled straight out of a kiln, glowing-hot, using long metal tongs, and are placed into containers of combustible materials like sawdust, leaves, and feathers. The smoke creates course and unpredictable, metallic, and crackled finishes. To survive this, the clay is wedged (combined) with grog and/or sand (20-30% of the total volume). Grog is pre-fired, crushed-up ceramic powder). Without this treatment, any normal clay would crack and shatter.

Soft Paste Porcelain was an 18th century English attempt at reproducing Chinese porcelain. Originally containing soapstone and lime, they were fired at lower temperatures, making them softer, weaker, and non-vitreous. Later attempts used feldspar, quartz, syenite and other minerals that were better, but still not true porcelain.

Earthenware pots and tiles

Terracotta is a kind of earthenware, rich in iron oxide. The iron makes it red/orange/rusty in color. They come in a variety of colors depending on location and the specific ingredients (impurities). The prepped earthenware clay you typically buy for ceramics contains 25% kaolin, 25% ball clay, 35% quartz, and 15% feldspar.  Terracotta is the oldest and most common form of clay used by man. Most European ceramics up until the 17th century were earthenware, including all ancient Greek and Roman pottery, most Egyptian pottery, all faience, majolica, and Delftware. It's very porous and plastic. It's fired at lower temperatures, below 1,200 °C (2,190 °F), making it non-vitreous. Terracotta is tricky, and timing is crucial because it’s unforgiving - you don’t want it too thick or it’s heavy and could crack. If it’s too thin and wet it will collapse. It’s most commonly used for unglazed flower pots, roofing tiles, decorations, and cooking vessels. Terracotta cookware must be “seasoned” to withstand heat without cracking. Even then, never wash with soap as it will soak it up like a sponge. And avoid thermal shocks or it will crack.

Seasoning Terracotta involves 5 steps:

1.      Deep Soak in Water (12-24 hours)

2.      Thorough Air-Drying

3.      Lubricate with vegetable oil

4.      Heat Cure – either in an oven or on a stove top.

Yellowware: This is a buff-colored iron-tinted clay, used for thick, durable mixing bowls, pie weights, and baking dishes, as it was less likely to chip. It originated from the UK in the 18^th century, and deposits were found in the US, so it spread there. It was the “Tupperware of its era” until cheaper glass and plastic replaced it in the 1930s.

Which kinds of clay can you eat and drink from?

You need a high-fire ceramic clay, like stoneware or porcelain. When they are fired they become nonporous. Other clays can leach toxic chemicals or harbor bacteria. Never make an eating vessel from polymer clay, air-dry clay, craft oven-bake clay, or wild (harvested) clay. Earthenware clay may be used if it is seasoned, or coated with a non-toxic, lead-free glaze to block bacteria. Avoid glazes that crack and craze as food will get trapped in it.

Which kinds of clay are machine washable?

Stoneware, porcelain and commercial gloss glazes should be dishwasher safe. The glaze should be smooth and nonporous. Never put earthenware, matte or textured glazes, gold or metallic lusters, cracked or crazed glazes, or hand-burnished/unglazed pottery in a dishwasher.

The “Ring” Test – tap the edge of a vessel lightly with a spoon. If it’s dense stoneware or porcelain, you’ll hear a high-pitched, clear “ping”. If you hear a dull, heavy “thud” it’s probably earthenware.