Introduction to Ceramics

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

What exactly is clay? The chemistry explained:

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

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?

Two clay classifications are: primary and secondary (sedimentary)

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 porcelain (china clay).

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 clays and earthenware clays.

Earthenware pots and tiles

Earthenware Clay (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 nonvitreous. Some earthenware can be fired as low as 600 °C. It's useful for potted plants and outdoor decorations. It is easier for beginners to work with. 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.

Ball Clay is the most plastic of all the clays, and also one of the purest. It is a sedimentary clay that commonly consists of 20–80% kaolinite, 10–25% mica and 6–65% quartz, along with small amounts of organic matter. It's most often used for tableware, vases, and sinks. The term comes from how it was first formed into balls for transport, from the mines in Devon, England in the 18th century.

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 often contains impurities that make it look dirty, or grey. It's ideal for mugs, pots, and dinnerware, as well as industrial applications. 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.

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 very popular for dinnerware, tableware, teapots, cups, and figurines. Porcelain 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

Soft Paste Porcelain were 18th century English attempts and 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.

Red Waves, by Francoise Joris, with paper clay

Paper Clay is a mixture of clay with paper fibers (cellulose). All forms of clay can combine with paper. This paper allows the structure to be much firmer and stronger while sculpting. It reduces cracking, and the fibers burn away in the kiln. In fact, 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.

Bone China contains kaolin, bone ash, feldspar, and phosphates. It has mostly replaced true porcelain in modern times. It’s the strongest kind of porcelain. It’s very resistant to chip damage and has great physical strength. It usually produces a creamy white or translucent result. There is no standard for how much bone ash should be present. It can be anywhere from 5%-30%. The more bone ash present, the stronger and higher quality it is. Bone China is typically more expensive than true porcelain.