Different Types Of Clay Used In Making Ceramics
Though it might not seem too obvious at first, we live in a world dominated by ceramics. The house where you live in is most probably built – at least partially - using bricks, cement, and concrete. Our finest cookware and tableware for serving food are made from porcelain, while most sanitaryware pieces in the bathroom are made from white clay-based ceramics. Indeed, ceramics has a wide range of usage.
Ceramic is a broad term that encompasses a long list of materials such as glass, bricks, cement, pottery, porcelain, stoneware, earthenware, graphite and diamond. To succinctly put, ceramic is anything made from fired clay.
If you have ever attended any fine art classes, there is a high possibility that you would have worked with clay. This naturally occurring resource is abundant and is the basis for everything ceramic.
Generally, clay can be classified based on several factors such as geological origin and at what temperature the clay must be fired at, for it to reach its optimum durability and strength.
Regardless of its mode of classification, there are five common types of clay, namely; kaolin, stoneware, ball clay, fireclay and earthenware. The different clay types are used for varying purposes.
Kaolin is also known as Porcelain clay, or China clay, is used for the production of porcelain. This is chiefly down to its mineral purity. It is the purest clay type known to man. It is a naturally occurring clay called a primary clay because it is found very close to its source.
Although this clay type has some colour varieties, they are mostly light-coloured. They are only slightly plastic as compared to other clay types, making it difficult to work with. Consequently, it is usually mixed with Ball clay to help improve its workability.
Asides low workability, it also has a low plasticity level. The poor plasticity of kaolin is chiefly due to its relatively large particle sizes. Consequently, it tears when bent, if in a moist unfired state.
Of all classes of clay, Kaolin has the highest temperature maturity levels (high-fire clay) with temperatures as high as 1800ºC. In most cases, they are mixed with other clays to increase workability as well as lower the firing temperature. When fired, porcelain can become very hard and translucent. Also, its melted surface becomes very smooth and shiny such that a glaze is not needed.
Most porcelain items are made of a perfect mixture of kaolin and ball clay. Kaolin is widely used for the production of glossy paper. It is also commonly used in the production of morphine.
This clay type is highly plastic and contains only a few mineral impurities. Their colours vary depending on the state of the material. They are usually dark grey when moist and light grey or light bluff when they are fired. The temperature at which they fire to mature hardness isn’t as high as porcelain clays at 1280ºC.
That said, they do have a severe drawback, which makes them unsuitable for some purposes. For example, they cannot be used as standalone materials due to their excessive shrinkage that ensues during either drying or frying. However, when mixed with other clays like porcelain, they become extremely useful as they experience a marked improvement in workability and plasticity. They can also be combined with stoneware clays as well, which produces a unique finished look.
Ball clay is highly prized for its use in making sanitaryware, ceramics, wall, and floor tiles.
Stoneware Clays are highly plastic clays. These plastic clays are rarely found in their pure state and are usually hard and durable when fired. They are known as stoneware due to the stone-like characteristics they exhibit when they are fired. Stoneware clays are hard, dense surface with a variegated colour.
For the most part, they are fired to maturity at a temperature range of about 1176ºC and 1237ºC for mid-fire stoneware clay bodies and to about 1204ºC - 1224ºC for high-fire stoneware clays.
It takes on a wide range of colours when fired. Its colours range from tan or light grey to chocolate brown or dark grey. These colours are mostly affected by the firing used.
This clay type is an excellent option for functional pieces like dinnerware because of its hard and durable qualities. It is for this reason that it is the go-to clay type of production potters.
Stoneware bonds fittingly well with its glazes and is usually leak-proof when fired to maturity.
Earthenware clays are the earliest discovered type of clay, and are the most commonly found across the world. Earthenware clays are quite easy to work with and are highly plastic. Unlike Kaolin and Ball clays, which have only few mineral impurities, Earthenware contains iron as well as various mineral contaminants that help the clay reach optimal hardness at lower temperatures.
Speaking of low temperatures, they are fired to maturity at a temperature range of about 951ºC to 1105ºC. Because of its ability to be fired to maturity at lower temperatures, it produces softer, rather unvitrified porous ceramic with vibrant colours, low shrink factor and the ability to be stained easily.
Earthenware such as terracotta are unglazed and are usually not watertight or porous. It is used mainly for the making of tiles, plant products, and bricks.
Unlike stoneware, they usually do not function well for water-holding vessels since they are porous. To accommodate such non-viscous liquids, they need to be glazed.
Even when glazed, products made from earthenware are usually less durable than those made of glazed stoneware.
Essentially, earthenware clays can be found in their natural states as red, yellow, orange, or light grey. When fired, they usually take on any of buff, brown, red, orange, medium grey or white.
The colours earthenware clay takes on when fired usually depend on the type of firing and the amount of mineral impurities contained in it.
Fire clay possesses vast features. One of the most prominent being their high firing range, which is how the name fire clay was formed. They reach maturity stage when they are fired to around 1480ºC. Generally, they do not contain any mineral impurities. However, they exhibit a speckled look caused by spots of iron when fired.
When used in combination with stoneware clay bodies, they help increase the maturation temperature. In addition to this, they give the fired stoneware clay a bit of extra roughness.
Fire clay is mainly used for making refractory bricks and cement.
The type of clay you choose to work with will be hugely influenced by the kind of pottery you want to make. For example, while stoneware clay is excellent for kitchen and dinner wares because of its durability, unglazed earthenware clay is a poor choice for such items especially if the vessel is made to store liquids.
If you need to glaze your earthenware vessels to be able to accommodate liquid, you must ensure that both the clay body and glaze mature at the same temperature. With this, you can reduce the probability of developing defects in the manufactured item.
For the best result, you might need to combine two or more clay types. However, you should be aware of the right mix and proportions to strike a perfect balance between plasticity, portability, and durability.