Different Types Of Clay Used In Making Ceramics

Ceramic materials are everywhere - and in many cases need a high quality clay for their production, that is well suited to their intended use. African Pegmatite is the go-to industrial partner for the supply and processing of the broadest range of clay materials, as part of an uncompromising selection of superior grade minerals, suitable for any project.

Ceramics are everywhere, from the bricks that build houses and the tiles on the bathroom wall, to the dinner plates the world eats dinner from and the sinks we wash in. Pottery is an ancient handicraft. Today, skilled artisans create many valuable items using clay, and it is even used in home based handcrafts.

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.

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.

What Is Clay?

Clay is a term for any type of naturally occurring soil-like material that is rich in aluminium phyllosilicates. The balance of such phyllosilicates alongside other compounds, combined with the conditions of formation and crystal structure determine the physical type of clay, which is grouped into kaolin-type (including kaolin itself), smectite-type (including montmorillonite) and ilite-type (of which ilite is the only major example).

Perhaps more useful is grouping clays by their use type - which is indirectly related to their chemical and physical compositions. Most of the clay types discussed here are of the kaolin type.

Kaolin

Kaolin is also known as porcelain clay, or china clay, is used for the production of porcelain, with non-ceramic applications including in papermaking and in the production of plastics. 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. Two major colours of kaolin exist, white kaolin and red kaolin. The former is more common and is widely used as detailed below. Red kaolin is broadly similar, except has a red colouration due to the oxidation of iron oxide.

Asides from having a low workability, it also has a low plasticity level. The poor plasticity of kaolin is chiefly due to its relatively large particle sizes. Consequently, pre-fired kaolin ceramics can easily tear when bent, if in a moist unfired state (i.e. moisture levels are suboptimal).

Of all classes of clay, kaolin has the highest temperature maturity levels (high-fire clay) with temperatures as high as 1,800 °C. 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. One of the drawbacks of thin deployments of porcelain is that it is highly brittle and prone to chipping at the edges - another reason why kaolin may be mixed with another clay to make more durable goods. Conversely, kaolin is abrasion resistant in the cured form.

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.

For the production of kaolin containing ceramics and bricks, reports have shown that bentonite can be added in proportions of around 2% and has been shown to increase both strength and density of bricks produced without increasing cost. Bentonite is itself a form of clay.

Ball Clays

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 1,280 °C.

Ball clays often 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. Rheological stability is a known feature of ball clays, in addition to their whitish colour upon curing. 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. Ball clays contain kaolinite (20 - 80% by weight), mica (10 - 25%) with the balance being quartz sand or other accessory minerals. Ball clays are almost exclusively fine grained.

Stoneware Clays

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 surfaces with a variegated colour. For the most part, they are fired to maturity at a temperature range of about 1,176 °C and 1,237 °C for mid-fire stoneware clay bodies and to about 1,204 °C to 1,224 °C for high-fire stoneware clays.

Stoneware-type clays take 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 temperature of 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.

It should be noted that stoneware clays are far from uniform - their composition of fire clay, kaolinite, quartz, mica, feldspar and other minerals can vary wildly. The kaolinite that may be present will be highly disordered, with the mica and quartz being present in very small particle sizes. Flint may or may not be present.

Earthenware Clays

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, hence are popular choices for industrial scale production and artist/hobbyist production alike. 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.

they are fired to maturity at a temperature range of about 951 °C to 1,105 °C. Because of its ability to be fired to maturity at lower temperatures, it produces a softer, rather vitrified 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.To accommodate such non-viscous liquids, they need to be glazed. Mechanical strength for this clay type is typically lower than the others, and hence it is formed into thicker articles prior to curing. 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 buff, brown, red, orange, medium grey or white colour.

Modern earthenware clays can be thought of as compound clays. A typical contemporary earthenware clay will contain kaolinite (25% by weight), ball clay (25%), quartz (35%) and feldspar (15%). The colours earthenware clay takes on when fired usually depend on the type of firing and the amount of mineral impurities contained in it. High iron containing clays will often produce a redder fired product.

Fire Clay

Fire clay possesses vast features and are defined as refractory clays being composed of hydrated silicates of alumina. These clays are known for their particularly high alumina content. One of the most prominent features being their high firing range, which is how the name fire clay was formed. They reach the maturity stage when they are fired to around 1,480 °C. 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 (also referred to as fire bricks) and cement. Refractoriness is the ability of a material to withstand extreme temperatures without deforming, breaking or becoming less resistant to chemical attack. Refractory bricks are used in situations like in the linings of furnaces for metal and glass smelting, and tend to be relatively dense. Typical modern fire clay bricks are composed of 50 to 60% (by weight) sand/silica, 20 to 30% alumina (coming directly from the clay) and 2 to 5% lime. Refractory cement is a related ceramic compound that is composed partly of fire clay, however it is applied in paste form and cured in situ akin to conventional cement. Depending on additives, refractory bricks and clay can be some of the most highly performing refractory materials around.

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Summary

The type of clay that is chosen to work with will be hugely influenced by the kind of pottery, brick or tile desired. For example, while stoneware clay is excellent for kitchen and dinner wares because of its durability, whereas unglazed earthenware clay is a poor choice for such items especially if the vessel is made to store liquids. Post-firing glazing is a popular choice for finishing many ceramic types. Additionally, many other additives can be incorporated into ceramics - particularly those for bricks and tiles (and is detailed elsewhere on this website). For the best result, the combination of two or more clay types may be most suitable. However, you care should be taken to ensure the right mix and proportions to strike a perfect balance between plasticity, portability and durability.

Ceramics are a broad class of materials that are made from clay that has been fired in a kiln
Depending on the desired application different types of clay - or combinations of clays - may be used to give certain characteristics
Kaolin, ball clay, stoneware, earthenware and fire clay are some of the most widespread examples

Clay ceramics are a broad class of compounds, with virtually limitless applications. African Pegmatite is a leading supplier and processor of an extensive variety of minerals including a full spectrum of clay products for the most demanding applications.