Clays are used to make things such as beads, bricks, electronic components, flowerpots, insulators, knife blades, ornaments, roof tiles, tableware, and vases. Clays are inexpensive and versatile and can be used at a basic level with very little skill.

What Is Clay?

Clays are formed naturally over millions of years as rocks break up into minute particles. They consist of hydrous aluminium silicates and other compounds such as feldspar, iron oxides, mica, and quartz. Clays are collectively referred to as ceramics.

Clays are often divided into three main categories: earthenware, porcelain, and stoneware. Generally, they needs to be fired for several hours, although the exact chemical composition affects the firing temperatures and times, and the clays' colour, porosity, shrinkage, and strength.

All clays are created to mature at specific temperatures, and any variance can lead to unsatisfactory results in ceramic durability or color. If fired too high, clay can deform or even melt; if fired too low, your pieces will be dry, rough, and potentially unsolidified.

Historically, low-fire has been the most commonly used firing range due to limitations in kiln technology. Modern kilns are now capable of much more complex, high-temperature processes, but low-fire range continues to be popular because it allows ceramic artists to use a variety of colourants that either burn off or become unstable at higher temperatures.

The maximum cone rating of a stoneware or porcelain clay is the temperature at which it vitrifies. This is the hardening, tightening and finally the partial glassification of the clay. Vitrification results from fusions or melting of the various components of the clay. The strength of fired clay is increased by the formation of new crystalline growth within the clay body, particularly the growth of mullite crystals. Mullite is an aluminum silicate characterized by a long needle-like crystal. These lace the structure together, giving it cohesion and strength.

When clay vitrifies it gets very strong. This is especially important for dinnerware where pieces are exposed to a lot of abuse. Vitrification also makes the clay's porosity low.

What Are Earthenware, Porcelain, And Stoneware?

Earthenware is normally beige, red, or white. It has the lowest firing temperature of the three, usually lower than 1150°C. It's slightly porous, and stains and chips easily, so it's often glazed to protect the surface. Its porosity means it's good for making terracotta planters and oven steamers, but not good for jugs or vases.

Porcelain is composed of kaolin, or china clay. Kaolin doesn't melt until 1800°C, so other compounds are usually added so it can be fired between 1250°C and 1400°C. For example, bone china is made by adding bone ash to the clay. It's known for its whiteness, hardness, smoothness, durability, and translucency. When tapped, it makes a distinctive ping: or ming.

Named after a hill in China from which it was mined for centuries, kaolin is the purest form of clay and is the foundation of all porcelain clay bodies. Though pure kaolin clays can be fired, often they are mixed with other clays to increase both workability and lower the firing temperature, so if using a kaolin-based clay body, be sure to note how pure your material is, as this will change the required temperature.

As a clay body, porcelain is known for its hardness, extremely tight density, whiteness, and translucence in thin-walled pieces. Another difficulty with porcelain bodies is that they are very prone to warping during drying in the kiln

When fired, porcelain becomes a hard, vitrified, non-absorbent clay body, very similar to high-fire stoneware. It also develops a body-glaze layer formed between the clay body and the glaze. The absence of any iron, alkalies, or alkaline earths in the molecular structure of kaolin not only dictate its high-fire requirements, but are also responsible for its most identifiable characteristic: its white color.

Stoneware is normally beige, grey, or red-brown. It's usually fired between 1150°C and 1300°C. It's hard, durable, and resists thermal shock. Glazes bond well, so it can be made waterproof.

Stoneware is a plastic clay, often grey when moist. Getting its name from the dense, rock-like nature of the clay body when fired, stoneware is typically combined with other clays to modify it, such as ball clays which might be added for plasticity. It is important to note that stoneware is divided into two types: mid-fire and high-fire.

Like low-fire bodies, mid-range stoneware is relatively soft and porous and has a clearly separate glaze layer after firing. However, a mid-range firing results in increased durability of the ware as well. When fired, stoneware ranges in color from light grey to buff, to medium grey and brown.

Mid-range glazes typically mature between Cone 4 and Cone 6, and most commercial underglazes have a maximum temperature of Cone 6. These glazes are more durable, still offer a fairly extensive color range, and though not quite as harsh as low-fire glazes, can still be quite bright.

What Is Bisque?

Bisque is clay which has been fired once, without a glaze, to a temperature just before vitrification. Firing changes the clay into ceramic material, without fully fusing it. A second, slower, firing melts the glaze and fuses it to the clay body.

What Is Bone China?

Bone china is a type of porcelain composed of bone ash, feldspathic material, and kaolin. It's the strongest of the porcelain or china ceramics, having very high mechanical and physical strength and chip resistance, and is known for its high levels of whiteness and translucency. Its high strength allows it to be produced in thinner cross-sections than other types of porcelain.

From its initial development and up to the latter part of the 20th century, bone china was almost exclusively English, with production being effectively localised in Stoke-on-Trent. Most major English firms made or still make it, including Fortnum & Mason, Mintons, Coalport, Spode, Royal Crown Derby, Royal Doulton, Wedgwood, and Worcester.

In the UK, references to china or porcelain can refer to bone china, and English Porcelain has been used as a term for it, both in the UK and around the world.

What Is Polymer Clay?

Polymer clay is a man-made material: tiny particles of polyvinyl chloride mixed with plasticisers and pigments. When it's baked, at around 125°C, the particles fuse and the clay hardens.

What Is Raku?

Raku was originally a Japanese technique, but it's now become an internationally popular way to make decorative ware, with each piece having a unique blend of colours.

A bisque piece is fired to about 950°C, then glazed. It's removed from the kiln when red-hot, and put straight into a container of combustible material.

The flames, reducing atmosphere, and mix of chemicals stain the clay. When the piece is removed and quenched in cold water, interesting colours and shades remain: often unpredictable.

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