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.
|A BRIEF INTRODUCTION TO CLAYS
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.
|EARTHENWARE, PORCELAIN, AND STONEWARE
Earthenware, Porcelain, And Stoneware
Earthenware is normally beige, red, or white. It has the lowest firing temperature of the three, usually about 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 planters and oven steamers, but not good for pitchers 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.
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.
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.
Polymer clay is a man-made material: tiny particles of polyvinyl chloride mixed with plasticizers and pigments. When it's baked, at around 125°C, the particles fuse and the clay hardens.
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.