A guide to pottery clay

Do you want to learn more about pottery but you are not sure where to start? Well, you are in the right place! :)

What is the origin of pottery?

Let’s start with the basics! Pottery has been made for as long as the human race existed, with functional vessels for storage. Many of these pots were decorated in various ways, and in some countries such as Japan, China and Greece, pottery became a form of art, ever more refined and involving several decorating techniques, glazes and firing methods. 

What is clay?

Clay is unique in its ability to be molded into objects and shapes by hand and become permanent when exposed to high temperatures. It the result of decomposing rocks that contain several different minerals and metal oxides. When mixed with the right amount of water, it forms a cohesive mass and keeps its shape when moulded: this is what we call clay’s plasticity.

Clay becomes ceramic when exposed to extreme heat, which fuses all particles together and hardens the body, making it often more permanent than rocks themselves. 

Types of clay

Wanna make a start in pottery too? One of the things to do first is getting to know what types of clay there are and which one works best for you. The clay handling's strength will depend on its plasticity and you will be able to choose the one you like the most by discovering how it responds to rolling, pressing and pulling it.

Clays can be divided into many classes, based on the characteristics and the temperature they are fired in order to become hard and durable, as in mature.

The most common ones are earthenware clay bodies, mid-fire stoneware and high-fire stoneware clay bodies. You can easily purchase all three of them, although some people like to create their own clay (it's a bit of a messy job, but surely fun!)

There are also other types of clay, such as porcelain, paper clay and Raku clay. 

Earthenware clay

Earthenware pottery has been made since the earliest of times; it has always been favored for domestic and culinary use because of its good resistance to thermal shock, particularly over open fires.

The most common earthenware is Terracotta, which is still used to make bricks, roof tiles and plant pots. Terracotta is an Italian word and it means "fired-earth", and when fired, terracotta becomes porous and relatively soft. The red color comes from the presence of iron oxide in the clay and the higher the percentage of iron oxide, the richer the color. It is also very porous and this allows evaporation through the surface, keeping every liquid content fresh and cool in hot climates.

Earthenware is very high in plastic and is very easy to model. It can be found in several colours, such as red, yellow, orange, white, medium grey, and so on. This type of clay can reach its hardness at lower temperatures, they can be fired between 1000 C to 1080 C.

Stoneware clay

Stoneware bodies are composed of mixtures of minerals, clays and sand or grog. After high-firing it, it becomes dense and resistant and its weight give it a stone like feel, hence the name. its durability and strenght, as well as low water absorption rate, makes it ideal for domestic ware and outdoor use such as vases and pots, while fine stoneware clay is excellent for functional pottery such as plates and dishes.

Stoneware usually comes in a grey colour and fired colours can get affected by the type of firing. The difference between mid-fire and high-fire stoneware clay is the firing temperature the clay will need to reach its optimum hardness. Mid-fire clays can be fired between 1160 C to 1225 C, high-fire clays can be fired higher, from 1200 C to 1300 C.

Porcelain

Porcelain is usually white in color, very fine in texture and one of its characteristics is its translucency when thin. To reach the whiteness and density, it will need to be fired at a temperature of 1280 C. It's usually given a first bisque firing at 1000 C to make it easier to handle while decorating and glazing. 

Porcelain is extremely durable and has incredible strength. However (there's always a weakness!) it lacks f plasticity, therefore it's not easy to model: the working time is very critical with a narrow margin between the clay being too soft and too dry. 

Porcelain is very delicate and of course, it needs to be handled with much care before firing it.