Pottery clay is something that everyone needs to pursue their interests in pottery. There are plenty of different kinds of clay that you have to choose from so it isn't just a case of picking up dirt that covers the ground. Clay is a result of rocks that have decomposed over the centuries, creating a unique composition, both in terms of what it is made of and what it is capable of.
Clay has the capability of forming a beautifully cohesive unit, when combined with water in the right amounts. That amount of water can make it anything between a slurry and thick liquid to something that's mouldable and can retain the shape it's put into. Known as plasticity, it is this ability of clay that makes it extremely different inelastic forms of mud or even sand. Pottery clay, which is a name derived from one of the most popular uses of this form of clay, is also capable of becoming extremely hard when exposed to really high temperatures - allowing you to finish off everything you make with a relative amount of permanency.
Types of Pottery Clay
The most important characteristics of clay and the ones that define how and why they are chosen are the elasticity, the smoothness and the level of hardness they attain once fired. Based on all these characteristics, the most commonly used forms of clay are earthenware, mid-fire stoneware and high-fire stoneware clay. All three of these formats are available quite freely and are extremely popular in pottery.
Earthenware clay is the most common form of pottery clay and one that has been in use since the first time humans looked at pottery to create materials and utensils for their usage. The plasticity of this form of clay is extremely high and is often seen as sticky, but without making it difficult for you to work with them. Earthenware is not one of the purer forms of pottery clay and, in fact, there are a lot more impurities in earthenware clay, like iron, that lead to a higher firing temperature.
Earthenware clay normally fires between 950 and 1,100 degree centigrade. In terms of colours, you will find earthenware clay to be in the red or orange range of hues, while finding some that's either grey or even yellow isn't uncommon. Once fired, you are likely to end up with something that's brown or red or orange, while white and grey are also possible colours. This colour will depend on the kind of impurities that are present in the clay and the amounts they are present in.
Stoneware clays are also quite plastic in nature and are usually found to be grey in colour, especially when in a moist state. They are fired at around 1,160 to 1,225 degree centigrade, for mid-fire stoneware clay and between 2,200 & 2,336 degree centigrade for the high-fire stoneware variety.
There are some other types of clay as well but they are mostly used by experienced potters or potters who are specialising in a particular format of pottery. Ball clay is one of those clays that comes with an extremely high plasticity rate and goes to about 1,300 degree centigrade in the kiln. It is dark grey in colour, when moist, and ends up in a light grey colour once it's gone through its round in the kiln.
However, the problem with ball clay is that it cannot be used individually because the amount of shrinkage it undergoes, in the kiln, leaves it quite dry after firing. This increases the tendency for the piece to crack and, therefore, it needs to be used with other forms of clay that keep it suitably moist. This also helps in creating a plasticity-level that's easier to work with and definitely something that potters prefer as compared to pure ball clay.
Fire clay is something that has a lot of different properties, most of which are sought after by potters. While the clay itself fires at about 1,500 degree centigrade, there is a tendency for parts of the clay to contain some iron. Since fire clay, in general, is quite pure, these little iron deposits end up creating little specks that give it a unique appearance once fired. When used alongside stoneware clays, fire clay gets a bit more roughness because the firing temperature is further increased.
In the end, it all comes down to the kind of pottery you are planning to do. The choice of pottery clay is based on the kind of project you are about to undertake. If you are still learning about pottery, stick to something that's easy to use, which means one of the first three kinds mentioned above. If you are trying to find new ways of making this hobby interesting, then pick out your pottery clay from the other varieties mentioned above.
It all comes down to practice and what you do with the time you spend at the wheel. The more you practice, the better you get and the better chance you have of exploring the entire world of pottery clay.