Plasticity refers to how flexible a clay or clay body is. Any particular clay's plasticity is greatly influenced by the clay's particle size, water content, and aging. It’s also known as "the quality or state of being plastic; especially: a capacity for being molded or altered."
Assuming a New Shape
It’s incredibly important in ceramics as it works in relation to the clay’s ability to assume a new shape without any tendency to return to the old (elasticity). It’s the ability to knead, shape, push, pull and for the clay to remain in that state. The more workable a clay is, the easier it is to mold into whatever shape you desire.
There are two types of workable clay, one which has more elasticity, the other which has a wider variety of different minerals in it, for example, grog, which will make the clay a little easier to work with. There are three primary groups of clay minerals and each one of them is formed with their own distinct properties. They are kaolin, Illite, and Montmorillonite.
How Is Clay Formed?
Looking a little more in-depth into how clay is formed, UCL states that "clay is the common name for a number of fine-grained earthy materials that become plastic when wet." Clay is a completely naturally occurring mineral and is created through very long periods of the constant weathering and eroding of natural rocks.
Clay is made from feldspar, which is defined as "any group of rock-forming minerals that make up a large proportion of the earth’s crust." It’s no wonder clay is one of the earliest known building materials on earth. Clay rocks can contain up to 40 percent of water within them.
Clay vs. Clay Body
A pure clay (of which there are said to be around 30 types) is one that is not made of any other materials; a clay body is made up of many different clay materials like kaolin or grog. There are hundreds of types of clay bodies, as there are so many different types of combinations of clay minerals. The main three types, which you may well work with are porcelain, stoneware, and earthenware.
Individual clay particles are absolutely tiny and measure in at smaller than 0.004 μm. Flocculation happens when the particles clump together, and deflocculants (usually an alkaline material) are commonly used by potters to disperse these tiny particles repelling them against each other.
Plasticity in Clay
It is easy to find out how plastic a clay is. Make a coil of the clay and then wrap it around your finger. A plastic clay will not crack or break. Non-plastic clays will. The firing process for your clay is extremely important as all clay bodies "mature" at different temperatures. Essentially "mature means fired to the point for which they were formulated." As "when a clay body is vitreous, it means that water cannot be absorbed into the clay when it is fired to maturity."
Ball clays are very plastic as they’re made up of very fine particles, which makes them very sticky. Ball clay is great to shape and very strong, but beware they do shrink a great deal during the firing process.
What Can Affect Clay’s Plasticity?
Small organisms can grow within clay; these organisms help increase a clay's plasticity. Some potters believe that mixing clay with water and aging a clay can help increase its plasticity. It’s best to look at your different types of clay first to assess how plastic you need your ware to be.
It is important not to add too much water to your ceramic ware while you are making it (although it can be tempting if you are throwing on the wheel) and it’s also important to make sure you don’t dry your piece out too quickly. If you do this and the water leaves the clay body too quickly, it can crack.