All about pottery clay

First, clay is not dirt! Don’t get us wrong, dirt is awesome. Without dirt, there would be no celery or soccer fields. Dirt is made from the lovely organic stuff that makes things grow.  Clay is not. Instead, it is a brilliant confluence of alumina, feldspar, and silica. It is the stuff of geology. It is the stuff of history. It is the stuff of streets and palaces, of toilets and teacups. It can be as strong as a bull and as delicate as a butterfly, and, in the right hands, the stuff of the most exciting expressions in contemporary art.

Not all clays are the same. In fact, ceramicists often use the term ‘clay body’ to define a blend of different clays that are mixed to provide specific qualities (think coffee blends).

There are essentially three clay categories manufactured by most clay manufacturers. You can find effective throwing and sculpting clays in each category but they fire to maturation at different temperatures. This difference in temperature makes them categorically different and, as such, one of the most important things to pay attention to as a potter. You can open the doors of possibility while saving yourself some heartache by getting to know your clay.

Low-fire clay bodies, often referred to as earthenware, fire to maturation between ∆06-04 (1830 - 1940° F). They don’t entirely vitrify like stonewares or porcelains, but they do constitute a large, and some would argue the most colorful, part of ceramic history.

High-fire clay matures between ∆8 - 10 (2305 - 2345° F). Traditionally, they are fired in a gas kiln, which allows for the opportunity of reduction firing (without oxygen). High-fire stoneware and porcelain clay bodies are strong and durable at vitrification. Mind you, these are volcanic temperatures! 

It used to be that potters had to choose between the exciting color palette of low fire and the durability of high fire. Those days are mostly over, though, because mid-range clay bodies hit the sweet spot. Mid-range clay fires between ∆4 - 6 (2167 - 2232° F) and provides the durability that earthenware lacks without sacrificing access to colorful glaze options.

Read more in The Types of Pottery Clay and What They are Used For