When starting to work with clay you will have a lot questions. Numerous types of clay are sold with different compositions. How are these clays stored and fired? This excerpt from "Making Pottery You Can Use: Plates that stack, Lids that fit, Spouts that pour and Handles that stay on" answers those initial queries that can stop you form getting started.
Clay is generally classified into three main categories— earthenware, stoneware, and porcelain—each is defined by the temperature it is fired to and the consequent density and strength of the clay.
However, when you look in your clay catalog you will see lists of clays that have a very wide firing range, enabling them to be fired at earthenware 2,012–2,102ºF (1,100–1,150ºC), middle temperature 2,156–2,228ºF (1,180–1,220ºC), or stoneware 2,228–2,372ºF (1,220–1,300ºC). So how do you choose?
CHOOSING A CLAY
Here are some basic questions to ask yourself to help narrow down your options.
Standard red earthenware clay
This choice will be dictated by the firing capability of your kiln. However, be conscious that the higher the temperature, the more expensive it will be, no matter what fuel is used.
Grogged red earthenware, half-covered in clear glaze
Do you want to fire in oxidation (electric) or reduction (gas)? Some clays are more suited to reduction firings than others—use the manufacturer’s catalog to guide you to suitable clays for each type.
Low temperature white body, half transparent-glazed
Base your choice on the surface decoration to be applied. Red bodies will influence glaze color response. If you seek a contrast between clay and glaze color, this can be overcome by applying a light slip background. Alternatively, if you want a vibrant color response, choose a white clay and you will not need to apply a slip as a base. Alternatively, there are clays that have additions that give them a speckled appearance when fired. The speckles react with the glaze to produce a very distinctive surface. Manufacturers often suggest glazes that work well with this type of clay.
Buff stoneware, one quarter transparent-glazed, one quarter tin-glazed
The making method will influence the type of clay you choose. For throwing, use a smooth, plastic clay. Hand-building clays benefit from the addition of some grog, which gives increased resistance to warping and cracking. However, be aware that fine detailing or turning can be compromised as a result, adding yet another factor to be considered.
Smooth white stoneware, half transparent-glazed
Most catalogs have a suitability code to help you decide. It may read something like this:
• Clays good for domestic ware
• Clays suitable for larger domestic ware
• Clays suitable for big pots—bread crocks, etc.
• Clays for general-purpose use— throwing, slabbing, coiling, etc.
• Especially good for throwing
• Especially good for slabbing and hand building
• Clays for larger constructions and tiles needing good warp resistance
Here is an example scenario: “I want to fire to earthenware temperatures, in my electric kiln, using a white body, which will form a great backdrop for color decoration on my handbuilt ware.” From here, you would check the suitability chart and note all the clays that meet your needs. There will probably be fired examples for you to look at
Standard porcelain, half transparent-glazed
Having chosen your clay, you will then need to decide how much to buy. This will depend on your intended output and the way you work. Throwers generally get through clay much quicker than hand builders because the process is faster. In contrast, hand builders can make a few bags of clay last quite a long time, especially if they reclaim all their scraps.
One of the main factors dictating the quantity of your purchase will be storage space. You will find that clay is generally cheaper to buy in large quantities. But, if you have not tested the clay, resist the temptation to do this. First ask for a sample and test it to check that it meets your requirements—then buy in quantities that you can practically store.
Keep clay in tightly sealed bags to retain moisture, preferably in a dark, frost-free place.
For more clay advice and projects, check out: "Making Pottery You Can Use: Plates that stack, Lids that fit, Spouts that pour, Handles that stay on."