Richard Boyt, September 2003
Just how and why does plastic clay change as it dries? I know just enough to know that I don't know very much.
Books tell me that tiny particles of clay are a bit like playing cards. In a loose scattered stack, these platelets interlace and, if lubricated with water, easily slide over each other. This is why clay can take and hold most any shape you give it. This property is generally called "plasticity". Dry the clay by removing the water, and the particles can no longer slide. The drying clay now becomes hard and rigid, and loses its plasticity. Tests I have recently made indicate that clay shrinks quite steadily as it dries. As long as it retains its plasticity, rapid uneven drying does not present a serious problem. However, as the clay loses plasticity, care must be taken to dry it evenly, to prevent shrinkage stresses that can cause fractures in the clay to occur. As long as all portions of the clay remain plastic, you can safely air dry. Drying evenly usually means slow drying. This is particularly important if an object made of clay has great differences in thickness. Outside surfaces tend to dry faster than inside surfaces. Thin sections tend to dry more quickly than thick sections. Top surfaces tend to dry faster than bottoms.
If you want to dry clay evenly, it is hard to beat putting it inside a polyethylene plastic grocery bag with just a bit of an opening at the top, well above the top of the clay. As water evaporates from the surface of the clay, it cools and dampens the air in the bag surrounding the clay. The cool damp air sinks, filling the plastic bag. The slightly open top permits a very slow air exchange with warmer, drier outside air, and so the clay dries and shrinks slowly, evenly, and thus with minimal stress. After the clay has dried enough to loose plasticity, and to change to a lighter color, you can usually finish drying it in the open air.
So just how long should it take to dry a clay shape? It depends upon the size, the shape, and the thickness of the clay object. The temperature and humidity of the air, the openness (porosity) of the clay, the size and position of the hole at the top of the plastic bag, and just how much water the clay holds. How do you know when its dry? The most obvious clue is when it becomes hard enough to resist a fingernail impression, and it changes color, usually becoming a lighter shade. A typical coffee cup shape might safely dry in a day or two. A 3" diameter solid clay ball might take a week or two.
If, in spite of all your efforts, for whatever reason, your clay fails, as long as it hasn't been fired, you can reclaim it by re-mixing with water, and start over.
Note: See other articles by Richard Boyt
- Ceramics for Stoves Part 1- Finding Clay
- Ceramics for Cookstoves 2: Testing Unfired (green) Clay Richard Boyt (May 2003)
- Ceramics for Cookstoves 3A: Test firing local clays- primitive kilns May 2003
- Ceramics for Cookstoves 3b: Test Firing Local Clays- Re-discovery of a "Natural" Kiln (June 2003)
- Ceramic for Stoves 4- Drying Formed Clay Shapes (September 2003)
- Ceramic for Stoves Part 5a- Making Samples for Testing
Practical Tips For Potters Making Improved Cooking Stoves
- Part 1-- Forward and Introduction
- Part 2-- Finding and Selecting the Clay
- Part 3-- Testing the Clay for Cooking Stoves
- Part 4 -- Materials That Can be Added to Make a Better Mixture
- Part 5 -- Making Up and Testing Mixtures, and Clay Preparation
- Part 6 -- Forming Stoves (June 2005)
- Part 7 -- Drying and Firing the Stove (July 2005)