This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is also composed of fine particles, but much finer than in sand, and possessing different properties. The particles are soft and greasy to the touch when wet, and can be moulded into any form; they also float in water for a long time and make it "muddy"; they retain moisture for a long time, and will not allow it to escape readily. When dry, clay cracks and shrinks; when wet it expands, and becomes very slippery to the foot.
Clayey soil by itself is fit only for making bricks, pottery, etc, the finest chinaware being made of a whitish clay containing silica, alumina, and water. When burned, clay undergoes marvellous changes. It is no longer sticky, plastic, or impervious to water, and its particles are loose, porous, and brittle. Even when wetted, burned clay can never revert to its original plastic and slippery condition. In some places the clay soil is often burned with the object of making it lighter, warmer, and more porous.
The advantage of clay in a garden soil is that it detains moisture and manures, and prevents the temperature from rising too high in summer and from sinking too low in winter, owing to its poor conductive powers.