Soft mud bricks are made under little or no pressure, and are, therefore, not as dense as the stiff mud bricks. It is claimed, however, that in the soft mud bricks the particles adhere more closely, and that when the brick are properly made and burned they are the most durable of all bricks. Soft mud bricks, after having lain in a foundation on the shore of a river for fifty-four years, were found in as perfect condition as when laid. Soft mud bricks are also generally more perfect in shape than stiff mud bricks and better adapted for painting.
Those made by the plunger machine also sometimes contain voids caused by the air which occasionally passes with the loose clay into the pressure chamber, and, being unable to escape, passes out with the clay stream and renders it more or less imperfect.
The manufacture of stiff mud bricks, however, is constantly increasing.
The soft mud bricks take longer to dry, but are more easily burnt.