Clays coming from the pit very rarely contain enough water to be worked as they are, especially in summer; it is necessary to moisten them. This operation is called soaking' or moistening according as the clay is rich or thin.
Moistening differs from soaking in this: thin clays absorbing water much more easily than rich ones, do not require to be placed under the water; it is sufficient to sprinkle them when they come from the pit, and leave them to absorb the moisture uniformly, or to accelerate the equal partition of it by mechanical means. Moistening is also suitable for powdered earths which have to be damped in order to be transformed into ceramic products.
The moistening of clays requires much attention, for on it depends their satisfactory manufacture. The amount of moisture which a clay should contain depends upon its nature; nevertheless, we must not forget that, in the mechanical part of manufacture, if the clay is too damp the products lose shape and cannot be handled; if, on the other hand, the clay is not sufficiently damped, the products do not hold together, and fall to pieces. We must then choose a suitable intermediate state so as to get a fairly firm paste resisting manipulation and at the same time giving the maximum production, for the power used in working increases with the hardness of the clay. There is then a happy mean to be found, which only experience can indicate.
The uniformity of damping has of course no less importance than its amount; if there are in the clay some parts softer than others, the products will be defective.
To attain good results, the clay coming from the pit is spread, after being reduced to small fragments by some process or other, in a thin layer, then damped more or less according to the natural moisture it contains, which varies with the season and weather. Another layer will then be spread over the first, and if clays of different nature are used, they will be placed alternately, each layer being sprinkled in turn. The clays will now be left for at least twenty-four hours to become uniformly damped. A longer period will not be injurious if the surface is prevented by a covering from getting dry.
In order to work it, the heap will be cut vertically, so as to make a first mixture of the different layers. The clay will be placed in a pug-mill, either with a shovel, if the heap is near the machine, or by an endless band if it is far off, or by any other means. The clay may be laid in ditches, instead of on the earth.