The clay for the plastic process should be thoroughly ground and pugged. The methods of doing this may be any of the methods in use that do not leave the clay in a flaky or laminated condition, but the preferable method is that of the dry and wet pan. The dry pan should have slotted screen plates, with slots of not over one-sixteenth inch in width, and if grit is used it should be ground with the clay. If more than one kind of clay is used they should be ground together. After leaving the dry pan, the clay and grit should be elevated to a screen, either of the revolving or shaking pattern, or the screen may be a fixed, inclined screen. The mesh should be No. 10 and screen made of No. 20 wire. This gives about a one-sixteenth-inch opening between the wires. The preferable screen will be found to be a six or eight-sided revolving screen, with clappers or beaters so arranged that each angle of the screen raises them and allows them to fall on the flat side. This will keep the meshes of the screen clear, and it will do good service. A regular wire screen is much cheaper and will give better service than a perforated steel screen. The tailings from screen should be carried directly back to the dry pan, so as to insure a constant uniform mixture.
The dust from grinding the clay will be a great annoyance and injury to the men and machinery unless some precautions are adopted to prevent its getting into the parts of the building where these are located. A very good plan is to inclose the dry pan, except where the clay is fed into it, and then locate, in the face of this inclosure, a large exhaust fan or disc fan. This will cause a strong draft inward at the feeding point, and will carry all dust into pan. Leaders may be brought from other dusty points, and the one fan made to exhaust the dust from them also. The dust that is expelled by the fan may be carried in a large tube to a convenient place and discharged into a room with muslin sides; or this room may have a very large and low wooden stack leading outdoors. In the first plan the muslin will clog with clay dust, and will not allow the air to pass through, causing some extra labor and annoyance. In the second plan some of the very finest of the dust will escape and be lost, but if the stack is large enough so there is very little draft in it there will be very little lost, and still less if shelves are placed in it occasionally so as to form eddies. The dust which is collected in this way is often found useful for special purposes.
The wet pan is too well known to require description. The only caution I have to give is to obtain a mill with rather narrow-faced wheels. It is the best pugging device for clay out of which glazed material is to be made because it does not tend to laminate the clay, and is, at the same time, a thorough mixer. With a little experience the charges can be all brought to a uniform consistency. The next best device is a good pug mill, not one of the broad-bladed, sharp-angle, quick pug mills, but what might be called an old-fashioned mill, with not too wide blades, say four inches, and set at a very slight angle. The capacity of such a mill may seem small, compared with its size, and the possible extra power it may take, but it will be found to pay in the long run, or at least that has been the writer's experience; but of whatever pattern it may be, it must be a mill that does not laminate the clay that is being used, but must deliver a compact, solid stream, free from air and laminations.
In the wet pan a definite and fixed amount of clay and water can be mixed together every time, and a constant, uniform condition of pugged material be maintained. This is a very important element of success, and is the only true foundation upon which to erect any hope of uniformity in size of finished product
With the pug mill the same degree of certainty may be reached by using a soak pit This method, however, necessitates an extra handling of the clay and takes considerable room if the daily output is at all large. I have seen surprisingly uniform results from the use of a single pug mill, where a constant stream of dry, ground clay and a constant stream of water were admitted. This plan takes the entire attention of one man, and he must be a good one, but as the clay is entirely handled by machinery, it makes a very cheap method of pugging. A second pug mill fed by the first would give the man in charge of the pugging opportunity to correct any errors made in the first mill and would improve the clay.
The stiffness of the pugged clay will, of course, depend entirely upon the subsequent method of manufacture.