This is one of the most important processes in the manufacture of pottery; its object is to change the clay or mixture of clays and antiplastics into a homogeneous paste without break in continuity. Pugging is done by hand or machines; the former is peculiar to brick-making by the Flemish method. Besides the ease of working given to clays by pugging, it gives the products a remarkable resistance.
An experiment made by Colonel Gallon of the Engineers shows the influence of this process on resistance.
A brick made with clay once pugged, and another made with the same kind of clay twice pugged, after having been fired under similar conditions were broken under weights of 34 kilos and 64 kilos respectively, applied at the ends, the bricks being placed up on edge and supported in the middle.
Pugging was formerly effected by treading, workmen called treaders trampling on the clay with a sufficient quantity of water in tanks. This primitive process has now been abandoned, except perhaps in the manufacture of certain porcelains.
In the very numerous brickworks where hand-power is used, this is how the pugging is done.
A hoe and a flat wooden shovel (Figs. 52, 53, 54) are the tools used. The latter is slightly concave, and is about 40 centimetres high by 25 broad; it is made of a single piece of wood. The workman attacks the heap of clay with the hoe; he brings down and spreads evenly on the ground a certain amount, then sprinkles it with water, and generally scatters cinders over it. Afterwards he lets fall on this layer another which he waters and covers with cinders like the first. He thus prepares two or three cubic metres to a thickness of 40 or 50 centimetres. This heap is cut for the first time with the hoe, the workman pulling the clay towards him so as to disturb it, and more water is sprinkled on it. The second time the workman beats it with the heel of the hoe; when this beating is completed the shovel comes into use. The workman cuts the heap from top to bottom and throws his shovelful violently to the side to form a new heap. From time to time he dips his shovel in a bucket of water to prevent the clay from sticking to it. This shovelling is followed by another beating; and when these processes have been repeated several times the pugging is complete. The workman arranges his heap in the form of a large round slab, which he smooths over with a well-damped shovel to prevent the surface from getting dry.
Fig. 52. End view of Hoc.
Fig. 53. Side view of Hoe.
Fig. 54. Wooden Shovel.
Pugging is better performed by two men; one of them brings the water and the cinders while the other beats the clay and cuts it with the hoe. Afterwards one of them throws it with the shovel under the hoe of the beater. If the pugged clay is not used at once it is kept from the sun by straw matting.
This mode of preparing the clay is, as we can see, long and troublesome, and it has been rendered easier by the use of pugging machines, which are the same as those we shall describe under the head of mechanical pugging, except that they are worked by hand or horse power.
This is done with special machines called pug-mills, but mills similar to crushing mills and cylinders are also used. Each of these machines produces a special kind of pugging, and one cannot be replaced by another because their effects are different. For clearness' sake we shall subdivide the three classes of pug-mills according to their construction, as follows: -
Pug-mills with knives.
A. With gearing above
b. Worked by steam power.
B. With gearing below.
Pug-mills with cylinders.
Pug-mills with crushing cylinders -
C. With two cylinders.
Generally speaking, these resemble grinding mills, except that the pans are not perforated, and that there is no central sieve. This kind of pugging is, we believe, not very extensively practised. The special arrangement in Fig. 40 shows the application of fluted cylinders to this kind of machine.