The key E is preferably wedge-shape, as shown.
f are the lower scrapers, which are provided with tenons of half-dovetailed form, and are secured in the shaft in the same manner as the blades C.
In case of accidental breaking of one of the blades the broken blade can easily be removed and another one substituted.
The pug-mill and cylinder inclosing are so placed that the pivot or spindle at the bottom of the mill will be in the centre of the diameter of a semicircular pit which, to contain the clay for three gangs, measures eight feet from the centre of the pug mill shaft to the edge or brick face of the pit, which is four feet deep.
This semicircular pit is usually walled around with brick, which should be hard burned, and the bottom formed of two inch oak planks, cut wedge-shape.
Directly in front of the pug-mill there is a fan-shaped hole or pit, which allows the wheeler to cut the clay away with a spade as it issues from the hole in the side of the cylinder at the bottom, inclosing the mill. If the pug-mill is turned by a horse, it is usual, if the clay bank is too far away to be con veniently filled with wheel-barrows, to harness the animal to a cart, and haul the clay to fill the pit, after the work of grinding has been completed, which usually requires about six hours. A long pole fixed in a yoke in the top of the shaft is the lever age by which the pug-mill shaft and knives are turned.
The pit around the pug-mill, when the clay is ground by horse power, holds usually material sufficient to make seven thousand brick ; after the pit has been filled it is the duty of the temperer to see that sufficient water is let to the clay to soak it.
The clay in the pit is left to soak over night, and in the morning the temperer gets into the pit of mud, and first digs a hole in the clay, next to the pug-mill, throwing the material into the mill.
It is necessary to grind the same clay through the pug-mill several times, the first thing in the morning, before it comes to the proper degree of plasticfty for moulding; this operation is called " sizing the clay." The temperer having secured the proper plasticity, or "size" for the clay, he continues to shovel the mud into the pug-mill, each wheeler of a gang, in turn, spading it away from the bottom of the pug-mill, as it is forced through the orifice at the base of the cylinder.
During this process of tempering, a small stream of water is continually running into a barrel sunk into the ground, near the box of the pug-mill ; if some of the clay is very hard, there is used a large quantity of the water from the barrel ; but if it is soft, only a small quantity of water is used.
Sometimes mud, seemingly almost slush, will be thrown into the pug-mill ; but when it issues at the bottom, it is stiff and firm ; when this is done, the men call it " grinding the water out of the clay ;" but it is really grinding the water into the clay, and thoroughly mixing it.
The work of the temperer for the pug-mill is confined en tirely to the clay in the pit, and he has nothing to do with handling any brick, as is the case with the hand-temperer be fore mentioned.
The pug-mills are sometimes driven by steam-power, instead of being turned by horse ; in case they are driven by steam, there is a large bevel-wheel placed on the top of the pug-mill shaft, which bevel-wheel gears into a smaller pinion on a shaft keyed to a large pulley.
The pulley on the pug-mill of this character is generally about five feet in diameter, and eight inches face ; if the pulley is too small in diameter, the mill is liable at times to clog and stop, leaving the belt either to slip or break.
When a machine of this kind is used for tempering clay, the pit which surrounds it can be enlarged to any reasonable ex tent, to meet the requirements of a yard of almost any size.
The power of the small pinion into the large bevel-gear wheel is usually about one into six ; i. e., the pinion makes six revolutions while the wheel into which it meshes makes but one revolution.
The quantity of thoroughly tempered clay which a mill of this kind will turn out is surprising ; four men constantly shoveling clay into it cannot overstock it, if it is going at any kind of quick speed.
The speed can be regulated so as to travel at any required gait; but when it is intended for fast work, the pushers at the bottom of the pug-mill shaft must be very strong, and securely braced together with one-half inch iron rods.
The temperers who work at the pug-mills must be very able men, and thoroughly understand their business, and the nature of the clay in which they are working.
The tools used for each temperer are one Ames shovel No. 2, one hilling hoe, same as that used by the hand-temperer, and one bucket.
The pug-mill which has been described is the best thing that can be used for tempering brick or terra- cotta clays ; it packs the clay very closely, and the ware made from material tem pered in this manner is very homogeneous.