"The sifters do get very hard wear," answered Mr. Marwood, "and are the machines most liable to get out of order. They become clogged. Our sifters are self-cleaning. By that I mean they have an attachment which removes the waste obstructing them. Nevertheless, even with this improvement they still bother us at times. If you watch this sifting machine carefully you can see that the method is one of sliding the slip back and forth until it is forced through the straining ducts." "And then what becomes of it?" They walked on and stopped before another machine. "This is a rough agitator," explained Mr. Marwood. "Into it is pumped the liquid slip you just saw strained, and afterward this is brought in contact with a series of horseshoe magnets which extract from the mixture every atom of iron." "Iron?" repeated Theo. "All clay has metals in it," continued Mr. Marwood. "Should you leave any of these in a pottery clay they would cause you much trouble, for when the ware was fired the metals would melt and discolor your porcelain. Sometimes this happens with cheap chinas. I dare say you yourself have seen dishes that are specked with yellow, or have stains here and there. Sometimes you can also detect bluish particles. That means the cobalt has not been properly ground or sifted. In less expensive wares such defects are frequent. But there is no excuse for them when making fine quality porcelains." Theo listened attentively. "After this iron has been extracted," went on Mr. Marwood, "the slip passes into smooth agitators, where it is simply kept well stirred in order that the heavier ingredients in it may not settle to the bottom. Then the liquid is forced by means of a slip-pump into the filter-presses, and it is now that you begin to see an approach to the clay used for shaping dishes. Up to this point the slip has been only a thick creamy substance. Now the filter-press squeezes this through canvas bags until after having been pressed between iron plates you get your cakes of smoothly mixed clay of about the consistency of putty. Each cake is of regulation size, and it is supposed to be an inch-and-a-quarter in thickness, and to weigh forty-two pounds." "The clay is now ready for use?" Theo asked. "Practically so," was Mr. Marwood's reply, "although before it can be sent to the jiggermen to be modeled it must pass through the pug-mill to be made more plastic and workable. It is here that it gets its final kneading, all the air bubbles in it being eliminated by a series of steel knives." "I must say it is pretty thoroughly prepared," smiled Theo.