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.
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