be weighed out and mixed in
the 'blungers,' as we call the mixing tanks. Now this body formula, or
clay combination, is not entrusted to the ordinary workman. It is
kept secret. Therefore we have on the trucks that carry the clay
between the bins and the blungers what we call charging-scales, which
weigh automatically each ingredient in the compound without betraying
it to the loader."

"That is pretty clever," replied Theo.

"Yes, it is a very ingenious device," Mr. Marwood agreed. "The
blungers in which the clay is mixed are over there. You can see
them--those great machines near the centre of the floor. They are
heavy steel tanks lined with vitrified brick, and in the middle of
each one is a revolving contrivance, with steel arms and teeth that
grind the clay up very fine and blend it thoroughly. While it is being
mixed in this way water is added to it, and also a certain amount of
powdered oxide of cobalt to whiten it."

"Just as we put blueing in clothes," Theo ventured.

Mr. Marwood assented.

"This cobalt has already been pulverized and sifted most carefully, so
there will be no particles in it, and so it will readily
dissolve. After the clay mixture has had this mauling--for I can call
it nothing else--the blunged compound, or slip, flows in liquid form
into the sifter machines where it is strained through silk gauze or
else a mesh of fine copper wire."

"I shouldn't think you could ever strain such stuff," Theo declared.

"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 fire