THE FOURJDRINIER MACHINE 137
in the chest is being made. The pump and overflow may then be changed over
to the second chest.
Some of the older mills are compelled to work the stuff as it is put down
from the beaters, owing to their chests being too small to hold a reasonable
quantity. Then colour, consistency, and quality of the beating may change
with each beater. This occasions a great deal of trouble with 'shades5 and
'weights' in a making, as all these may alter several times in one shift. Where
a refining engine is used, the chest from which it works ought to be about
30' cwt. capacity and kept well filled with stuff. Then, as the beaters are put
down, the change in colour, etc., if it does happen, does so gradually, and the
machineman and colourman can make correction before it gets far off the
sample. In the same way, if a machine is provided with chests that are too
small, by emptying the stuff into one and letting it come gradually through
the connecting pipe to the other from which the stuff is being worked, the
chance of differences is somewhat lessened.
But it is the worst policy for a mill making 'fine' papers from rags and
mixed furnishes to have to do this, for no matter how skilled the beaterman
may be, irregularities in makings (weight and especially shades) must always
follow. Also, as no two beatermen treat their stuff in the same degree as to
length, wetness, and even the quantity of water they let down with the beaters,
there are bound to be at least three upsets in the colour, make, and weight
every 24 hours. Where two chests are used alternately, it is possible to get
more strength from a mixed furnish; for instance, a paper made from J strong
canvas, J cotton seconds, J wood, J broke, and intended for a water-marked
bank, about 13 Ib. Large Post substance.
By putting these fibres into separate beaters, the utmost value in length and
wetness may be got from each. But if they have to be beaten together in
one beater before the strong canvas is milled and properly cleared, the
other and softer fibres will be reduced far below their greatest possible
strength.
Agitators.—There are two types of agitators or stirrers used in chests. The
old type is a gate-shaped contrivance with cross and diagonal bars, which
simply stirs or mixes the stuff by travelling round. The newer and more
efficient is composed of two blades, preferably copper-covered, shaped like the
propellers of a ship and fixed to a driven shaft near the bottom of the chest.
The circulation and movement of the stuff may be observed when colour is
added. Put in at the side of the chest, it travek round in a spiral streak until
it reaches the centre, when it is seen to disappear downwards. Therefore,
besides the travel of the stuff round the chest, it is being thrown up from die
centre bottom to the top sicfes, and from there round, inwards,- atfed down tt>