234 MODERN PAPER-MAKING
When an equal finish across the web cannot be obtained, the spread of
the stock on the wire should be looked to. Too much pressure on the coucher
and press rolls will make the edges of the web too dry, because these rolls also
are cambered to a certain pressure and spring in the centre.
A badly worn felt which is most worn in the centre gives a high finish in
the centre and low finish at the edges, owing to the edges pressing the paper
against the cylinders harder on the edges than in the centre, this giving an
unequal moisture content. Constant attention is required to keep the finish
correct; when the exact finish is obtained the attendant should note the position
of the carrying roll over which the web passes after leaving the cylinders. The
spring will be found to tighten up when the paper is too dry, and to slacken
off when it is too moist.
The average position should be noted, and if necessary marked, and the
least change in moisture content will be quickly apparent. Another good
guide is the amount of steam coming from the last dry felt as it leaves the paper.
When altering the speed of the machine or the substance of the paper, the
man in charge should always tell his assistant so that he is not taken unawares,
but can shut off or put on steam beforehand.
Water-Marking.—-The water-marking of paper is carried out in two distinct
ways, depending on whether the paper is hand-made or made on a Fourdrinier
machine.
In the case of hand-made paper, the design is formed on the wire cloth
of the mould by sewing or soldering designs or letters in wire upon it. When
the mould is immersed in the stuff, the wire designs or letters cause a thin place
in the stuff, and this appears as a transparent place in the finished sheet.
A variation of this effect may be achieved by pressing or counter-sinking
a design into the wire cloth, and in this case a thick place is made in the stuff—
i.e., more stuff is deposited there—so that a dark or thick mark appears in the
finished sheet. A combination of thick and thin places may be produced on
the same sheet and very fine effects may thus be obtained.
The water-marking on some currency and bank-note papers made by hand,
and also on the machine, is very beautiful, and requires a high degree of skill
on the part of the wire-workers who make the moulds.
Tfcye second method of water-marking is that used on the machine, and it
differs from the previous one in that the impression is made on the web after
a good deal of the water has been removed and the web has become settled in
plaoe on the wire% It is carried out with the aid of a wire-covered skeleton
dram made of brass and supported in brackets fixed to the wire frame. These
brackets have a fine adjustment screw, so that the drum or dandy roll, as it
is called, usay be very finely adjusted upon the surface of the stuff. The roll