23o MODERN PAPEK-MAKING
and large as can be run on the substance should be employed. Where letters
or devices are on the roll, and require to be spaced for registering accurately
a certain size of sheet, the size of the roll has to be accommodated to the size
of the sheet. The letters or devices, being raised above the surface of the roll
cover, impress themselves on the stuff on the wire, and by making thin places
where they touch, cause the design to be transparent when looking through
the sheet.
This is called the 'water-mark'. The clearness of the water-mark depends
upon a great variety of circumstances, among which the chief are the amount
of water left in the paper after passing the suction box or boxes in front of the
dandy; the quality of the beating; the quality of the fibres and their length;
the weight and diameter of the roll; the thickness of the wire forming the letters
or designs; the speed of the machine; the substance of the paper, and the skill
used in closing up the paper on the wire. Subsequent operations on the machine,
and the finish, also affect the water-mark. The couch rolls and presses all
contribute a little towards taking away the clearness, and so does a very high
finish, which may go so far as nearly to obliterate it altogether.
The proper amount of water to leave in the paper to get a good impression
from the roll is altogether a matter of experiment. The suction is altered until
the desired result is obtained. If too much water is left, the paper is crushed by
the dandy roll; if too little, the roll makes a faint water-mark. Very fine stuff
gives the best results in wove papers. The wires sink into the body of the paper
without encountering any long or hard fibres. Well-fibrillated stock is re-
quired for the good water-marking of thin substances. A good laid mark
requires longer stuff.
The fine fibrillse on the surface take the impression well and long fibres
on the under side help to sustain the weight of die roll and prevent 'pick up'.
Soft, fine fibres such as esparto are easily water-marked. The wires of a design
on a small diameter roll stand at a more obtuse angle from the circumference
than they do on one of greater size. This makes them have a 'digging-up*
action, which is very apt to cause 'pick up'. The figures on a large roll fall
on the wire with more of an impressing motion, and are also supported by a
greater plain area, which helps to press the water through the wire and prevents
it returning to the impression and filling it up again.
Troubk is sometimes experienced from air bells with a wove dandy. A
stream of air or steam, impinging on the roll as it rises from the sheet, stops
dae trouble, but if froth bells are coming along the wire their source must be
found aini stopped; usually more water will stop them. A laid roll with the
laid lines arranged spirally round the circumference is used occasionally for
cheap papers to attain a higher speed, but it makes but a poor water-marL