WATER-MARKING 233
to the bulk and water-mark and the elimination of felt and wire marks. Finish
on the machine is a question of moisture, pressure and number of rolls used, and
practical experience and experiment on the part of the machineman. Moisture
content is controlled by the drying operation, and this again by the steam heat
admitted to the drying cylinders. Several systems may be used by the dryer-
man. A certain pressure may be admitted to several or all the cylinders and
controlled by the main*valve; or he may use the cylinders separately, admitting
more or less pressure, according to the feel of the paper, without altering the
main valve. The third and best way is to keep the pressure constant by using
the main valve and to make finer adjustments separately on each cylinder.
The heat of the cylinders should be so graded that the paper gets no sudden
heat, but is gradually raised in temperature as it travels towards the dry end.
It is very difficult to get and keep a regular finish on a paper-making machine.
There must be a certain quantity of moisture left in the paper before it enters
the calenders, and the condition must be the same all across the sheet. Any
unequal pressure of the couch and press rolls, worn and damaged wet or dry
felts, dirty or corroded places on the drying cylinders, unequal spread of the
stuff on the wire or ridges in the latter will affect the moisture content of the
paper and show up in the finish.
A change in the beating from wet to free, or pice versa, however slight,
changes the conditions of the pressing and drying and upsets die finish. Con-
stant attention and good clothing are necessary if the finish is to be kept accurate
and constant to the sample.
Great care is required where using steam-heated rolls, as heat affects the
camber, sometimes expanding the ends of the rolls more than the centres. It
is best to heat the rolls thoroughly before starting the machine.
When the paper is put through, the result will enable the marhineman to
alter the heat, or put on more or less pressure to get a level finish* If the rolls
are not heated in time there will be the greatest difficulty in heating them
afterwards, owing to the cooling effect of the moist paper. As calender rolls
are cambered to a certain pressure, any pressure over or under that point will
cause uneven fitii^ across die sheet. If the finish is not correct with the pressure
intended for the rolls, tiae spread of .tbe stuff on the wire must be slighdy
changed to suit the pressure required* Hms if an unequal spread gives a finish
too high in the centre, by putting down the slice in the centre of the wire
the paper will be slighdy less in substance and will come along the machine
dryer and take less finish. Too much pressure on the calender rolls, even when
well cambered, gives high finish at the sides and low finish in the centre, because
the rolls axe sprung on the centre. This seems incredible with steel rdfe ol
the usual diameter, but can be easily proved to be correct by experiment.