306 MODERN PAPER-MAKING
weights and lifted apart, so as to prevent the line of pressure producing a
flat place.
The newer type of calenders have a lifting device for that purpose. They
must be washed frequently with warm water and soap, or a little soda, to clean
the paper rolls from dust, grit, and resin or gelatine size. Care must be taken
not to use too much heat in the iron rolls while this is being done, otherwise
they will take the top skin off the paper rolls.
The latter being of a softer nature, any hard substance or wrinkle in the
paper is liable to make a bruise or an impression on their surface. For this
reason, extra precautions are necessary when working on tub-sized papers;
any 'slap' or defect on the deckle edge should be marked with a ticket at the
drying machine and damper, and watched for at the calender, as a folded,
wrinkled or doubled corner will make a mark in the paper rolls. Hard knots
of rag fibre, "rolls' from the machine apron and ragged deckle edges are to
be avoided.
In cheaper papers, the greatest danger lies in particles of metal or grit which
become embedded in the rolls and may pass from one roll to another, making
so many impressions that the marks become continuous all round the rolls.
This is the case when there are no doctor blades on the iron rolls. Many
calenders have none except on the bottom roll. A long run on a narrow width
web is bad for the paper rolls, as it upsets the camber, the pressure being heaviest
on the width covered by the paper. After such a run, the rolls should be cleaned
and run as long as possible with frequent applications of hot water and with
no weight on.
This will cause the compressed part to swell out to its normal size. Bruises
and imprints, if not too deep, may be cured by blowing a steam jet on the
damaged place or by applying cloths saturated with hot water. Any deep cut
or mark may be improved by a judicious touch up with very fine emery or
gkss paper and afterwards washing it with hot water. This is a temporary
repair and only lessens the mark made on die paper, the proper remedy being
to have the roll rebuffed.
This is necessary in any case at longer or shorter periods, according to the
hardness of the rolls and the conditions they show after a certain time.
As an additional damping arrangement, it has been found of great assistance
to pass the paper through a cloud of steam and over a felt-covered roll before
it enters the first roll. Great heat and pressure give a high but not a permanent
glaze; therefore, for good class papers, it is better to put die paper twice through
the rolls with less heat and weight. When finishing tinted papers, pressure and
heat should be kept very regular from reel to reel, as a great many 'shades'
may be caused by varying finish-