M.G. MACHINES	245
It may be taken as a general rule that the purer the furnish used the easier
will it be to keep the cylinder clean, and the making of common papers from
refuse containing grit, oil, tar, etc,, is very detrimental to the surface of the
cylinder, as is also the use of bleached pulp from which the bleach residues
have not been properly removed.
It has already been stated that about three-quarters of the cylinder is used
for drying the paper; the other quarter is taken up by the cleaning and polishing
apparatus, which must always be in as accessible a position as possible, in order
to facilitate the changing of doctor blades, wire brushes, etc.
The principal piece of apparatus for keeping the cylinder clean is the ordinary
doctor, preferably a Vickery, and there are usually two of these. One is fitted
in position just before the cylinder comes in contact with the web at the press
and the other one immediately after the paper has been pulled off the cylinder.
This latter doctor removes the end of paper which comes over first, prior to
its being wound on to the reel.
Between these two doctors may be fitted all kinds of cleaning devices to
suit the individual tastes and fancies of the paper-maker.
Another effective burnisher is a stiff wire brush, made by winding carding
cloth round a wooden roll, and nailing it in position. The roll is driven at a
high speed (250 revolutions per minute), the wire bristles cleaning and polishing
the surface of the cylinder.
A wooden doctor covered with a piece of damp felt is also used, and this is
very effective in damping the fluff and dust which dry on to the cylinder, thus
enabling the steel doctor blade to remove them. The felt may be frequently
changed if necessary7.
A 'water' doctor is also very effective; it consists of a perforated pipe, which
forces wet steam against the surface of the cylinder immediately in front of
the iron doctor bkde.
Apart from these devices for keeping the cylinder clean while it is in motion,
it is also necessary periodically to grind it or buff it, or it may be cleaned with
emery powder and oil. For grinding it, a perfectly true iron roll is fitted in
position in place of the press roll, and driven at a higher speed than the cylinder
by means of very tight belts in the presence of water.
This is a risky business, for unless the belts are kept very tight all the time,
the motion of the cylinder will be bumpy and will result in flat places being
formed, which will show on the surface of the paper later.
The best way to cany out this work is to buff the cylinder with proper
buffing apparatus.
When the cylinder has to be stopped for any special cleaning, it should fet
be allowed to cool down, as the dirt is then more easily removed.