THE LITHOGRAPHIC PRESS
Mechanical Principles--Constructive Details--Scraper--
Tympan--Practical Suggestions--Elastic Bedding.
It is not a little surprising to find that the mechanical principle of
the lithographic press in general use to-day is almost identical with
that which the pioneers of the craft employed so successfully. This is
an interesting fact which either reflects much credit upon the ingenuity
of the early lithographic printers or points to an unreasonable
conservatism on the part of the present-day craftsmen. A discussion of
this phase of the question would be of doubtful interest, for the
practical printer has long been accustomed to regard it simply as a
convenient appliance for the production of a heavy and readily
adjustable pressure.
A brief examination will prove to what extent these requirements are
fulfilled by the modern lithographic press (Fig. 5).
The simplicity of its construction suggests a first point for favourable
criticism. In fact, its general mechanical arrangements are so
exceedingly simple that the merest tyro might readily understand their
principles and purpose.
The adjustability of the pressure by means of the screw D (Fig. 8) is
both effective and necessary, owing to the constantly varying thickness
of the lithographic stones.
The pressure of the boxwood scraper B on the surface of the stone is
perfectly rigid, and yet, owing to the intervention of the tympan C, is
sufficiently elastic to ensure the closest possible contact. Figs. 6 and
7 show one or two constructive details by which the hand lever A and the
cam motion E bring up the cylinder F to the bottom of the carriage or
bed of the press, Fig. 8.
[Illustration: FIG. 5.]
It is in this position that the movement of the carriage gives the
necessary pressure required to pull an impression. The shaft H runs
across the press and operates a similar cam to E on the opposite side.
These two cams raise the brass block G and give the requisite support to
the cylinder F when the pressure is applied.
These are the chief characteristics of the lithographic press, and as
such they require not a little attention and intelligent manipulation.
It is practically impossible to secure a steady and uniform pressure
unless the scraper and tympan are carefully adjusted.
[Illustration: FIGS. 6 and 7.]
The former must be perfectly true with its V-shaped edge nicely rounded,
and the latter tightly stretched on the frame C so that it will not sag
or bulge when pressure is applied and the scraper passes over it. To
reduce the enormous friction caused by this pressure the back of the
tympan is usually dressed with a mixture of tallow and plumbago, a
dressing which requires frequent renewal. The plumbago possesses but
little body, and its salutary effect soon passes away. To prevent this
and to increase its adhesiveness it is sometimes mixed with a little
gum. A mineral black which is found in large quantities in the west of
England is even more effective than plumbago for this purpose. It forms
a strong and flexible dressing for the leather, is peculiarly adhesive
and provides an efficient lubricant.
It is a decided advantage to have two tympans in use, one for small
stones and another for the larger sizes. It is obviously unwise to pull
a number of impressions from small stones with a large tympan, for if
this practice is persisted in the tympan leather not only loses its
shape, but becomes perceptibly thinner on such parts as may have been
most subjected to pressure.
For similar reasons it is advisable to have a number of boxwood scrapers
of different sizes. The "dents" produced by a small stone on a large
scraper can only be removed by planing.
In lithographic press work some form of elastic bedding placed
underneath the stone will not only materially assist the pressure, but
will also minimise the risk of breakages. In fact, the pressure is
frequently so keen and of such a direct character as to render this
arrangement little short of a necessity. Extra thick linoleum will serve
this purpose admirably, and a zinc covering for this bedding will
complete the equipment of the lithographic press.
[Illustration: FIG. 8.]
The operations directly associated with lithographic press work are of
sufficient importance to warrant a full description of each, and will
form the nucleus of the following chapter.