SUBSTITUTES FOR LITHOGRAPHIC STONES
Metal Plates--Preparation--Manipulation--Descriptive
Details--Machine Printing--The Printing Bed--Rotary
Printing Machine.
Metal plate, as a substitute for stone, is now such an important factor
in lithography that the printer who wishes to consider himself
thoroughly efficient must possess a fairly comprehensive and practical
knowledge of its manipulation and possibilities.
The prejudice which has hitherto checked the progress of this branch of
lithography was not altogether of an unreasonable character. The plates
themselves were far from reliable, and the difficulties resulting
therefrom were a fruitful source of trouble and expense.
Metal, as a printing surface, is even yet a comparatively new factor in
lithography, and the majority of printers have been working with
lithographic stones from their apprenticeship till the present time. It
is not surprising, therefore, that in relation to the use of stones
almost every possible contingency has been provided for, but with metal
plates a little fresh knowledge must necessarily be acquired before the
workman can claim the same familiarity of manipulation which he may feel
towards the parent process. This is, in fact, the point upon which the
whole question usually turns. Good work can be produced from zinc and
aluminium plates,--of that there is not the slightest doubt,--and it is
equally certain that the advantages offered by their use are of a
substantially practical character. They can be handled with ease and
with absolutely no fear of breaking. They are much less costly than
stone, and require less storage room.
A grain of a finer and sharper texture can be imparted to metal than is
the case with stone, and what is even of greater importance, the
character of such a grain remains unaffected for a considerable time.
Surface inequalities are rarely met with in metal printing surfaces, and
consequently uniform pressure is to a certain extent guaranteed. In
photo-lithography it is possible to make a print from a negative direct
on to the plate (Chap. XVII. page 100). This ensures an original of
exceptional clearness and strength, especially in half-tone subjects.
Although an ordinary zinc plate, which has been carefully polished to
free it from every trace of grease, can be used for lithographic
printing, the best results are obtained from plates which have been
specially prepared. A slight de-polishing with pumice sand and a piece
of felt may impart the requisite "tooth" to the face of the plate, or
the following method may be adopted:--Clean the plate with pumice sand
and felt, and immediately immerse it in a hot bath containing:--
18 oz. Water.
1/4 oz. Alum.
1 dram Nitric acid.
Keep this liquid in constant motion over the face of the plate until it
assumes an even, silvery-grey appearance, and then wash it thoroughly
with a plentiful supply of clean water. Dry at once, and quickly.
These plates can also be sand-grained by specially constructed
machinery, or a variety of grains and stipple can be imparted to their
surface by etching or sand blast.
A novel yet practical idea, which has met with considerable success, is
to electrolytically prepare the surface of the plates. The value of this
preparation has been amply demonstrated by its extensive adoption and
successful use. Another distinctly progressive feature is a deposit of
alumina on the zinc, which for printing purposes gives it all the
advantages of an aluminium plate.
[Illustration: Plate-graining machine, showing oscillating motion.]
Transfers can be made on metal plates in much the same manner as on
lithographic stones. For press work mount the plate on a piece of
cardboard its own size, then, having gummed a sheet of brown paper on
the face of the litho-stone, place the mounted plate near the centre and
fasten it with paste or gum. This will prevent it moving about, and also
raise it sufficiently from the stone to enable the printer to use his
damping-cloth and roller freely. Before mounting, the back of the plate
should be carefully dusted to free it from grit and dirt.
After the drawing or transfer has been made cover the surface of the
plate with strong gum, and while this is still wet add to it a solution
of bichromate of potash. Leave a slight film only of the gum bichromate,
and then dry it quickly and thoroughly. Wash out the work with
turpentine, _not with water_. The gum, being insoluble in turpentine,
remains unaffected.
[Illustration: Plate-graining machine, showing interior of trough.]
Roll the plate up _solid_ in black ink, sponge it over with water, damp
it with a cloth, and then roll it up in the usual way. When the work
becomes fully charged with ink, dust it over with a mixture of resin and
French chalk, and etch it with the acid etching solution.
Every maker of zinc and aluminium plates supplies this special etching
solution, prepared for a certain quality and character of metal. It is
advisable, therefore, to use the preparation recommended, and thus avoid
the many pitfalls which beset the path of the experimentalist.
"The plate is now well washed over with clean water, and _thinly_ gummed
up. When the gum is dry wash out the work on the top of the gum with dry
flannel or felt till all the work is removed, sprinkle a few drops of
water on the plate, and with a rubbing-up rag and a little ink and turps
rub up the work till it is gently charged with ink, then roll up till
work or transfer looks strong and sharp, when the plate is ready for
printing."
For etching and rolling up chalk drawings on grained plates: "Pour in a
saucer some of the etching solution, and add about the same quantity of
fresh, strong gum. This solution is evenly distributed all over the
plate with a camel-hair brush, and left to dry. Then the etching is
removed with water, and the plate very thinly gummed up and left to dry.
Now the work is washed out with turps and a piece of dry, soft felt or
flannel, _without water_, till all the work disappears, and the plate
rolled up solid black. Then sponge over with water-sponge, damp over and
roll up till work appears nice and sharp. Now dust over with resin and
French chalk, and etch again with the etching solution, full strength.
After the etching has dried remove the same with water, and gum up
again.
"When drawings are to be washed out for proofing or printing, the plate
should first be thinly gummed up. This gum layer is left _to dry
thoroughly_, and is not removed when the work is washed out (without
water) with turps and a dry piece of flannel or felt. Next wipe away all
the black ink and turps, still using no water for this operation. Should
any black work be left, use some more turps on the _clean dry rag_ till
all the work has been removed, then sprinkle a few drops of water on the
plate, and use a clean rag to remove the gum layer and loose black ink
left on the plate, then damp over as usual and roll up.
"Plates treated in this way retain the grease contained in the drawing,
and the work rolls up easier and quicker, and none of the finer work
gets weakened or lost; also, the plate keeps cleaner and free from scum.
"When alterations are necessary the work should be rolled up with a
strong black ink, and dusted over with French chalk (powdered talc).
Remove the part which requires altering with a mixture of equal parts
etching solution and turpentine on a small piece of felt or flannel, and
rinse well with water. Pour some special re-preparing solution in a
saucer, and apply this solution with a camel-hair brush to the parts
where the work has been removed; after the solution has remained for a
few seconds rinse well with water. This operation is to be repeated
several times till the surface looks clean, no work being visible; and
after the plate is dried with clean white blotting paper it is ready to
receive the additional work. When the alteration has been made, the
rolling up should be done with the bichromate solution, as per general
directions.
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
"Where the alterations involve additional work only without erasures
roll up the design first with a good, strong ink, and dust over with
French chalk. The special re-preparing solution is used in the same
manner as acetic or citric acid is employed on a litho-stone. For this
purpose we recommend that the special solution should be diluted with an
equal quantity of clean water, and the solution applied with a
camel-hair brush. It should remain on for a few seconds, and then be
quickly rinsed with clean water. This operation may with advantage be
repeated two or three times for securing a clean surface for the
additional work. When the plate has been thoroughly dried with clean
blotting paper it is ready to receive the additional work. The rolling
up should be done with the bichromate solution, as per general
directions."
Machine printing from zinc or aluminium plates requires but a slight
modification of well-known methods; and although it may in some respects
present entirely new features, the making ready of work in which several
printings are employed is simplified, and consequently much time is
saved. In the ordinary type of litho-printing machine the printing bed
(Fig. 22) is levelled in the machine for the first printing, and, if
this is carefully arranged, no alteration will be required throughout
the whole series; for, in changing from one colour to another, the
printing plate can be slipped from the bed and another substituted in a
very few minutes. The most scrupulous care should be exercised at all
times to prevent dirt or grit of any kind insinuating itself between the
surface of the printing bed and the back of the plate.
It is not in connection with the flat-bed printing machine, however,
that the most decided progress is likely to be made in surface printing
from metal plates. "The change which is already foreshadowed in the
printing mechanism of to-day is shown by the growing demand for the
rotary in place of the slow and tedious movements of the flat-bed
press."
The mechanical principle of the rotary machine (Fig. 23) at once
suggests an absolute precision of movement which it is scarcely possible
to guarantee in the flat-bed press. There is no appreciable lift in the
gearing of the cylinders when the impression is made, and practically no
risk whatever of _slogger_, such as that described in Chap. VII. page
38. The uniform velocity of the cylinders, which is to a great extent
due to the points already indicated, considerably reduces friction,
produces perfect registration, and enables the machine to be worked at a
high rate of speed. The machine shown on page 63 presents many novel and
essentially practical features. The side elevation (Fig. 24) gives a
fairly accurate idea of its general mechanical principles.
[Illustration: FIG. 24.]