APPARATUS FOR PRINTING UPON ZINC.
A suitable negative (the subject being in line, _not half tone_) having
been obtained, the next stage toward producing a relief block is to
make from that negative a print in ink upon a sheet of zinc. To do this
a sheet of polished zinc is first of all subjected to the action of a
solution of nitric acid, alum, and water. It is then thoroughly washed,
placed in a machine called a whirler, then coated with a solution of
albumen (white of egg) and bichromate of potash; this coat or film of
bichromated albumen should be dried over a spirit lamp.
The negative is then placed, face uppermost, in a printing-frame, the
coated side of the zinc in contact with the face of the negative. The
negative is now exposed to the action of light.
The exposure being complete, the frame is taken into a room lighted
by yellow light, gas, or lamp-light, the zinc plate is removed, and
its surface covered with a thin film of transfer ink, by means of a
type-printer’s roller; then the zinc plate is immersed in clean cold
water and the image developed by gently rubbing it with a pledget of
cotton-wool.
POLISHING THE ZINC.
Zinc plates suitable for photo-engraving can be obtained either in
large sheets, or cut to size as ordered, and, when new, require
polishing with a damp rag dipped in levigated pumice powder—_i. e._,
pumice powder washed in water, so as to get rid of all grit.
Lay the plate upon a board covered with muslin and polish with a
“from-and-to” the body motion, not circular; do not use a great deal
of pressure, the object being to get the plate highly polished with a
perfectly smooth surface.
Plates that have been printed upon, and are not satisfactory, may be
cleaned and repolished.
Plates that are scratched require polishing with emery cloth. Scratches
interfere with the picture. |40|
To remove scratches, sprinkle the face of the plate with turpentine,
then rub with a piece of FF emery cloth stretched over a carpenter’s
cork-covered rubber. Use the same motion, to-and-fro, until all the
turpentine has evaporated and the surface of the zinc is polished. If
the scratches are removed, the plate is ready for use, but if not,
then the operation must be repeated. If the scratches are too deep for
emery cloth alone, first use a piece of fine pumice stone, followed
by snake stone, pumice powder, rotten-stone, and rouge. Too much care
cannot be taken with the zinc plate at this stage, for, after the block
is etched, if it is discovered that the plate has not been properly
polished, all will have to be done over again, entailing a loss of
time, labor, and materials.
Remember, that no matter what polishing agent is used, the plate must
be polished with a to-and-fro from the body motion, not a circular
motion such as is usual in polishing metal.
GRAINING THE PLATE.
The next operation is to give the plate a slight tooth so that the
sensitive solution will flow evenly over the surface. Do this by
removing all traces of grease.
This graining is done in a wooden tray, 24 inches by 18 by 6, lined
with asphaltum or gutta percha and mounted upon rockers; it is quite
necessary that the graining tray be of large size, as if only a little
larger than the plate the returning wave of acid water will mark the
edges of the zinc.
Into this tray pour a quart of clean water, and add one drachm of
nitric acid and one ounce of a saturated solution of common alum. Place
the plate in this, face up, and commence rocking it at once or else
the acid will mark the surface, and it will require repolishing; rock
slowly for five minutes, during which time the polished surface will
give way to a fine matt, like fine frosted silver. Now remove the plate
and rinse it under the tap, rubbing it gently with a fine sponge, or a
pledget of cotton to remove the scum or deposit formed by the acid. The
surface at this stage should be quite smooth; if it is at all rough,
the acid is too strong, and the solution must be diluted with water; if
the action of the acid is very slow, then a little more must be added;
but it will only be necessary to add more alum when the plate is a
bluish color instead of being a pearly gray.
A drop or two of acid will be required each time plates are to be
grained.
In rubbing with sponge, or a pledget of cotton when under the tap, care
|41| must be taken not to scratch the surface of the zinc, or else it
will need repolishing.
COATING THE PLATE WITH SENSITIVE ALBUMEN.
If the solution of albumen was poured over the plate and drained and
dried, the film would be too thick at the bottom and too thin at the
top; it is, therefore, necessary, to insure an even film of sensitive
albumen, to subject the plate to a quick circular motion, so as to
spread the film of albumen evenly, and to get rid of the surplus
solution. This is effected by placing the grained zinc in the jaws of
an instrument called a whirler and made as follows:
Two pieces of wood, each half an inch thick, twelve inches long, nine
inches wide at one end, and six inches at the other; upon the narrow
end of one piece fix four screws, a piece of iron the shape of a ┻,
the top end of which is shaped to fit into the socket of a carpenter’s
brace (which must have the centre handle revolving, not fixed); the
cross piece must be flat, and pierced with four holes, countersunk to
admit of stout screws.
Place the two pieces of wood together, the ┻ piece between the two,
and with a piece of stout leather hinge the two together, nailing the
leather not only on the outside, but upon the top also; now screw a
narrow batten on each of the pieces, about an inch from the widest end,
to keep the wood from warping.
Four inches from the top (the hinge being the top) bore a hole through
both pieces, and pass through a couple of strong leather laces; by
one end fix these on the outside of the cheek to which the iron ┻ is
screwed, and on the outside of the other cheek, seven inches from the
top, hinge a piece of wood (so as to fall toward the bottom) 5 inches
by 2 by 1/2; and in the centre of the batten screwed on to prevent
warping, fix a broad-headed nail or a turn-button, over which the free
end of the leather laces can be passed.
Now about half an inch from each end drive through the wood, six or
eight wire nails one inch long in a line, and so that the points
project on the inside of each cheek. Next fix the top of the iron into
the socket of the brace and the whirler is ready for use.
To use this instrument, lay it upon the bench, raise the upper jaw,
place the grained zinc behind the teeth (face out) close the jaw,
and after seeing that the face of the zinc is close up to the teeth,
tighten the shoe lace, pass it over the hinged piece (which is lying
down close to the cheek), and around the batten with two or three
turns, then pass it back and tie the loose ends around the iron shank
at the top; then raise the hinged piece, which by straining on the |42|
leather laces will cause the jaws to grip the zinc plate tight and
hold it in position. (See Fig. 4.)
[Illustration: Fig. 4. The Whirler.]
Now lift up the whirler from the bench, grasping the handle of the
brace firmly in the right hand. Hold it at arm’s length, and by a
series of quick jerks set the plate revolving. This will require a
little patience to do evenly and rapidly, but with practice it will
become quite easy.
The plate is coated twice, whirling it after each application of the
sensitive mixture. The whirler is then laid upon the bench, and the
hinge piece let down. This relaxes the strain upon the leather laces,
and allows the jaw to be lifted and the plate removed.
An inverted revolving egg-beater may be made to serve for the same
purpose.
DRYING THE COATED ZINC.
The film of albumen now requires drying, which should be done over a
spirit lamp. The best form of lamp is one of the small pocket spirit
stoves sold by ironmongers, and having the three supports for a kettle
cut away. The zinc is held over the flame and kept in constant motion,
so as to dry the film as quickly and evenly as possible; no fear need
be felt at applying too great a heat so long as the metal can be held
comfortably in the fingers. When dry, the plate is ready for printing
upon.
For drying large plates, two or more of these stoves would be required,
but it is very easy to extemporize a suitable arrangement, by laying a
large pad of cotton saturated with spirits of wine in a tin dish, and
setting it on fire; by this means a stove of any desired dimensions can
be prepared quickly. To extinguish such a flame, cover it with a large
sheet of zinc.
It must be borne in mind that the film on the zinc is now very
sensitive to |43| light, therefore the preparation of the film must be
done in a yellow light, as must also the operation of placing it in the
printing-frame, inking up, and development; gas or lamp light may be
used, as the film is not sensitive to artificial light of a low actinic
power.
THE PRINTING FRAME.
The printing-frames used for this purpose must be of the box pattern,
fitted with stout plate-glass fronts, the cross bars behind being
fitted with wooden screws, instead of springs, as absolute contact can
only be obtained between the surfaces of the glass negative and zinc
plate by means of screw-pressure.
The front glass of the printing-frame must be kept perfectly clean, and
especial care must be taken that no grit be upon either glass, inside
the frame, or on the back of the negative, else the negative will be
broken.
TIMING AND EXPOSURE.
The time of exposure to light is measured by means of an actinometer,
which is simply an instrument in which is a strip of sensitive
paper, exposed under a graduated series of different thicknesses of
translucent paper, each division or gradation being distinguished by a
number, thus: No. 1 has only one thickness of paper over it; No. 2, two
thicknesses, and so on. But on account of the ever varying intensity
of the light, a screen actinometer that is reliable is almost an
impossibility. The best must be closely watched in a bright light, so
that the tints are changed evenly.
Johnson’s actinometer is very simple and handy; it consists of a
cubical box with two lids, the inner one serving to press the paper in
contact with the glass of the outer lid; the outer lid has a circular
opening with a narrow rectangular strip in the centre transparent,
the rest of the glass being covered with pigment the color assumed by
the sensitive paper after exposure to the light. When the strip of
sensitive paper inside has assumed this color, it is called one tint;
the strip of paper is then pulled forward, and another portion, quite
white is brought under the transparent portion of glass, and when the
light has turned that the color of the pigment, that counts two tints,
and so on. The paper used in the actinometer is ordinary albumenized
paper.
The exposure of zinc in the printing-frame under a good line negative,
will be about six or eight tints in diffused light, but in direct
sunlight the exposure may be timed by the watch, a suitable line
negative never requiring more than from three to five minutes. |44|
INKING THE EXPOSED ZINC.
The sensitive zinc having been exposed to light under the negative,
next cover the surface with a thin coating of litho-transfer ink, stone
to stone.
For this purpose we require a type-printer’s composition roller,
mounted upon a litho stock. This roller must be perfectly smooth and
cast in a solid mould, as the line along the side, formed by a split
mould would render it useless for inking up a zinc plate. A roller
eight inches long by five or six inches in diameter is a useful
size. We also require an inking slab, a palette knife, a bottle of
turpentine, with the cork cut so that the turpentine can be sprinkled
out without having to remove the cork, a tin of transfer ink, and a
linen cloth or two.
The inking slab may be made of smooth iron, or a suitable lithographic
stone is good.
|45|