NEGATIVES FOR PHOTO-LITHOGRAPHY.
An unconditionally good negative is required for photo-lithography,
and those who have not seen one produced for the purpose by an expert
should endeavour as a first step to do so, or they will scarcely
be likely to devote the attention necessary for producing one of a
thoroughly suitable character. Not only must the lines of the copy
be represented in the negative by clear glass, and the white ground
of the original be represented by opacity, but the utmost sharpness
of definition of the finest dots and lines to the very corners of
the plate is absolutely necessary to render the negative thoroughly
serviceable. To produce such a negative a lens entirely free from
distortion, and capable of giving microscopic definition, is, of
course, a first necessity.[U]
[U] Lenses of the rectilinear or symmetrical class are best.
=The Wet Collodion Process= is better adapted than any other for the
production of suitable negatives, possessing the necessary density
and freedom from fog or veil. The bath must work with the utmost
cleanliness, and the collodion should not be used too soon after
iodising. The developer works better when not quite fresh, and should
contain a sufficiency of acid, and the negative, after fixing, should
be subjected to a vigorous intensification.
Professor Husnik thus comments upon negatives for photo-lithography:—
“As in photo-lithography only subjects in black and white line or
stipple can be reproduced, to the entire exclusion of half-tone, the
negatives should represent the copy by clear lines and opaque spaces.
This is seldom the case except in instances where the drawings are
specially prepared, and the subjects somewhat bold and coarse. For
example, in copying a proof from a fine steel engraving the graduated,
grey, hair-lines, appear in the negative somewhat veiled, whereas the
intermediate spaces between close shading, and which are somewhat
toned in the print, are not developed actually opaque in the negative.
On this account the fine hair-lines will take longer to print in the
copying-frame than the bolder shadow lines. The latter may develop
faultlessly, while the ink may leave the hair-lines. The intermediate
spaces between close shading may not be sufficiently opaque in the
negative, consequently the print is over-exposed in these portions,
and during development remains covered with ink. Such faults must be
overcome as far as possible by shading the negative during printing in
the usual well-known manner.”
=The Colour= and condition of the original copy will greatly influence
the class of negative it is possible to obtain, and if allowable it
will be advisable
=To Bleach= old engravings or other copies, either yellow from age
or stained, by immersing them in eau-de-javelle[V] 10 cc., water
100 cc., wash in water for two hours, and finally in a weak solution
of hyposulphite of soda.
[V] Chloride of lime 1 oz. Water 15 oz.
Carbonate of potash 2 ” Water 5 ”
Mix the two solutions together, boil, and filter
before use.
=Intensifying.=—Having developed a clear, sharp negative, wanting
only in density, it must be intensified to the utmost limit—without
fogging—first with citric acid, pyrogallic acid, and silver. This
solution may be applied either before or after fixing, preferably
the former; if this course is adopted, after washing and fixing, and
again thoroughly washing, the final intensifying operations may be
proceeded with. Opinions vary as to the actual best method to adopt,
but preference is usually given to an intensification by a salt of
mercury or lead; under any circumstances there must be a good deposit
of silver to work upon, or the necessary density cannot be obtained.
In fact, a negative too thin, submitted to mercurial combinations,
particularly the bichloride, will become more bleached and spotty, and
the after application of sulphide of potassium will not again restore
it; the deposit will assume a dark brown appearance, but the spots
arising from the mercurial salts will still be apparent. The two
mercurial salts referred to are—(1) Bichloride of mercury, corrosive
sublimate, a well-known and highly-poisonous substance used in solution
in water, and (2) iodide of mercury, which, used in intensification,
acts more evenly, though with somewhat less energy. This substance,
which is of a beautiful red colour, is insoluble in water, but is
soluble in sublimate, and also in a solution of iodide of potassium. It
is recommended for intensification, and is easily obtained by dropping
into a solution of bichloride of mercury—with agitation—a concentrated
solution of iodide of potassium until the red precipitate as first
formed is again dissolved. This yellow solution may be diluted as
required by an addition of ten times its volume of water. If, instead
of flowing the solution over the negative, a dipping bath be used,
into which the negative is steadily lowered for a few seconds, a more
uniform intensification is likely to ensue. Carefully watch the plate,
and stop the action before it goes too far, or the silver deposit will
lose strength in parts. An important gain in density may be obtained
by exposing to sun or daylight a negative which has been developed and
intensified with pyrogallic acid, fixation taking place after drying.
The negative must be thoroughly washed before exposure to light, and
with ordinary care there is little danger of fogging.
=Illumination of the Copy= should be perfectly even, a front light
being made use of, and direct sunlight if possible. The latter is
seldom admitted direct into the studio, and against outside work wind
and rain are almost fatal objections. Where much copying has to be done
it will be found an advantage, if at certain hours of the day the sun
shines direct upon the studio, to remove all blinds, screens, or other
obstructions, and so allow the rays to fall direct upon the print or
drawing to be copied.
=A Side Light=, particularly if direct sunlight, must be avoided, as
any unevenness or grain of the paper will be exaggerated and rendered
more apparent if lighted in such a manner.
=Under Exposure=, if slight, is a fault much to be preferred to an
error in the opposite direction.
=New Transfer Paper for Photo-lithography.=—In the photographic office,
Calcutta, a new method of preparing the photo-lithographic transfer
paper with arrowroot has quite recently been introduced, which, as
is said, has the advantage of being much cheaper than the method of
coating with gelatine usually adopted, and of giving results quite as
good, if not even better, than by the old proofs. Bank post paper is
coated in the usual way with two coats of the following mixture:—
Arrowroot 140 parts
Bichromate of potash 70 ”
Water 3,500 ”
After exposure to light, the prints are coated with transfer ink in the
press as usual, the transfer ink used being composed of—
Hard re-transfer ink (plate to stone) 100 parts
Lithographic chalk ink 100 ”
Palm oil 7 ”
After inking in the prints are washed off with hot water, hotter than
is required for gelatine transfers. The arrowroot transfer prints are
found to go down well on the zinc without sticking, and give clear,
sharp transfers.
=Gelatine Plates= have hitherto been considered almost useless for
photo-lithography. Messrs. Mawson and Swan, however, have recently
introduced a special make, known as the “Photo-mechanical” plate, which
gives negatives of great density, and at the same time clearness in
the shadows. The makers recommend the ammonia-pyro developer, with
meta-bisulphite of potash. It will be found of considerable advantage
if, before proceeding with the development, the plate be submitted
for one minute to a bath of water 2 ounces, gallic acid 6 grains;
this, without prolonging the development, will add considerably to the
density of the resulting negative.