PHOTO-LITHOGRAPHY BY DIRECT PRINTING ON TO STONE OR ZINC.
The processes of photo-lithography previously described are based upon
the printing on to bichromated gelatine or albumen paper development
with a greasy ink and subsequent transference of the greasy print to
plate or stone. A process may also be used in which the stone or plate
is printed on direct. There are in this various methods, of which the
principal are printing on to light-sensitive asphalt, or an organic
substance, gelatine or albumen, in combination with a bichromate
salt. The reason is the high sensitiveness, which is possible, of the
substances used.
1. THE ASPHALT PROCESS.
(A.) THE LIGHT-SENSITIVE ASPHALT.
Light-sensitive, or so-called Syrian, asphalt, which is prepared for
photo-lithographic and photo-zincographic purposes, can be obtained
from various firms who deal in photographic goods, and one has only to
dissolve the same according to the prescribed directions. This kind of
asphalt is, however, not very sensitive, and in use must be printed
for some considerable time. If, therefore, an asphalt of greater
sensitiveness is desired it is not sufficient to merely dissolve Syrian
asphalt, but it must undergo a preliminary preparation.
Only that part of the asphalt is sensitive or useable for
photo-lithographic work which does not dissolve in ether. The
insensitive part must, therefore, be separated out, which is done by
finely powdering the asphalt, sifting it through a fine sieve, and
then treating it with ether. All that is soluble in ether goes into
solution; the marc which is not soluble in ether is then dissolved in
benzole and used as wanted.
(_a._) HUSNIK’S PROCESS.
Professor Husnik has improved this process in so far that he does not
dissolve the asphalt powder in ether, as, according to his view, a
complete solution of all the insensitive portion is not attained by
this process, but he dissolves coarse asphalt powder completely in
rectified spirit of turpentine to the consistence of a moderately-thick
syrupy mass, until no hard particles or a residue are contained in the
solution. To this viscous asphalt solution a plentiful quantity of
ether is added gradually, and with constant stirring, and a big flask
should be used for this purpose. After a sufficient quantity of ether
has been added a pitch-like deposit {61} separates out at the bottom.
In order to test whether there is still any light-sensitive asphalt
in the supernatant liquor a small quantity is poured into a glass and
mixed with ether, when if a precipitate ensues more ether must be added
to the big flask.
After standing for about twenty-four hours the ether contains all
soluble insensitive constituents of the asphalt; the pitch-like residue
in the flask, after pouring off the solution, is again treated with
ether, so that all turpentine is extracted and any stray insensitive
particles are dissolved. The pitchy residue is now removed from the
flask and allowed to stand in a porcelain dish in a warm place, with
frequent stirring, till it is quite freed from ether, and has formed
a hard, brittle, black shining substance, which can be easily broken
up into powder with the hand. This product is now the light-sensitive
asphalt, which is dissolved for use in anhydrous benzole. As benzole is
only to be obtained anhydrous with difficulty, some chloroform is added
to the solution in order to prevent the running together in drying,
whence the film would become unequal and patchy.
The coating of the stone or plate with the asphalt film is very simple;
it should be placed horizontal, and then whirled rapidly on a whirler.
Printing under a clear negative takes from a quarter to half an hour in
direct sunlight.
The picture is developed with turpentine.
(_b._) VALENTA’S PROCESS.
Valenta recommends for photo-lithography asphalt which has been
sulphurized by the wet process, as it possesses a considerably higher
light-sensitiveness than that which is not sulphurized.[3]
[3] Eder’s “Jahrbuch der Photographie,” 1892, p. 241.
The process is as follows:—100 g. of raw Syrian asphalt are boiled in
a retort with an equal quantity of raw pseudo-cumene, which has the
formula C_{6}H_{3}(CH_{3})_{3}, and a boiling-point of about 170° C.,
with 12 g. of flowers of sulphur, which should have been previously
dissolved in the pseudo-cumene. When after about three or four hours’
boiling the evolution of sulphuretted hydrogen has ceased, the
pseudo-cumene is distilled off and the black pitchy residue dissolved
in benzole in the proportion of 4 : 100, and used for the preparation
of the plate or stone.
The sulphurized asphalt prepared in this way is almost insoluble in
ether, but dissolves fairly readily in benzole, toluene, xylene,
cumene, and turpentine, and is very sensitive to light.
With this asphalt good prints are obtained even in bad, cloudy weather.
For use four parts of the sulphurized asphalt are dissolved in 100
parts of benzole, the solution filtered and diluted till a thin film
on a zinc plate shows a golden yellow colour. Exposure of the asphalt
solution in an open flask for from half to one hour in sunlight is
advisable.
For developing the asphalt image rectified oil of turpentine free {62}
from acid is used; the best is French or Austrian. The development
may be accelerated by adding Hungarian or Russian turpentine, which,
however, attack the asphalt image; it can be restrained by the addition
of ligroin, benzine, or wood oil to the turpentine.
Zinc plates are best developed in a dish, stones by making a wax margin
round the drawing. The oil of turpentine is poured on and rocked to
and fro. Rubbing with a pad of cotton wool, etc., should be avoided.
After development the plate should be well washed under a stream of
water, and before gumming, the plates should be exposed for some time
to light, as this makes the ground more resistant to the etching.
(B.) COATING THE STONES.
The stone or zinc plate is now coated with one of the above-described
asphalt solutions, or with the following:—
Asphalt 20 g.
Chloroform 300 g.
Benzole 100 g.
Oil of Lavender 20 drops.
The solution must be thin, so that it flows over the stone well and
does not form unequally-covered patches.
The subsequent manipulations must be conducted in the dark.
The stone which is to be used must be well ground, and polished with
dry pumice stone, well dusted, and be absolutely flat. It ought not to
have any depressions, as the negative would not lie in contact there.
The places would not print sharp, and therefore a bad result would be
obtained.
The necessary quantity of the solution is now poured into the middle
of a well-polished stone, and distributed as evenly as possible by
rocking backwards and forwards. When thoroughly distributed the stone
should be placed on the whirler and kept for some time in motion.
The asphalt solution must be as thin as possible, so that it appears
yellowish-brown and the colour of the stone appears through it. If the
stone is only covered all over the film can be very thin; on the other
hand, if it is too thick it will not print through, and will develop
badly, as too much of the asphalt will dissolve. If zinc plates are
used for printing they should be prepared as previously described on p.
6. Zinc plates for deep etching must be well ground and well polished,
and ought not to repel water. The zinc plates are coated the same as
the stone.
(C.) PRINTING.
When the film is absolutely dry, which will be in about fifteen
minutes, printing may be proceeded with. For this purpose the negative
is either laid on to the asphalted stone, film side down, and pressed
down with screw bands, or it is laid, the glass downwards, in a
specially-constructed and very strongly-built printing frame, which is
provided with a sheet of plate glass, and the {63} asphalted side of
the stone laid on this, the printing frame closed, and then exposed to
light. The exposure continues according to the strength of the light
and the nature of the negative from 15 minutes to 1½ hours. If the
film of asphalt be thin and the negative clear it may be printed to
18 or 20 degrees in Vogel’s photometer. This kind of printing always
requires some experience, as various factors, such as the thickness of
the asphalt film, the density of the negative, also the fineness of the
drawing, have to be taken into account. It will be understood that the
asphalt film must always be coated as evenly as possible.
If it is printed too long the stone will not develop at all, or only
in parts; if it is printed too little the drawing wholly or partially
dissolves. In both cases an unsatisfactory result will be obtained.
Negatives very clear in the lines of the drawing, and well covered in
the ground, are here more essential for success than with the chrome
gelatine process.
(D.) DEVELOPING AND INKING UP.
When properly printed the frame and the stone should be brought into
the dark room and development proceeded with. For ordinary Syrian
asphalt rectified turpentine can be used, and one of two methods may be
adopted. The stone is laid in a pan as straight as possible, and then
flowed over with turpentine, which should be allowed to remain on it
for some time, and then poured off into the pan. Fresh turpentine is
now applied till the drawing appears clear and distinct. The turpentine
which is allowed to run off can be used for washing purposes. An edging
of warm gutta-percha may also be made round the drawing, and then
turpentine poured on to it and the stone rocked up and down till the
drawing is developed.
The very sensitive kinds of asphalt of Husnik and Valenta are much
more readily soluble than the ordinary Syrian asphalt. In developing
one must go carefully to work, and not allow the turpentine to stay on
the stone a long time, but to pour it over the stone, which should be
inclined so that it can run off. This can be repeated if required. It
can be well and safely developed if half and half of turpentine and
linseed oil is used, which, however, has the disadvantage that the
stone becomes greasy, but this can be taken off by the expert printer.
After developing the stone should be well washed under a strong stream
of water, then allowed to dry in the light, as by this the asphalt film
becomes more capable of standing the etching, and then be coated with
neutral or very slightly acidulated gum. After gumming, the stone is
again dried and allowed to stand for some hours to rest, after which it
can be inked up and etched, and finally sharp etched.
The zinc plate is, after development, also well washed, then allowed
to dry, treated with thin gum solution, then with an {64} etching
solution of tincture of galls or gallic acid and phosphoric acid, and
inked up.
Prints on zinc plates for deep etching are gummed after developing, and
if the asphalt film is perfect without rubbing up with a pad, which
thickens the drawing, etched for some minutes in very dilute nitric
acid.
Many lithographers are accustomed, and indeed it is necessary with some
transfers, to rub the prints up with greasy ink. The process is as
follows:—Greasy transfer ink is diluted with some turpentine, and after
the stone has been gummed and allowed to dry, then washed, the drawing
is wiped over with a soft pad with this dilute ink, when the ink
adheres to the drawing, strengthens it, and if great care is not taken,
thickens it also. With asphalt prints this is absolutely useless, and
for other direct prints, as well also as for good chromated gelatine
prints, in most cases unnecessary. The exposed asphalt combines so
intimately with the stone that strengthening with a greasy ink is quite
unnecessary, is indeed absolutely purposeless, as the ink can neither
penetrate the hard asphalt film nor combine with it. The film of
asphalt alone is so resistant to every etching solution that it is for
this reason absolutely useless to strengthen it.
2. OTHER PROCESSES.
Besides the asphalt other mixtures of other light-sensitive substances
may be used for direct printing, and these are mostly organic
substances in combination with a chromium salt, and possess also a
greater sensitiveness to light than asphalt.
The solutions which have been specially used in our establishment
consist of—
(A). Albumen 5 g.
Ammonium bichromate 3 g.
Distilled water 500 g.
Ammonia 15–20 drops
(B). Chloroform 250 g.
Benzine 50 g.
Alcohol 20 g.
Mastic 2 g.
Aniline red 2 g.
Solution A is strongly sensitive to light, and is used for coating
the stone or zinc plate; with solution B the stone is coated before
developing. The details of the process are as follows:—
(A.) COATING THE PLATES.
After the stone has been carefully polished and well dusted it should
be first flowed over with distilled water, in order to increase the
possibility of spreading the sensitive solution. When the water has run
off, the stone whilst still damp should be flowed over with solution A,
and care should be taken that the same is evenly distributed and that
no bubbles form. The stone should be {65} placed immediately on the
whirler and rotated till the solution is dry. Obviously a thick film
must be avoided, as in such a case the print would not adhere to the
stone.
If a zinc plate be used it must be well prepared and be rendered free
from grease. To prove this the plate is flowed over with distilled
water, and when this spreads equally all over and is not repelled
from any part, the solution can be poured on. If, however, the plate
does not willingly take water everywhere, it must be again cleaned or
polished. The other operations are the same as for the stone.
(B.) THE PRINTING.
When the film is quite dry, printing may be proceeded with. The
duration of printing depends obviously also here again on two important
factors—the quality of the negative and the strength of the light.
Under a good negative with absolute bare glass in the lines of the
drawing I print to about 10° Vogel, for example:—
In direct sunlight (middle of April, 11 a.m.) 2 min.
In the shade (middle of April, 11 a.m.) 5 min.
In electric arc light (3000 c.p., at a distance of
50 cm. = 20 ins. without reflector) 25 min.
The picture is faintly visible. If the negative was too thin, or if
it be over-printed, the image develops badly or not at all. With
under-printing the film washes right away.
Immediately after printing the stone or plate is coated with solution
B, which is distributed as evenly as possible by rocking backwards and
forwards. It is not necessary, however, to obtain even distribution
with a whirler. The whole of the coated surface becomes deep violet.
(C.) THE DEVELOPMENT.
When solution B has become completely dry, the stone is placed in a
suitable vessel with pure water. After about five minutes the places
not affected by light dissolve if the stone is rocked, and it is only
necessary to go over the drawing with a pad of cotton wool to clean
the same completely. The drawing now appears a deep violet on a bright
ground. The development is continued without rubbing strongly with the
pad—which might rub up the drawing—till all the details of the drawing
appear clear and distinct. This especially applies to the details in
the deep shadows.
(D.) FURTHER MANIPULATIONS.
When the stone or plate is correctly developed, which can be easily
told, it should be well rinsed and again exposed to make the film
adhere firmly. It should then be gummed and treated as previously
described in the asphalt process. The “rubbing up” of the stone or
plate is not necessary; it is better to roll up the drawing with an ink
roller. Zinc plates for deep etching can be lightly etched, then dusted
with fine asphalt of resin powder, heated and then further etched. {66}
R. J. Sachers, of Toronto, Canada, published a sensitive solution which
has the following composition:—
Distilled water 150 parts
Fish glue 50 parts
Ammonium bichromate 2 parts
Ammonia 2 parts
Chromic acid 1 part
This solution is brown, not very transparent, with an olive green
tinge, and must stand and settle at least twelve hours before it can be
decanted, filtered, and used. The time of exposure is about—
Direct sunlight 2–5 minutes
Diffused light 12–15 minutes
Electric arc light (3000 c.p., 50 cm.) 30 minutes
The image appears a bright brown on the golden yellow ground of the
plate. The development is effected in an aqueous solution of methyl
violet, when the image assumes a beautiful blue violet colour. The
developed image is hardened with alcohol. The results by this method
are very good, and although this is recommended for half-tone etching,
it is also suitable for photo-lithography.
Excellent results also attend the sensitive “Marine glue solution,”
prepared by A. Hebensperger, of Munich, to which a second solution
called “Chlorogutt-losung” is added. The sensitiveness to light is very
high, and the treatment about the same as with the first described
substance. The “marine glue solution” consists of albumen, bichromate
of soda and ammonia. The dye solution of Victoria blue dissolved in
benzine.
The process published by Waterhouse consists of coating a prepared zinc
plate with a thin chromated solution, and then drying in a few minutes
with a gentle heat—
(A.) Arrowroot 2 parts
Potassium bichromate 9 parts
Water 700 parts
(B.) Alum 1 part
Water 1 part
For use mix—
Solution A 40 parts
Potassium bichromate 5 parts
Solution B 15 parts
This is printed under a reversed negative for about six minutes in
the sun. The plate is then laid for about half an hour in cold water
in order to remove the chromium salts, and then cleansed from all
soluble parts with a pad, and after well washing set up to dry. It
is then inked up with transfer ink, and after about fifteen minutes
washed with turpentine. This process, as, indeed, all direct printing
processes, offers many advantages for preparing colour plates from a
coloured object with one negative. The register of the separate plates
is very exact, which is not always the case with a transfer process. In
reproducing line drawings the unnecessary parts on the stone or plate
can be covered.
{67}