THE PHOTOGRAPHIC PROCESS.
1. THE STUDIO.
The arrangement of the reproduction studio is essentially different
to that for ordinary portrait work. The general points of such
arrangements are described in detail in the handbooks of Drs. Eder and
Vogel, and these I may therefore omit so far as nothing novel is to be
observed.
We distinguish now between daylight and artificial light studios;
further, those in which a camera is used for making the negative and
those in which a dark-room itself is the camera. The first will, of
course, be used where other things besides reproductions have to be
made; the arrangement without a camera presents many advantages for
reproduction work only. In the arrangement with artificial light the
illumination of the object to be taken is effected as a rule with a
source of light which can approximately replace daylight, and which
also remains as constant as possible, and the electric light is at
present the best. Although other sources of light are sometimes
used, the electric light in the form of the arc light for continuous
practical use has the advantage. For copying oil paintings daylight is
to be preferred under all conditions, and for this the best arrangement
is the revolving studio.[1]
[1] See Eder’s “Jahrbuch für Photographie,” 1893, p. 231.
The description of a modern studio with electric light as used in the
K.u.K. Militar-geographischen Institut in Vienna, and from which the
studio of the K.K. Hof- und Staatsdruckerei was copied, is given here.
This studio lies seven metres below the level of the street on the
south front of the building, and is shown in Fig. 2.
The preparation and dark rooms are illuminated with white, yellow, and
red light by means of two 50-c.p. incandescent electric lamps for each
colour, and besides this are provided with window screens of glass of
the same colours for daylight.
The windows open into an area 50 cm. wide which runs round the
building. The lighting is so arranged here that besides the ordinary
collodion plates, very sensitive gelatine plates can be worked. The
room C, where the original is placed, is fitted up {31} with four
Franzen arc lamps of 3,000 c.p. each for illuminating the original,
and the lamps are so arranged that the light falls centrally on the
original. The four lamps are fastened by ball
[Illustration: Fig. 2.
A is the room for the preparation of the plates, for the wet
collodion plates, and the silver bath.
B the developing room.
C is the room in which the original is placed, where is found a
support for the original TT′, as is shown in Figs. 3 and 4.
D is the dark-room with the focussing table EE′ (Figs. 2 and 3), and
is separated from C by a wall of 15 cm. thickness. In this wall is
found the photographic lens in a metal flange built into a stout iron
box.
F is the washing and polishing room for the glass plates.]
and socket arms to an iron frame which rests on rollers; they can be
raised or lowered on the frame, and for taking small objects can be
pushed closer together. The arms are fastened to the round pillars of
the frame, being provided with a screw grip. The lamps can be placed as
close as 0·5 m. to the original. As a rule, however, they are worked at
a distance of one metre, as then the intensity of the light is about
equal to diffused daylight. The axes of the carbons in the lamps are
so arranged that the glowing crater formed in the positive pole is
turned to the original, by which means the illumination is intensified.
The positive carbons have a diameter of 20 mm., the negative carbons
8 mm. The lamps stand in pairs one above the other at LL′. In the two
upper lamps the positive carbon is at the top and the negative carbon
below it, so that the light is equally distributed over the whole of
the subject. With this arrangement of the lamps all reflections are
avoided, and neither the grain of rough drawing paper, the relief of
an engraving, nor the edges of pieces stuck on are felt. The current
is produced by a dynamo in the house; it enters into the place at N,
whence it is divided into two circuits of 20 ampères, in which are two
switches, an ammeter and a rheostat, and the two lamps on either side.
The wall in which the objective O (Figs. 3 and 4) is placed, as also
the brick socle _aa_ and _ee_ (Fig. 2), on which the support for
{32}
[Illustration: Fig. 3.]
{33}
[Illustration: Fig. 4.]
{34}
the original and the focussing screen rest, run on rails, and are
absolutely isolated from the brickwork of the building in order to
avoid any possible vibrations.
The stand for the original TT′ is provided with screw fittings, cams
and wheels, which make it possible to raise the original, to lower
it, to move from right to left, and _vice-versâ_, vertically and
horizontally, so as to place its centre axial with the optical axis of
the lens, as well as parallel to the focussing screen in the dark room.
The whole of the mechanism lies at the back of the stand, so that there
is absolutely nothing in front of the original.
The focussing table EE′ is constructed in a similar manner to the stand
for the original, and is movable in every direction in the same way. It
carries in front two wood clips, in which the board with the original
is placed. The table for the original, like the focussing table, is
constructed on rollers, which run on the already-mentioned rails. In
order to be able to fix these as soon as a sharp focus is obtained a
brake is fitted. The placing of the original table at the distance from
the lens is accommodated according to the size in which it is to be
reproduced, and has already been estimated. There remains, therefore,
only to trouble about the fine focussing, which is very quickly
effected.
For taking line or wash drawings and for enlargements up to 80 × 80 cm.
a Steinheil wide-angle aplanat is used, which gives no distortion. The
duration of exposure is with such subjects from eight to ten minutes.
For smaller subjects, and principally for photographs on wood, an
orthoscope by Voightländer is used, and the exposure varies from four
to six minutes.
A Zeiss anastigmat is used for making autotypes. Duration of exposure
from three to five minutes. Coloured objects, oil paintings, etc., are
taken in the daylight studio. They are printed in the daytime in the
open air, and in bad weather or under pressing circumstances in the
night by the electric light.
2. THE GENERAL QUALITIES OF NEGATIVES FOR PHOTO-LITHOGRAPHY.
The negative for photo-lithographic work, for whatever method it may
be used, must before all things possess two principal qualities; when
looked through it should be as clear and as clean as possible, on
the other hand the ground as well covered as possible. The deposit
must not be of a black colour; from a well-drawn original absolutely
satisfactory negatives can be prepared without the black colour. When
a drawing is reduced which contains grey lines, dots, and points,
as well as full black ones, toned lines will be visible as well as
the transparent; with careful treatment, the negative may be so far
corrected that it may be used. If this is not effected, or is not
possible, the retouching on the stone will be somewhat troublesome.
The toned lines print later than the transparent; these will therefore
be overprinted {35} when the former have scarcely reached the correct
degree of printing. The choice is then only left either to weaken the
lines which are too strong or to draw afterwards the faint or not
printed lines.
If the ground is not sufficiently opaque it will print through. This
can be remedied by intensifying the negative, or when this is not
possible to paint over it as well as possible.
Transparent spots are formed on the negative by an impure silver
bath or by dust flying about the room. These must be spotted out if
increased work later on on the stone or zinc plate is not desired.
In reproducing fine copper engravings, it generally happens that the
fine grey hair-like lines and the light interstices, if the plate was
not properly polished, appear less transparent in the negative than
the other parts of the picture. In developing the print the result is
a partly broken image which can only be retouched with considerable
trouble. The negative can be corrected by taking care to paint over the
transparent parts. Professor Husnik recommends, when the picture is not
too large, to fasten a transparent paper on the back of the negative,
and to cover over the glassy places with a soft lead pencil. These
parts then print simultaneously with the hair-like line and develop
also equally. If, however, the negative is large, and the picture very
complicated, a glass positive picture is prepared in the following
manner: A polished sheet of glass is levelled and coated with the
following solution:—
Gelatine 6 parts by weight.
Sugar 1 part by weight.
Ammonium bichromate 1 part by weight.
Water 80 parts by weight.
When the solution has dried the glass plate is laid on the negative
and exposed till the deepest shadows have printed, and it is then
developed in water. When dry this glass positive is laid on the back of
the negative so that every part of each picture fits. The plates are
then bound round the edges with paper strips so that they cannot shift.
By this—although somewhat troublesome—operation can an unsatisfactory
negative be rendered more even.
This correction can also be effected in the following manner: The back
of the negative is coated with raw collodion which is faintly coloured
with aniline red, eosine or fuchsine, allowed to dry, and then with an
engraving needle or a narrow knife the less transparent lines may be
scraped out, so that the red collodion only remains on the transparent
places. Obviously also this can be done in the reverse, by painting a
faint red on the transparent places with a brush.
By this last method a tolerably extensive retouching may be {36}
effected on unequal negatives, which are the result of not quite
equally black or also very close drawn originals, from subjects
with very fine lines, such as copper-plate engravings, or from
finely-executed lithographs with very great reduction. Finally it
should be mentioned that with intelligent retouching any bad negative
can be used for photo-lithography, and the only question now is
whether it is not better to take the shortest way and prepare a
suitable negative when the original really permits of its being done.
In printing on chromated gelatine paper a good result can be easier
attained from somewhat foggy or thin negatives than by printing with
asphalt or other light sensitive substance.
The following kinds of negatives are used for the various
photo-lithographic processes:—
For the transfer process with chromated gelatine paper or direct
printing on stone or zinc direct, a reversed line or autotype negative.
For Orell and Füssli’s process, a reversed ordinary half-tone negative.
For Bartös’ process and for photogravure on stone, a direct ordinary
half-tone negative.
3. PREPARATION OF THE DIRECT GLASS NEGATIVE.
The preparation and cleaning of the plate glass is as follows:—First
of all the plates are roughened with a piece of sharp pumice stone on
the extreme edges to the breadth of about 0·5 cm. in order to prevent
the frilling or floating off of the collodion film. This is very little
trouble considering the advantage it offers. After being edged the
plates are placed in a solution of
Nitric acid chemically pure 1 part
Water 10 parts
Instead of nitric acid, chromic acid may be used, but this is not
absolutely necessary.
After one or two hours the plates are taken out of the acid bath, well
washed under a strong water rose, and dried in a place free from dust.
Then the plates are well rubbed with 1 : 10 tincture of iodine solution
with a pad of clean filter paper and then polished with 50 per cent.
alcohol. This preparation must be done with absolute accuracy, and thus
they make all substrata of albumen, india-rubber solution, etc., for
direct negatives unnecessary.
Plates which have been previously used are placed in pure water till
the collodion film has become loose, and are then scrubbed with a stiff
brush, treated for some hours to a bath of caustic soda, and then
treated as above, when they may be used again. {37}
For reproduction of simple line drawings iodized collodion is used
which consists of 2 per cent. raw collodion composed of—
Ether 1,000 ccm.
Absolute alcohol 1,000 ccm.
Pyroxylin 40 g.
Iodizing is done with
Sodium iodide 15 g.
Cadmium iodide 15 g.
Cadmium bromide 8 g.
Potassium iodide 1 g.
The last salt is dissolved in a few drops of distilled water, the other
salts are dissolved in one part of the absolute alcohol. The iodizing
solution is filtered into the raw collodion, which has already been
allowed to settle and filtered, and allowed to stand for some days.
The sensitizing solution is nitrate of silver in the proportion of
1 : 10, with larger sizes 1 to 12–1 to 14, and the silver bath is
faintly acidulated with chemically pure nitric acid.
For a monochrome subject the exposure lasts according to the size of
the reproduction, and the rapidity of the objective, from three to ten
minutes.
For development an iron developer is used, consisting of:—
Water 4000 ccm.
Ferrous sulphate 150 g.
Copper sulphate 75 g.
Alcohol (40 per cent.) 250 ccm.
Glacial acetic acid 125 ccm.
The iron developer, with addition of sulphate of copper, is to be
preferred for short exposures to iron and lead developer. If the
plate has been correctly exposed the image gradually appears after
a short time, and very soon gets vigorous. When the details appear
in the deepest shadows the development must be immediately stopped,
otherwise fogging will ensue. If the image does not gain vigour by long
development it was under-exposed; with over-exposure the image appears
quickly and remains thin. The developer, when fresh, works fairly
quickly; older developers work slower, but give very clear negatives.
For very fine line drawings or reproductions of copper-plate engravings
the developer is used somewhat weaker, and one-fifth of its volume of
water added. The development takes somewhat longer; the fine lines
remain, however, very clear.
After developing the plate is well washed and then intensified first
with a solution of—
Water 5000 ccm.
Ferrous sulphate 180 g.
Citric acid 90 g.
{38}
to which a few drops of a 1 : 12 silver nitrate solution have been
added. Care should be taken that in the first intensification the
glassy places remain clear. When, therefore, the desired opacity
is attained the plate should be immediately washed. Too long
intensification fogs the finer lines.
The plate should be fixed with 5 per cent. solution of potassium
cyanide. For this, however, a concentrated solution of hyposulphite of
soda can be used.
After well washing, the negative will show, when examined by
transmitted light, whether the ground is opaque enough, or whether
a further intensification is necessary. With correct manipulation
this first intensification will be sufficient for fine, well-prepared
drawings.
If, however, the opacity proves to be too little, after thoroughly well
washing, the second intensification or blackening is effected with a
concentrated solution of mercuric chloride in distilled water, which is
allowed to act till there is a distinct precipitate and the film has
become grey. After well washing the plate is flowed over with solution
of thiosulphate of gold till the grey quicksilver precipitate has
become absolutely black. The gold solution consists of—
(A.) Distilled water 1000 parts
Chloride of gold 1 part
(B.) Distilled water 1000 parts
Hypo-sulphite of soda 10 parts
Immediately before use solution A is added with constant stirring to
solution B, and a few drops of ammonia added. If the desired blackening
is not effected with one application the operation must be completed.
The solution of gold, when kept in the dark, will keep a long time.
For fixed negative the following intensifiers have been used with good
results:—
1. Intensifier with pyrogallol.
(_a._) Silver nitrate 5 parts by weight
Distilled water 75 parts by weight
To this solution is added—
(_b._) Citric acid 5 parts by weight
Distilled water 75 parts by weight
(_c._) Pyrogallol 1·25 parts by weight
Distilled water 150 parts by weight
2. The hydrochinone intensifier recommended by Baron Hübl:—
Hydrochinone 10 parts
Water 1000 parts
To this as much acid is added as will keep the solution clear for {39}
five minutes, when it is mixed with a third of its quantity of 1 : 30
solution of silver nitrate. The addition of a few drops of sulphuric
acid or 5 parts of citric acid is generally enough. This intensifier
acts somewhat more delicately than the silver and iron solution,
and has the advantage over the pyrogallol that it can be applied as
frequently as desired in full daylight to the fixed negatives without
any danger.
3. The metol intensifier.
The negative should be developed with the above-mentioned iron
solution:—
(A.) Metol 15 parts
Citric acid 10 parts
Distilled water 1000 parts
(B.) Silver nitrate 10 parts
Distilled water 100 parts
The negative must be fixed before intensification with hypo and well
washed.
A small quantity of solution A is poured over the negative to drive off
the water, and then it is intensified in the ordinary way with 10 parts
of solution A and 1 part of solution B.
Further intensification is effected, as described, with mercury, the
blackening with thiosulphate of gold.
When the negative is finished and dried it is coated with a solution of
gelatine 1 : 50, or a 1 : 15 solution of gum, in order to protect the
film, and the negative should be levelled and allowed to dry.
Good opacity in the ground and clear glass in the lines of the drawing
are the fundamental requirements of a negative for photo-lithography.
But drawings are not always prepared in such a manner that this can be
completely obtained. With flat drawings which are drawn in grey instead
of black the first intensification is only continued till the finest
lines appear clear. For further intensification the negative after
drying is given a coating of a light sensitive solution of—
Gum arabic 17·5 parts by weight
White sugar 17·5 parts by weight
Potassium bichromate 5·8 parts by weight
Glycerine 1·8 parts by weight
Water 35 parts by weight
Ammonia is now added till a light colour is formed. After drying it is
exposed from the glass side to 15° Vogel’s photometer, washed for about
a quarter of an hour and then soaked in warm water till a relief is
formed. By this operation a transparent relief is formed on the lines
of the drawing, which absolutely protects the same from fogging. The
intensification is then effected with mercuric chloride and chloride of
gold, or an intensifier of {40} solutions of uranium and ferridcyanide
is used, which is composed of two solutions—
(A.) Uranium nitrate 8 parts by weight
Sugar 8 parts by weight
Water 350 parts by weight
(B.) Potassium ferridcyanide 8 parts by weight
Sugar 8 parts by weight
Water 350 parts by weight
The negative is first flooded with the uranium solution and then with
the red prussiate.
Another process of intensification recommended by Prof. Eder and Toth
is the so-called lead intensifier.
The somewhat under-exposed negative ought only to be developed with
iron, fixed with hypo, and must then be well washed. It is finally well
rinsed with distilled water, and then placed in a filtered solution of—
Lead nitrate 4 parts by weight
Potassium ferridcyanide 6 parts by weight
Distilled water 100 parts by weight
After a few minutes it becomes absolutely opaque in the covered parts.
The negative is allowed to remain in the bath till it has attained
the desired density. On the silver molecules of the image a pale
yellow precipitate has formed by the action of the lead bath. The
ferridcyanide of potash is reduced by the silver to yellow ferrocyanide
of potash, and gives then, with the lead salt, an insoluble compound,
ferrocyanide of lead. By the action of ammonium sulphide the unstable
lead salt is converted into a stable salt.
As soon as the negative has been taken from the lead bath and well
washed, till the washing water no longer gives a blue precipitate, it
is flowed over with dilute ammonium sulphide 1 : 5. Sodium sulphydrate
may also be used. The image becomes black instantaneously. The negative
becomes generally vigorous; if, however, it was exposed too long the
fine hair lines and points veil over very easily.
If the negative is not sufficiently intensified, which may happen with
very much under-exposed negatives, it should after well washing, and
before treating with ammonium sulphydrate, be laid in a bath of—
Cadmium sulphate 10 parts
Distilled water 100 parts
in which it becomes still whiter, and should then be treated with the
ammonium sulphide. The lead, the cadmium, and the silver of the bath
are converted by the ammonium sulphydrate into sulphide. The negatives
treated with cadmium have a yellowish tinge, whilst those treated with
lead are pure black. For {41} clearing up the white-lead image when it
is too vigorous or is foggy hyposulphite of soda or a dilute solution
of potassium cyanide can be used. This manipulation, however, requires
great care. The negative treated with ammonium sulphide if foggy can
be cleared up with hydrochloric acid, used weak and repeatedly poured
on and off, with careful watching of the action. After this it must be
well washed.
The lead solution will remain clear for weeks, and if it begins to
work slowly can be freshened up with addition of lead nitrate and
ferridcyanide of potash.
If stripping negatives are to be prepared, it is advisable, with the
lead intensified negatives, to give the glass plate a preliminary rub
with vaseline oil, to well wash, and then to coat with raw collodion.
Husnik’s graphite intensification is still to be mentioned, which, in
some cases, may be of great service, but requires practice and care.
The expert photo-mechanical worker will adopt one or the other methods
for his work, and as soon as he can work safely and without hesitation
he will adhere to that.
4. THE PREPARATION OF AUTOTYPE NEGATIVES OF BLACK AND COLOURED DRAWINGS.
As regards the preparation of autotype negatives from simple black
originals—wash drawings—the above photographic processes are quite
sufficient. For taking painted originals—such as oil paintings,
water-colour drawings, fresco, and pastell drawings—an orthochromatic
collodion must be used if a correct rendering of the tones in the
negative is to be obtained.
In our studio we have obtained excellent results with two kinds of
colour sensitive collodion. These are the isochromatic collodion
emulsion prepared by Dr. E. Albert and that suggested by Major von
Hübl. In using the former, the glass plates as soon as they are taken
out of the acid bath and well washed must be given a substratum of the
following solution:—
White gelatine 5 parts
Water 500 parts
to which is added—
Glacial acetic acid 15 parts
Alcohol 10 parts
The solution should be filtered and coated whilst warm, and the plate
dried in a room free from dust at least 15° R.
For the production of the sensitiveness, or for the production of the
correct reproduction of the colours, two eoside of silver dyes, R. and
P., are used. The P. dye gives great sensitiveness, and is especially
suitable for portraits, landscapes, etc. In using this a {42} correct
reproduction of the red must be abandoned, whilst the R. dye gives
an absolutely correct colour rendering but requires about double the
exposure. Ten ccm. of the eoside of silver solution are added to 100
ccm. of the emulsion. When coloured the emulsion will only keep for one
or two days.
The time of exposure with the R. dye is normally from one-and-a-half to
three minutes.
After exposure the plate must be well washed under a tap and then
allowed to thoroughly drain, or otherwise developing streaks will show.
Hydrochinone developer is used for developing, consisting of three
solutions:—
(A.) Distilled water 500 parts
Sodium sulphite 200 parts
Potassium carbonate (pure) 200 parts
(B.) Hydrochinone 25 parts
Alcohol (96%) 100 parts
(C.) Ammonium bromide 25 parts
Distilled water 100 parts
This developing liquid is concentrated, and is used as follows:—
Solution A. 100 parts
Solution B. 5 parts
Solution C. 7 parts
To obtain hard negatives solution B is increased from 1 to 5 times. A
mixture of 150 parts of the above solution with 1000 parts of distilled
water forms the actual developer. By the addition of more concentrated
developer or increasing the quantity of water the developer may
be suited to the character of the subject. The developer must be
freely flowed over the plate, and then the plate fixed with sodium
hyposulphite. Intensification is effected with—
(A.) Pyrogallol 7 g.
Citric acid 7 g.
Distilled water 1500 ccm.
and when dissolved
Glacial acetic acid 25 drops
are added.
(B.) Silver nitrate 10 g.
Distilled water 100 ccm.
Immediately before use 100 parts of solution A are mixed with 5 parts
of solution B, and the intensification continued till the desired
density is obtained. Negatives can be reduced by placing them whilst
damp in a solution of—
Potassium permanganate 1 part
Distilled water 300 parts
When this has acted sufficiently, the plate is well washed, {43}
flowed over with a five per cent. solution of potassium cyanide, and
then well washed. When the dyed emulsion is used this must be done,
after developing, in red light.
A very excellent method of preparing an orthochromatic collodion
emulsion has been described by Hübl,[2] which can be used either wet
or dry. Repeated trials made in the Imperial Military Geographical
Institute, in our establishment, and in the Photographic Institute
in Vienna have proved the advantageous use of this method for
orthochromatic work. The raw emulsion, according to Hübl, is prepared
as follows:—40 g. of silver nitrate are dissolved in 50 ccm. of warm
distilled water, and as much ammonia added as will give a clear
solution; then 100 ccm. of alcohol are added and the whole allowed to
cool. 30 g. of ammonium bromide are dissolved in 35 ccm. of water and
70 ccm. of absolute alcohol with gentle heat.
[2] “Encyclopædie der Photographie,” Heft 3, and Eder’s “Jahrbuch,”
1892, p. 387.
To make the emulsion 450 ccm. of four per cent. raw collodion are
placed in a strong glass flask which will hold a litre, and to this in
the dark room the silver solution added. Some of the wool separates
out, but by vigorous shaking this can again be dissolved. A part of
the silver salt also remains suspended in the liquid in the form of
fine crystals. The ammonium bromide is added in three or four parts
whilst warm. After shaking thoroughly for several minutes the emulsion
is precipitated by distilled water, which is repeatedly added till the
separation is complete. The emulsion, which is now powdery, is placed
on a linen filter, washed several times with distilled water, squeezed
to press out the last of the water, damped two or three times with
alcohol, and then pressed. Whilst still damp with alcohol the emulsion
is dissolved in from 800 to 1,000 ccm. of alcohol and ether, mixed with
0·5 grs. of codeine, and allowed to rest for some days, when it will be
ready for use.
The preparation of the eoside of silver solution is effected as
follows:—10 g. of yellow-shade eosine are dissolved in 250 ccm. of
boiling water and 5 grs. of silver nitrate in 50 ccm. of water added
hot. The solution is allowed to settle, then filtered, and then washed
first with boiling water and then with alcohol on the filter, and then
dried in a dimly-lighted room.
To prepare the dye solution 0·5 g. of the dry eoside of silver and 1 g.
of ammonium acetate are gently heated together in 30 ccm. of alcohol
till dissolved, then 120 ccm. of alcohol and 10 ccm. of glacial acetic
acid are added, and the whole filtered. For plates to be exposed wet,
5–10 ccm. of glycerine are added. The emulsion is sensitized by adding
one-tenth of its volume of dye, and should be well shaken before
coating the plates. The glass plates should receive a preliminary
substratum of gelatine or india-rubber, so that the frilling of the
film and impurities of the glass plates may be totally avoided. {44}
For small sized plates the emulsion is used somewhat thicker than for
larger, and for the latter it should be diluted with alcohol and ether.
After coating they should be dried at about 15° C., and then for some
minutes exposed to a temperature of 30° C.
The plates should be developed with the following mixture:—25 g. of
sodium sulphite should be dissolved in 40 ccm. of warm water, then
10 g. of glycine added and 50 g. of potassium carbonate, at first in
small quantities in consequence of the evolution of carbonic acid. When
cool about 75 ccm. of a thin pasty liquid will be obtained, which may
be kept as a stock solution, and before use must be well shaken and
diluted with from 12–15 times the quantity of water. Wet plates should
be washed before development, and the developer flowed on; dry plates
should not be washed, and should be developed in a dish.
They should be fixed in sodium hyposulphite. The intensification may be
effected with metol (see p. 39), and the blackening as described on p.
38.
Obviously the collodion emulsion may be used for ordinary work undyed.
The qualities of the cross-lined screen and the distance of the same
from the sensitive plate will be described in Chapter V.
5. PREPARATION OF REVERSED NEGATIVES.
For the different photo-lithographic work direct and reversed negatives
are used; for the transfer process direct negatives are used; for
printing direct on to the stone or plate, reversed negatives.
For making the latter various methods are used, the principal of which
are as follows:—
(A.) EXPOSURE THROUGH A PRISM.
This method is, on account of its inconveniences and difficulties, only
used when it cannot be possibly avoided.
[Illustration: Fig. 5.]
{45}
Fig. 5 shows the arrangement in which the prism _a_ is placed in front
of the lens _b_. The prism is three-cornered and the angle turned to
the objective is a right angle. The hypothenuse is silvered. It is
mounted in brass or nickel; on the mount is a screw, by means of which
it is fastened to the objective, so that the side opposite to the
hypothenuse lies upright to the side of the camera, and this position
must be an exact right angle to the focussing screen. The camera itself
must be absolutely horizontal. The subject to be taken, instead of
being in front of the camera, is placed sideways to the prism. The
image passes through _a′_, falls on the silvered hypothenuse _a″_, from
there through the objective _b_, through the box-shaped extension _c_
and the camera _d_, on to the sensitive plate, which is not visible in
the diagram. The exposure with the prism is about 1/5th longer than
usual, because a portion of the light is absorbed by the prism.
(B.) PLACING THE PLATE THE WRONG WAY ROUND IN THE SLIDE.
Generally, for making reversed negatives of line drawings, it is
quite sufficient to place the sensitive plate the wrong way round in
the dark slide or in the carrier, so that the film side, instead of
being towards the objective, is turned towards the operator. It is,
of course, absolutely essential to use only glass plates which are
absolutely clean and which contain no particles of sand, air bubbles,
etc., which would show on the prepared negative as points. After
sensitizing the plate, the back of it must be thoroughly cleaned, for
any smudges or drops would be visible on the negative. Obviously great
care should be taken also in cleaning the plate to make as little dust
as possible. In the Imperial and State Printing Works nearly all the
reversed negatives are prepared most satisfactorily in this way.
The photographic manipulations, the preparation, sensitizing and
developing of the plates, etc., is exactly the same as with direct
negatives.
(C.) STRIPPING NEGATIVES.
There are various directions for making stripping negatives.
An india-rubber solution, consisting of equal parts of benzole and
chloroform, in which an equal part by weight of india-rubber is
dissolved, is used as a substratum. When the solution has become quite
clear, the unreversed and unvarnished negative is coated with it. When
the film is dry, a coating of raw collodion, to which some drops of
castor oil have been added, is given. When this film is quite dry, it
is cut through near the edges with a sharp knife, and the glass plate
and film placed in a dish filled with water, when the negative film
will loosen from the glass and after some time will swim in the water.
In this operation the use of force must be avoided. The film is lifted
from the water by placing a card {46} underneath it, carefully dried
with fibre paper, then laid flat and pressed somewhat between tissue
paper. The negative prepared in this way has many disadvantages. First
of all, the film is easily injured, and, therefore, the edges should
be bound with strips of paper in order to protect it from tearing;
further, the film is easily folded and does not remain accurately of
the same size, which makes it useless for certain purposes. Moreover,
this process is somewhat costly. It is now almost entirely replaced by
a process in which a gelatine substratum is used.
This process is as follows:—The unvarnished negative is accurately
levelled with a level and coated 2 mm. thick with a solution of—
White gelatine 20 parts by weight.
Distilled water 500 parts by weight.
Glycerine (chemically pure) 25 parts by weight.
Glacial acetic acid 10 parts by weight.
Alcohol 36% 50 parts by weight.
The above quantity is quite sufficient for at least four negatives
21 × 26 cm. Many omit the acetic acid, but I have found that it is
an advantage for the slow drying, the pliability, and the keeping of
its size of the negative, to use acetic acid in moderate quantities.
To a certain degree the addition of glycerine also effects this, but
not sufficiently enough. Before pouring on the gelatine solution, the
plate must be absolutely dry. Any air bells formed whilst coating must
be pushed to the edge of the plate with a piece of card. The plate
must be dried spontaneously in a room as airy and as free from dust
as possible. In order to protect the film from dust particles, it is
advisable to give it a paper cap or cardboard cover, which allows the
access of air, and obviously it ought not to touch the film.
When the gelatine film has become thoroughly hard, it should be coated
with negative varnish, and, after again drying, should be cut round the
edges with a sharp knife right through to the glass and stripped from
the plate.
If the glass was well-cleaned and prepared, as has been previously
described on page 41, any gelatine film thus prepared will well and
safely strip without a substratum.
With dirty or old plates, a substratum of a 1–2% solution of
india-rubber should be used.
After stripping, the gelatine films are best kept between filter-papers
and under glass plates.
{47}