NEGATIVES SUITABLE FOR COLLOTYPE.
A Collotype plate may be readily produced from any negative, but the
highest results are naturally only obtained when the negative is
prepared with a view to its special use. First should be considered the
question—May the negative, without detriment to the finished print,
be utilised without a reversal? In the not very frequent case of this
important point being immaterial, it is simply necessary to produce
the negative on patent plate, and it is at once available for printing
from. As the negative may easily sustain damage in any of the various
operations, it should be thinly and evenly varnished; this, while
affording protection, does not mar the sharpness of the resulting print
to any appreciable extent.
=Reversed Negatives.=—In the majority of cases, the resulting copy has
to exactly correspond with the original as regards the position left
and right, and in such cases the negatives will have to be reversed.
This is usually effected in one of four different ways—
1. Reversal of the image by reflection with a prism or mirror, or
reversing the plate.
2. Stripping the negative from the glass.
3. By the dusting on or powder process.
4. By the use of negative films.
The second is that usually adopted. With regard to the special features
desirable in negatives for Collotype, one giving a good silver print
will answer equally well for Collotype—they should incline rather to
softness than the other extreme. It has been remarked that they should
be as free from veil or fog as possible, and may be intensified or not
according to the judgment of the operator.
=Portraits from Nature=, with their fine half-tone, and at the same
time necessary contrast of light and shade, are the most difficult to
reproduce in Collotype. The negatives should be perfect in gradation,
and such as are usually only obtainable from a talented operator who
has the lighting of his studio entirely under his control.
=Black and White= subjects, on the contrary, for their satisfactory
reproduction, require a negative of the utmost density, but perfectly
clear in the lines. Such are better produced by the collodion process,
as directed for photo-lithography. For subjects in lines or dots
entirely free from half-tone, photo-lithography is more suitable for
reproduction than Collotype, where price is a consideration.
=Reversal by Reflection=, by means of a mirror or prism, necessitates
a special and rather expensive apparatus, and the exposure being
somewhat lengthened, it is, although quite successful, more suitable
for lifeless objects.
=Stripping the Collodion Film= is a safe and cheap procedure for
producing the necessary reversal, and in the subsequent printing
operations the danger of breakage, when compared with glass, is
reduced to a minimum. The glass plate, free from defects, must be well
cleaned, and it is safer, and renders the stripping more certain, if
it be covered with a fine film of pure beeswax. This is best applied
by rubbing over the surface a few drops of a solution of beeswax in
benzole and carefully polishing, at the same time avoiding the entire
removal of the wax. The piece of linen used in the operation should be
rubbed upon a lump of wax previous to the polishing. The plate must on
no account be gelatinised or albumenised, except round the edge only,
as a precaution against premature slipping.[J] Use a tough collodion,
and otherwise proceed as usual for the wet process. After the negative
is finally washed and dried it should be varnished with a thin solution
of gum arabic, to which has been added a few drops of a solution of
chrome alum. After complete drying the margin or edges of the plate
are rubbed with a little tallow. To prevent overflow the plate is
accurately levelled and flooded with a solution of 10 parts gelatine
soaked in 60 parts of water and dissolved by heat, to which is added
20 parts of alcohol and 6 parts of glycerine, using 1 dram of the
solution to each four square inches. A few drops of phenyl, carbolic
or salicylic acid may be added to the solution as a preservative, and
the whole before use well filtered, while hot, through linen. The
hotter the solution during the pouring on the more evenly the coating
will run upon the glass, and it may be assisted in its flow by careful
tilting of the plate. The use of paper strips is dangerous, and,
notwithstanding the gum, extremely liable to damage the negative.
[J] Instead of wax the plate may be dusted over with powdered
French chalk, all traces of which are removed before applying
the collodion. After such treatment the film is scarcely
likely to leave the plate before required. But as a further
precaution, if the edges of two plates be drawn across each
other a rough edge will be produced, to which the collodion
coating will adhere tenaciously.—TRANS.
=Reversal of the Plate=, either collodion or gelatine, is another
method of securing reversed negatives, and consists simply in inserting
the sensitised plate in the dark slide with the uncoated side towards
the interior of the camera. A modification of the dark slide may be
necessary, and it will be obvious that the ground glass focussing
screen must be reversed, or the focus corrected after the insertion
of the dark slide by shortening the camera a distance equal to the
thickness of the sensitive plate. Dry plates present great advantages
in this method, but, in development, density must be judged entirely by
transmitted light, as the image is formed in, not upon, the film. As
the rays of light pass through the glass to the sensitive surface, it
is necessary that the uncoated surface be quite clean, and the glass
itself should be perfectly free from bubbles, scratches, or other marks.
The plate should remain in its levelled position for about fifteen
minutes to set and then be carefully removed to a safe and airy
position, gelatine side outwards, and allowed to dry. This operation
will probably take two days; the hot gelatine layer should be about 3
mm. thick. The negative may then be coated with a toughened collodion
(one or two per cent. of castor oil added to plain collodion). The
plate is placed aside, and in an hour will probably be dry and ready
for the next and final operation. Cut with a knife through the film,
take it by one corner, and, without pause or hesitation, strip it from
the glass.
Any necessary retouching should be done upon the gelatine surface after
roughing it sufficiently with pumice powder to enable it to take the
pencil, and all such work must be completed before removing the film,
particularly if it has been damaged, as in such places the colour used
in retouching would cause an adherence, and the film would almost
assuredly tear in stripping.
Upon the quantity of gelatine solution used the nature of the film
depends. It should be entirely free from air bubbles. Too thick a
layer almost always cockles, and does not lie flat during printing,
in consequence of which portions of the picture will not be sharp in
the print. On the other hand, too thin a layer is extremely awkward
to handle in the larger sizes, and it is preferable to transfer it in
a reversed position to a sheet of plate glass coated with indiarubber
solution.
=Gelatine Plates for Stripping= are in the market. In their
preparation it is necessary to first coat the glass with a solution of
yellow wax in benzole, or an indiarubber solution, before coating with
emulsion, such plates being known as strippers in America and Germany.
When finished, the negative is placed, together with a gelatine film—or
“skin”—in a solution of glycerine and water, both are withdrawn
together, a squeegee applied to remove the superfluous solution, and
the negative, with its adhering skin, placed aside to dry. Afterwards
the edges of the film are cut through, and the negative removed from
the glass.
STRIPPING THE FILM OFF GELATINE NEGATIVES FOR COLLOTYPE.
In the _Photographisches Archiv_, Mr. Wilh. Otto, a well-known German
Collotype printer, describes the following process:—
“The glass plates are first cleaned by being well rubbed with a
solution of caustic potash, after which they are thoroughly rinsed and
polished with tripoli. The next proceeding is to wax them by rubbing
one face over with a solution of 12 grains of beeswax to one ounce of
ether, this being applied by means of a piece of cotton or linen cloth
saturated with the solution. In polishing off the excess of waxing
solution, care must be taken not to destroy the continuity of the
extremely thin and almost invisible film of wax which it is necessary
to leave on the glass to ensure the easy and complete separation of
the film. Some operators prefer to wax the plates by warming them to
a temperature somewhat over the melting point of wax, then rubbing one
face over with a lump of the article, the excess being now polished off
with a piece of flannel before the temperature of the glass falls below
the melting point of the wax. It may be mentioned that, before waxing,
it is as well to mark the working surface of each plate by making a
diamond scratch in one corner.
“A small piece of cloth, moistened with ether or benzole, is now used
to remove all traces of wax from the edges of the plates, a margin of
the width of an inch being sufficient; and it is well to paint this
margin with albumen and to allow it to dry. Unless the edges of the
plate are entirely free from all traces of wax, there will be a great
probability of the film separating from the glasses at too early a
stage, and adhesion at the edges is made more certain by the use of
albumen. A moderately diluted solution of silicate of soda may be used
instead of albumen.
“The waxed side of each glass is now coated with collodion containing a
little castor oil; eight grains of tough pyroxyline and three drops of
castor oil to each ounce of mixed solvents (equal volume of alcohol and
ether) being a convenient preparation.
“The collodion being dry, the plates are coated with emulsion. The
emulsion flows very badly on the collodionised surface, but by using a
bow made of a piece of sewing cotton stretched across an arc of thin
iron wire the difficulty may be readily overcome. The thread is to
be drawn over the surface of the plate after the required quantity of
emulsion has been poured on, and it is scarcely necessary to say that
the glass should be supported on a levelling stand.
“Plates prepared in this way are developed and fixed in the ordinary
manner, and when a plate is dry it is sufficient to cut through the
film inside the edge which has been cleaned of wax, when the film
negative can be easily stripped off. In this case, however, the film
is very thin, and it is often desirable to thicken it somewhat before
stripping. For this purpose a piece of thin commercial sheet gelatine
is taken, and soaked in water till it becomes quite flaccid, and this
is laid on the negative, care being taken that no air is enclosed
between them. A sheet of wet paper, or of mackintosh, is now laid on
the gelatine film, and all excess of water expelled from between the
negative and the soft gelatine film by stroking the upper surface of
the paper, or of the mackintosh cloth, with a squeegee—that is to say,
with a strip of indiarubber set in a wooden handle.
“It is now easy to remove the paper or the indiarubber cloth which was
used to protect the soft gelatine from the action of the squeegee,
and the plate can be set up to dry. When dry, the film is cut through
within the border from which the wax was cleared off, and the film will
separate readily; but should there be any tendency towards a too early
separation, owing to the contractile force of the fresh thickness of
gelatine, the edges of the plate may be bound with strips of gummed
paper.” (_Photographic News._)
=Stripping Ordinary Gelatine Plates.=—Any make of dry plate may be
stripped by the following method:—The plate is coated with strong
tough collodion upon the gelatine side; as soon as the collodion has
set, it is plunged into cold water and washed until all greasiness
disappears. In the meantime, a piece of “flexible support,” as used
in carbon printing, is carefully waxed by rubbing upon its surface
a solution of turpentine 140 cc., beeswax 2 gm., resin 6.5 gm. This
should be applied evenly by means of a piece of flannel, the solvent
allowed to evaporate, and the waxed surface polished with a second
piece of flannel. The support should then be placed in cold water
and allowed to become quite flexible, in which state it may be used
at any time. Remove the negative from the clean water, and place in
a bath of methylated spirits 150 cc., glycerine 3 cc., water 30 cc.,
hydrofluoric acid 3 cc. An ebonite, guttapercha, or lead dish should
be used to contain this solution, with which the fingers should come
in contact as little as possible. The film should be watched narrowly,
and when loose upon the glass—but not actually detached—the plate,
still bearing the film, should be placed in a bath of clean water, well
washed, and all surface moisture absorbed by careful blotting. Take
the flexible support from the water in which it has been standing, and
without removing any adherent moisture, carefully lower it upon the
film, which is still resting upon its original glass plate, squeegee
it into contact with the negative film, avoiding air bubbles, place
under a weight, and allow to stand for ten minutes or a quarter of an
hour. The film will then be found to adhere to the flexible support,
and may be readily removed from the glass. It should be allowed to
dry spontaneously upon the support, no attempt being made to remove
the film until it is “bone” dry, when it will readily part with the
temporary support, remain perfectly flat, and may be printed from
either side with perfectly satisfactory results. Should any of the
waxing solution remain on the surface of the film, it may be removed by
the careful application of a little turpentine applied on a small linen
pad. Do not use methylated spirit for this purpose, as it may cause the
film to “buckle.” Any stripped films may be conveniently preserved and
kept flat between the leaves of a book.
=The Powder Process= is one of importance for reproducing and, at the
same time and at one operation, reversing negatives, but for some
reason is far more resorted to upon the Continent than in England,
where few only have been able to thoroughly master it. Failures are
usually attributed to changeability in the climate, but, although
success is by no means uniform, it is a thoroughly workable process.
The basis is a chromated solution of gum arabic, with which a plate is
coated, dried, and in a thoroughly dry condition exposed to the light
under a negative. The greater the extent to which the light acts, the
smaller will be the amount of moisture the coating of the plate will
absorb when exposed to a damp atmosphere. If the surface of the plate
be now dusted over with a fine powder, as drop black, or graphite, the
greater the amount of moisture certain parts of the film have absorbed,
the greater will be the amount of colour such parts will retain, and so
exactly reproduce the light and shade of the original negative. Simple
as this process reads, the difficulties must not be underrated, and
the operator will probably find plenty of opportunity for the exercise
of his patience and ingenuity. A consideration of the process as just
described will show that a plate which has only been subjected to a
short exposure will, on dusting over and development, be found to
exhibit greater density than one that has received a longer exposure,
a careful adjustment of which will give to the operator great control
over the density and characteristics of the reproduced negative, a
power certainly too important to be overlooked. A reliable formula for
the preparation of a gum-chromate solution for this process is due to
Dr. Leisegang:—
Rain water 1 litre.
Dextrin or glucose 50 gr.
Gum arabic 50 gr.
Honey 10 gr.
White sugar 20 gr.
Glycerine 5 gr.
Cold saturate solution of
bichromate of ammonia 100 gr.
which, after careful filtration, is poured over a well cleaned and
dusted glass plate. After draining a little by standing on blotting
paper, dry over a spirit lamp, and place, while still warm, under
a negative in the printing frame—one minute in the sun, 10 to 20
minutes in the shade, is about the necessary exposure. As previously
mentioned, the shorter the exposure, the denser will be the resulting
negative, a result at variance to any other photographic printing
operation. To develop the picture the glass plate is placed, coated
side upwards, upon a sheet of clean white paper, then, with a soft
camel-hair brush, dust over the picture some very finely-ground
Siberian graphite, working the same backwards and forwards over the
plate until the necessary strength is attained. If the picture should
develop too quickly, and take the colour all over, the cause is under
exposure; should the powder adhere unevenly, or the picture present a
smudgy appearance, the coating must have absorbed too much moisture,
and must be again warmed before further development. This precaution
is particularly necessary during the winter months. If a prolonged
development be necessary, and after continuous dusting a picture
lacking in contrast be the result, it is occasioned by over exposure.
On completion of the development the picture should be finally and
carefully dusted with a soft brush to remove the superfluous powder,
coated with a thin plain collodion, and placed in water, which should
be frequently changed, until it fails to show any trace of yellowness.
The plate should be afterwards retouched and varnished.
=Negative Films= have now been before the public some time, and
have been considerably used as reversed negatives by printing with
the back of the film in contact with the plate. The four principal
varieties are, in the order of their introduction—1st, Eastman’s; 2nd,
Froedman’s; 3rd, Pumphrey’s.
=Eastman’s Stripping Film= consists of a substantial paper coated
with a layer of soluble gelatine, upon which is spread a coating of
insoluble gelatine emulsion; they may be exposed in contact with a
sheet of glass in an ordinary dark slide in the film carriers or on
the roller slide. There are no special precautions necessary in the
development, except that no alum may be used previous to stripping. For
use as reversed negatives the ordinary operations are greatly shortened
and simplified. A sheet of plate glass is coated with collodion,
allowed to set, and then placed in a dish of cold water until all
greasiness has disappeared. After fixing and washing, the negative,
still supported upon the paper, is placed over the collodionised plate
in the same dish, and the two brought out face to face. Upon the back
of the negative paper place a piece of blotting paper, cover it with a
sheet of oiled paper—as used in copying letters—and apply the squeegee;
place a dry sheet of blotting paper upon the back of the film, cover
it with a sheet of glass or other flat surface, upon which place a
weight. In fifteen minutes the plate may be placed in warm water—about
130° F.—and in a short time the paper backing will float off. By gentle
washing with hot water remove the soluble gelatine from the film
resting upon the glass, and place the negative aside to dry. It is then
complete, and reversed.
=Froedman’s Films= consist entirely of gelatine, require no stripping,
and may be printed from either side. The film is somewhat difficult to
handle in the developer, and has to be dried by spirit. One feature
against their employment in Collotype is a yellow stain, which probably
arises from the bichromate with which the film is hardened, and
although more apparent where the negative is protected by the rebate of
the dark slide, it undoubtedly pervades the whole picture.
=Pumphrey’s Lifting Films= are the latest and best of a series of films
introduced by the same maker. They consist of a basis of paper coated
with a waterproof varnish on one side, upon the other being spread an
indiarubber coating possessing considerable adhesive power; to this
is attached the gelatine film proper, which consists of two layers,
one of plain gelatine, and the other the sensitive emulsion. The films
should be exposed in the camera between two glass plates, and the
spring of the dark slide should be of considerable strength to insure
flatness of the film, and consequent sharpness of the image. The films
during development may be treated exactly as an ordinary plate, the
manufacturer recommending Beach’s developer. When in the bath they seem
little liable to damage, and may be handled with great freedom and
facility. Upon completion of the development, fixing, and washing, the
paper backing or support is attached by means of paste to a substantial
and smooth card. As a precaution, not always necessary, but usually
advisable, against the film prematurely separating from its support,
around the edges gummed paper strips may be attached to the card, and
allowed to extend over the edges of the gelatine film to the extent
of 3 mm. When dry the film may be varnished, and afterwards lifted
from its support by first cutting through the gelatine and passing the
blade of the knife round the edges between the film and the temporary
paper backing, the latter remaining attached to the card. The two are
by these means readily separated, and the resulting negative film may
be printed from either side without disadvantage, and is of sufficient
strength to withstand a considerable amount of rough handling.
=Other Methods of Reversal.=—There are processes of copying, usually by
first producing a transparency and from that again a negative. If the
wet process is used throughout, a copying camera is essential in both
stages. Should dry plates, however, be used, the transparency may be
produced in the camera and the negative by contact in a printing frame,
or _vice versa_, or both may be produced by means of the camera. If
the transparency be produced by the carbon process a negative may then
be made from it by contact, and the necessary reversal will be brought
about without the use of the copying camera.