PREPARATION OF THE COLLOTYPE PLATE.
The well-polished glass plate must be thoroughly cleaned with alcohol
and ammonia, and it should be ascertained that the surface is entirely
free from scratches. It is then coated with the substratum, consisting
of a dilute solution of silicate of soda and albumen. The following
formula is in constant use by many practical workers of experience, and
answers every requirement:—
Distilled water 8 parts.
White of egg—whisked 7 ”
Silicate of soda (of the consistency
of syrup) 3 ”
This mixture should be well beaten to a froth or put in a bottle with
pieces of broken glass, well shaken, and placed aside to settle for
half-a-day. Then filter through paper, which should be replaced so soon
as the pores become choked. To facilitate the operation the filter
paper should before use be moistened with distilled water. The surface
of the plate should be flooded with water, and coated as evenly as
possible with the above solution, a small glass rod being made use
of to assist the flow. Air bubbles must be removed, and by quickly
tilting the plate the superfluous solution run off; this should not be
used to coat other plates without being again carefully filtered. If
the substratum be thin, it is not necessary to wash the plate after
drying, but after standing aside two days to harden, with dusting and
warming, they may be coated with the chromated solution of gelatine.
If small crystal formations appear on the surface the plate should be
well washed and again dried before proceeding further. The following
formula for substratum is due to M. Thiel, senior; it contains less
silicate, and the washing before applying the sensitive coating is
never necessary:—
Distilled water 36 parts.
White of egg 20 ”
Silicate of soda 6·2 ”
=Stale Beer.=—Albumen is sometimes replaced by beer, from which by
frequent outpourings the carbonic acid has been removed, and to which
caustic potash has been added until it fails to show an acid reaction
when tested with blue litmus paper (about three to five grammes of
caustic potash to the litre of beer will be necessary). To a litre
of beer so prepared is added 1/3 kilo. of silicate of soda. The
main points in this and also the following operations are careful
filtration, even coating and drying in a clean place entirely free from
dust. Plates previously used or somewhat scratched must before further
use be thoroughly cleaned and the face reground with the finest emery
powder.
=The Sensitising Solution.=—This is manifestly the most important
matter in the Collotype process, for on it depends not only the
durability of the printing plate, but also the method of procedure
in the subsequent operations. It may be here intimated that plates
prepared by the very best formula cannot be expected to yield good
impressions if the whole of the manipulations are not carried out with
the greatest care, and it should be noted
1. That neither too much nor too little of the chromated gelatine
should be allowed to remain upon the plate. It is impossible to lay
down a hard and fast rule, and the exact amount must be determined by
the experience of the operator. Of equal importance, and naturally
connected with this operation, is
2. The even distribution of the solution, the flow of which may be
assisted or directed by the aid of a small glass rod, and by allowing
it to run off more or less at each of the four corners of the plate.
Considerable experience has demonstrated that the coating is applied
most easily and in the evenest manner by resting the plate—previously
warmed to about 45° C.—as horizontally as possible on the cork of a
heavy bottle, the latter standing in the centre of an earthenware dish.
A sufficient quantity of the solution, carefully filtered, and heated
to 45° to 50° C., is then poured on to the centre of the plate and
rapidly spread over it with a glass rod, previously well warmed. So
soon as the surface of the plate has been entirely covered a further
supply of the hot gelatine solution is quickly poured on and partially
run off at each corner. A scarcely perceptible inclination should be
given to the plate, otherwise the layer will be too thin; it ought to
be about the thickness of four or five sheets of Rive paper of 8 kilo.
per ream. It should be borne in mind that the addition of isinglass
will necessitate a comparatively thinner layer than when gelatine is
used alone.
To obviate the gelatine coating becoming uneven great care must be
exercised in removing the plates—when coated—to the drying-box, the
levelling screws of which have been adjusted with great accuracy. The
box should be heated to a temperature not exceeding 50° C.
To satisfy the adherents of isinglass, two eminently satisfactory
formulæ are here given—
WITH GELATINE ONLY.
Collotype gelatine 6 grams.
Water 700 ccm.
The gelatine is first soaked in the water for one hour, and then
complete solution effected at a moderate temperature, and the solution
filtered through best filter paper. Boiling heat is never required.
Japanese filter paper is the very best. Heat will be necessary during
the operation, which may be conducted either in the drying box or a
warm oven. When filtered, add, with constant stirring, 8 ccm. of a
solution of bichromate of ammonium in water (1 in 8, so that 8 ccm.
solution contains 1 gramme of the ammonium salt), which should also be
thoroughly filtered before addition to the gelatine.
The combined solutions being well incorporated, clear and free from
air bubbles, should, as previously described, be heated to 40° or 50°
C., poured upon the warmed plate, and at once conveyed to the drying
box, the temperature of which has been slightly raised. The box should
be closed, and gradually increased to 50° C., at which heat it should
be steadily maintained until the drying is complete, which usually
occupies one to one and a-half hours, varying somewhat according to
the thickness of the coating and the strength of air current, which is
necessary to the proper and rapid drying of the films. It may, under
some circumstances, be necessary to augment the admission of air; this
may be done by slightly raising the lid of the drying box. The current
must freely circulate in all parts of the box, and remain as constant
as possible. If subjected to sudden fluctuations in the air current or
temperature, the plates will dry unevenly, and endless troubles ensue.
=The Grain.=—The so-called grain which is necessary for the printing
first shows itself on the completion of the washing of the plates
after printing in the copying frame. It is a peculiar, net-like
formation, varying in fineness, and similar in character to the
unwelcome appearance known to carbon printers as reticulation. It
arises in consequence of the uneven distribution of the soluble layer
in comparison to the substratum. In Collotype, its production is
promoted by thicker layers and slower drying at a temperature of 45°
to 50° C. The addition of isinglass also seems to aid the production
of grain. Certain varieties of gelatine which possess great power of
absorption give a smaller grain. Creutz’s gelatine may be mentioned as
an example of this class. An addition in small quantities of dilute
acid, particularly those of organic origin, as acetic or tartaric, tend
likewise to the production of grain. Insoluble powders, as of glass
or flint stone, may be added, but only in the very finest state of
division, and in very small quantities.
FORMULA WITH ISINGLASS.
Two grammes of the best Russian isinglass are cut up into the smallest
possible pieces with a pair of sharp scissors, soaked in 25 cc. of
water for 12 hours, heated nearly to boiling point until solution is
effected, and then filtered while hot through close flannel. Cover six
grammes of good Collotype gelatine with 50 ccm. of water and allow to
stand, heat nearly to boiling point and thoroughly incorporate the two
solutions, again, while hot, filter through paper or close flannel.
To this is added 11 cc. of the filtered solution (1·8) of bichromate
of ammonium, as previously referred to. The complete solution is then
heated to 50° C., or should it exceed that temperature it is allowed
to cool to it, and the plates warmed, coated, and dried in the manner
previously described. It is not advisable to overcrowd the drying-box
by attempting to dry too many plates at the same time: it necessitates
a too frequent examination of the contents and so interferes with the
evenness of the drying, and as this is a point of even vital importance
no risk should be incurred.
The plates being properly dried allow them to remain in the box
for some time longer, the source of heat being either removed or
extinguished, and the whole allowed to cool gradually. If the plates
are removed too quickly in their fully-heated condition—particularly
when thickly coated—the too rapid contraction of the glass will crack
and destroy the film; by gradual cooling such accidents are avoided.
When finished the plates may be kept in a cool, dry place for two or
three days. Many operators keep them this length of time purposely,
believing they thereby obtain finer results. The plates so kept must,
of course, be thoroughly shielded from actinic light.
=Negatives=, with regard to their suitability for Collotype, may here
be considered, as the after result naturally depends considerably on
their adaptability to the process. They should be quite free from
fog or yellow stain, and for half-tone subjects not too intense.
The greatest difficulties will present themselves in the production
of portraits or such work from nature by the Collotype process.
Photographer and printer ought, so to speak, to work hand-in-hand.
The negative must show the finest gradation from clear glass in the
shadows to the necessary density in the high lights, quite free from
veil or fog, and intensified very little, if at all. They should be
produced in a studio where the light is thoroughly under control. If
not to be stripped all negatives should be taken on patent plate and
protected by a coat of varnish, sufficiently thin to offer no obstacle
to the production of sharp prints, and yet sufficient to obviate in a
great measure the risk of damage by the pressure it is subjected to
in the pressure frame in contact with the unyielding surface of the
Collotype plate. The risk of breakage at this stage always renders
stripped films preferable, their elasticity better enabling them to
withstand the pressure. The gelatine layer applied to the negative
before stripping must be as thin and even as possible, as from thick or
uneven films it is almost impossible to produce a sharp print. If the
negative is of a line subject, and it is desirable to have clear whites
that the impressions may resemble as much as possible the original,
the lines in the negative must be clear, but the remaining portions of
great density. The ink for such work should consist only of pure chalk
printing ink, which will be generally found to give stronger contrasts
than the usual mixed Collotype inks.
=Retouching Negatives and Plates.=—The negatives must have all defects
made good by the retoucher, all blemishes being moderated as far as
possible. Retouching on the Collotype plate or print should not be
required; but this is seldom so, as during the printing the negatives
frequently sustain slight damages. Retouching the printing surface
is seldom very successful. The alteration easiest effected is the
removal of small light spots which refuse to take the ink, and such may
be removed by touching with a fine brush dipped in a strong aqueous
solution of tannin, this alteration being made upon the finished
plate after it has been washed and dried. With this tannin solution
inscriptions may be inserted which have to appear black in the print.
Or a solution of gum arabic, to which has been added a solution of
bichromate of potassium, may be used, and after such addition the
plate is once again exposed to light. Writing which has received too
much exposure, and which should appear in the finished print black on
a white or light ground, often appears white. This defect arises from
the gelatine on which the light has had little or no action swelling
to such an extent that the letters are buried or sunk into the plate
too deep to be touched by the roller when the plate is inked. It may be
rectified by further thinning the ink with varnish. The removal of dark
spots from the print layer can never be completely effected. Ammonia,
cyanide of potassium, oxalic and other acids have from time to time
been recommended, but this treatment will prove effective for only a
few impressions.
Negatives should therefore be very carefully and systematically
retouched and carefully preserved from damage during the printing.
Another method of applying writing, such as the names of firms, &c., is
to get them printed from small type on thin gelatine films and attach
them to a clear or erased portion of the negative during the printing
in the pressure frame. Writing so added will appear as white letters
on a black ground. It will often be found practicable to attach the
written or printed matter to the original while photographing, and so
include it in the negative at one operation.
=Printing under the Negative.=—The printing of half-tone plates is best
performed in diffused light; to hit the exact exposure is as important
as it is difficult, and depends not only on the actinic power of the
prevailing light, but also on the quality of the negative and the
sensitiveness of the layer. The greater the amount of bichromate the
latter contains, the greater its sensitiveness, and consequently the
exposure is shorter than for one containing less of the sensitising
salt. A thinner layer must not be printed so long as a thicker one,
or dark prints, lacking in contrast, will result. On the other hand,
a thicker layer may be submitted to a comparatively longer exposure
without harm; the exact time must not be greatly exceeded or it will
happen that the blackest portions, written titles for instance, will
not take the ink. During printing, one may with advantage shade such
portions a little. The examination of the back of the Collotype plate
is the safest method of judging the correct exposure during the
copying, but every precaution must be observed during such examinations
that neither the plate nor negative be moved from their original
position. Only when the negative is very clear may the picture be seen
by reflected light as a positive; it will in such cases appear on a
dark ground. It will be found difficult, particularly for a beginner,
when making an examination in the copying frame, to distinguish between
the dark original picture and the pale brown image of the Collotype
plate, as they naturally completely cover each other. By the dark brown
tone of the uncovered margin of the print layer a certain indication is
afforded of the completion of the copying.
=An Actinometer= may be used as an exposure gauge, as in carbon
printing, and as a slight indication of the necessary exposure. When
using the small cube[H] photometer with chloride of silver paper,
five to seven tints may be printed, but the different modifying
circumstances previously referred to must all be duly considered. Under
a clouded sky in winter, using a dense negative, it may take a whole
day to complete the copying, while in clear direct sunlight, perhaps,
from the same negative, a quarter of an hour would suffice.
[H] The scale actinometer is one requiring less attention.—TRANS.
=Masking= should be resorted to to form a margin at least an inch wide
round the four sides of the negative, either by painting round the
same with Indian ink, or by attaching to the negative strips of opaque
paper, or, better still, thin tinfoil.
=Sunning.=—When sufficiently exposed, _i.e._, when the brown picture
with all its half-tones and details is complete, the plate is removed
from the copying frame and placed, face downwards, upon a piece of
black velvet, and exposed for a short time—five to ten minutes—to a
good but not direct sunlight, so that it may pass through the glass
plate and harden the back of the print layer. A sheet of ground
glass may be placed over the plate to thoroughly diffuse the light,
and prevent any scratches or damages to the back of the glass plate
producing any detrimental effect upon the printing surface.
=Washing the Plate= is the next operation. The plate is removed into
the dark room and placed in clean cold water, which should be often
changed—every half-hour—unless an automatic washing apparatus[I]
is used; this is continued until the plate no longer has a yellow
appearance, and the picture is visible only as a slight brown image.
[I] Jefferies’s Patent “Perfect” Washer is a very suitable
appliance, and may be used for Collotype plates, negatives,
or paper prints.—TRANS.
=Hardening the Film= is effected by soaking it for a quarter of an hour
in a one per cent. solution of alum. The plate is again washed, and the
surface evenly dried with fine tissue paper, and placed aside for at
least two days that it may thoroughly dry.
=An Alcohol Bath= is used by many operators to immerse the plate in for
half an hour after drying it with paper, perhaps with the intention
of drying the film quicker, or to strengthen the layer and assist the
formation of the grain, but there is no gain by this procedure, and, as
the bath is an expense, it may be omitted.
=The Damping Solution=, or so-called etching fluid, is of much greater
importance, and it is necessary that the plate be submitted to it
before printing be attempted.
=Tinted Grounds= often enhance the appearance of prints, and may be
imparted to them by the addition of an aqueous solution of red, yellow,
or other suitable colour, to the damping solution, which is applied in
the ordinary manner. The plate is superficially dried, inked up, and
printed from in the usual way.