CHEMICALS AND MATERIALS FOR COLLOTYPE.
=Gelatine.=—The basis of the Collotype process is gelatine—the finest
purified animal glue—and the important part it plays in the process
demands an acquaintance with its properties, which are found to differ
widely, according to its origin and method of preparation. Gelatine
suitable for Collotype should be free from all impurities—particularly
alum, which is not infrequently added in small quantities during
manufacture. Dr. Eder, in particular, has made most minute examinations
of various samples, more especially with regard to their suitability
for Collotype, Carbon, and the Galvanographic processes, and his
opinion, agreeing with that of other experimentalists, is here quoted:—
The two principal modifications of gelatine are those produced from
bones, sinews and skins of animals, and from the air bladder of the
sturgeon, the latter variety, better known under the name of isinglass,
represents, from a chemical point of view, glue in the purest form.
On account of its high price and ever-varying quality as found in
commerce, it must in practice give place to a glue or size containing
chondrin, and prepared from bone and gristle. Another objection is,
that plates prepared with an admixture of isinglass are less durable
than those containing gelatine only. On the other hand, it certainly
possesses advantages of its own, which will be more fully considered
hereafter. A gelatine suitable for collotype must absorb plenty of
water and swell out, but not liquefy after having been soaked in cold
water for twenty-four hours. Gelatine or isinglass prepared from the
bladder will not stand this test, but, on the contrary, partially
disintegrate. The consistency and bearing capacity of a jelly formed
by dissolving gelatine in water by means of heat and allowing it to
cool, form, in the opinion of Lipowitz, a safe and reliable means of
forming a judgment as to its suitability for collotype. Dissolve by
first soaking five grammes in 45 cm. of distilled water for several
hours, afterwards apply heat, and allow to stand twelve hours to set.
For purposes of comparison this should always be done in a glass of the
same diameter. A small tin or zinc slip is placed across the top of
the glass vessel containing the jelly; through the centre of the tin a
hole is pierced, through which a wire moves easily up and down. To its
bottom end is soldered a piece of tin, of about 15 cm. diameter, the
convex side lying upon the surface of the gelatine. On the other or
upper end a small funnel is placed, large enough to contain 1-1/2 kilo.
of small shot. The weight of the wire and funnel being ascertained,
it will be a simple matter, by also weighing the shot, to calculate
the weight required to sink the instrument into the jelly, and to
ascertain with accuracy its weight-bearing capacity and proof of the
relative value of various samples for Collotype printing. If otherwise
suitable, the variety capable of supporting the greatest weight
before yielding will be the one to select. The power of absorption
of different samples varies considerably; for example, the Collotype
gelatine of Creutz absorbs in two hours 30 parts of cold water poured
over it. Nelson’s patent opaque, in the meantime, only absorbs five
parts, but after a further period of 4-1/2 hours the difference is
equalised, as it will absorb 25 parts more of water, making altogether
exactly 30 parts. This experiment proves the latter to be a harder
quality than that of Creutz, and it necessarily requires a somewhat
different treatment for the production of the best results.
=Isinglass= or fish glue, although, as before stated, a finer quality
of glutin than the ordinary gelatine of commerce, is, on account of its
high price and varying quality, comparatively little used. The addition
of this substance also softens the gelatine layer. On the other
hand, it possesses certain properties which render its use at times
desirable; for instance, it does not dry so completely as gelatine,
the layer prepared with it consequently gives cleaner and more even
proofs, a point particularly noticeable in the earlier impressions.
Such prints will also exhibit greater brilliancy, and this is of
especial consequence in work produced from photographs from nature,
in which class of work the half-tones are too often wanting. Some
operators take even equal parts of isinglass and gelatine, but such
layers must be very thin, and dried quickly. Thick layers are liable
to crack or fly, and in use to refuse to take the ink, which must for
such film be used in greater quantity and thinner than when gelatine
alone is employed. A solution of isinglass is not so readily prepared
as one of gelatine. It is necessary to allow it to soak in cold water
for from twelve to twenty hours, and the temperature then gradually
raised to nearly boiling point, at which heat it should be maintained
for some time longer, and then filtered through flannel into the
gelatine solution. In printing from these plates, particularly when the
proportion of isinglass exceeds one-third the amount of gelatine, there
is a disagreeable tendency of the printing paper to adhere in the high
lights—that is, in those parts of the print layer which are represented
in the negative by entire opacity—as in the sky of a landscape or a
background which has been “stopped out” on the negative. On other
portions of the plate where the light has more fully acted—during the
printing in the printing frame—this troublesome defect will not be
observed. The fault is more noticeable when the dull enamel paper is
used, while a hard, well-sized letter paper presents the other extreme.
Further details of printing papers will be found in another chapter.
In the introductory chapter of this work the photo-chemical
qualifications of the bichromates of ammonium and potassium with
regard to collotype are considered.
=Bichromate of Potassium= is found in the well-known rich orange-red
crystals. It is readily soluble in cold water (1.10). It may be easily
recrystallised and purified readily out of either a hot, warm, or
cold solution, and forms in thin layers of fine crystals. Therefore,
in using it for collotype, care is needed not to exceed the strictly
necessary proportion, as the plate may be hopelessly spoiled by the
partial crystallisation of the salt, which will probably show during
the process of drying. The maximum addition of bichromate of potassium
in proportion to the weight of the gelatine may differ according to
the peculiarities of the latter. Dr. Eder considers about 3/10 to 4/10
by weight of this salt in a dry state may be added to a gelatine,
which will absorb from six to ten times its weight of water, without
fear of crystallisation. There are, however, many gelatines that are
incapable of working with more than 2/10. Rapid drying of the plate in
a current of air, and at a temperature of 40° C., will, to some extent,
obviate this objectionable feature. By reason of its comparative
insensitiveness to light and this facility of crystallisation, and
notwithstanding its comparative low price, it is now far less used in
collotype than
=Bichromate of ammonium=, the crystals of which somewhat resemble
those of the potassium salt. In colour they are of a browner hue, and
readily distinguishable, since when heated on platinum deflagrates,
emitting sparks and leaving a residuum of very finely-divided oxide of
chromium. The ammonium salt does not, however, so readily crystallise
on account of its considerably greater solubility in water compared
with bichromate of potassium. By comparison, 12 parts of the ammonium
salt are equal to 15 parts of that of potassium. At least 1/3 to 1/4
more of the former may be used without disadvantage or fear of it
crystallising out; it will, on the contrary, improve the quality of
the film or layer. The ammonium salt will also be found comparatively
more soluble in hot water, but care must be exercised that the
solution never reaches the boiling point, neither should the heat of
the drying box ever exceed 50° to 60° C., or the films will present a
pale brown appearance, and will prove alike insensitive and partially
insoluble. The single and tri-salts, offering no advantages over those
already described, may be dismissed without further consideration.
It may be remarked for non-chemists that the addition of any acid
to the chromated gelatine solutions inclines to the formation of
trichromates of the alkalis; and, on the contrary, the addition of
free ammonia, or any other free alkali, produces simple chromates.
The addition of ammonia to the chromated solution has been strongly
advocated in many quarters, but as the result of numerous experiments,
it may be definitely stated that the supposed advantages are more than
counter-balanced by the numerous disadvantages arising from such
addition. The mixing, in equal or other proportions, of the potassium
and ammonium salts has been recommended, but there is no real gain
or sufficient reason for preferring any of these combinations to the
ammonium salt alone. Besides the chromates of the alkalis, several
other salts as well as acids are used in Collotype printing, although
seldom, and then only in small quantities and for special purposes,
such, for instance, as hardening the print layer, or for the purpose
of facilitating the retention of moisture in the film. For the former
purpose small quantities of chrome alum are sometimes added; while for
the latter chloride of sodium or chloride of calcium are sometimes
added to the chromated gelatine solution at the time of coating the
plates, or are added as ingredients to the so-called etching or damping
solution which is frequently applied to the plates during the printing.
Glycerine answers for this purpose, and is to be preferred. It should
be borne in mind that all additions to the film to facilitate its
retention of moisture lessen its adhesiveness to its support. And with
regard to the direct addition of the substances mentioned to the film
during the preparation of the plates, it is not very apparent how such
readily-soluble matters can remain in the film after the prolonged and
careful washing the plates should be subjected to after exposure in the
printing frame. On these grounds the addition of such substances to
the damping solution is more rational and certainly more effective.
=Alum= and =Chrome-Alum= causes the gelatine to set sooner without
precipitation, and harden the layer considerably after drying, and
although gelatine, as well as glue, always contains more or less
chondrin, which is precipitated by alum, great care is necessary in
the addition of the latter; and it is better that such addition should
be made immediately before the hot solution be poured upon the plates,
otherwise a precipitation will ensue, which, although very finely
divided and at first scarcely visible, will become more apparent
during the drying of the plates, and finally show in the picture in
innumerable white spots.
=Silicate of Soda= is almost indispensable as a substratum in the
preparation of collotype plates, although many practical workers still
adhere to the method originally proposed and practised by J. Albert, of
Munich, who used as a substratum a chromated solution of gelatine and
albumen, rendered either partially or wholly insoluble by exposure to
light. The silicate is here considered as having about the consistency
of syrup. If more fluid the adhesive power of the substratum will not
realise expectations, and the plates will require very careful handling
if even a few hundred impressions are required.
The composition of the film, the exposure of the plate through the
back, and the careful treatment of the layer in the actual printing
operations are all matters which considerably affect the “life” of the
plate, and to which we shall again return.
=Chloride of Zinc= is used for hardening special parts of the collotype
plate, and is easily prepared by dissolving—to saturation—sheet zinc in
hydrochloric acid, filtering, and reducing as circumstances may demand.
=Tannin= and =Tannic Acid= in strong solution harden the film, and
may be applied to certain parts to cause it to “take” the ink. For
instance, to remove, or rather harden, spots which refuse the ink and
consequently show upon a dark ground; and also to write names or titles
to the plates. Written additions of this character must, of course, be
reversed, as in lithography. The addition of tannin to the chromated
gelatine before coating the plate is recommended by Waterhouse, he
considering such addition confers greater lasting power to the films so
treated.
=Cyanide of Potassium and Ammonia.=—All alkalis or carbonates of the
alkalis have a directly opposite effect upon the printed layer to
that last referred to. By the judicious use of one or the other of
these agents spots printing light or dark may be either removed or
considerably modified.
=Glycerine and Sugar= and similar easily soluble organic substances,
added to the chromated gelatine, would assist the easier and evener
damping of the plate, but render it more susceptible to damage, and
the desired effect is better attained by suitable additions to the
damping water.
=Organic Acids.=—Citric acid and tartaric acid are recommended as
additions to the chromated gelatine, partially as influencing the
so-called “grain,” and also as additions to the damping solution to
facilitate the production of cleaner prints. Experience with these
substances shows that the print layer suffers from their use, and for
clearing up a plate inclined to yield dirty impressions a weak solution
of ammonia is far preferable.
=Nitric Acid= has even been recommended for occasionally damping the
plate in certain cases, after which the most delicate half-tones should
be reproducible. An experienced collotypist ought never to be under the
necessity of adopting such desperate remedies, which more often than
not rapidly and entirely spoil the plate, and it is better to at once
prepare a new one.
=Ox Gall= remains to be mentioned as exercising a beneficial influence
upon the print layer under certain circumstances. It considerably
hardens the gelatine, and tends to reduce the annoying tendency of
the film to adhere to the paper in the high lights. Ox gall was first
used at the Royal Printing Works in the “glass” printing process there
utilised for the reproduction of line subjects. Some operators of
eminence consider that in collotype its use should be strictly confined
to lineal representations, as by its continued use the half-tones are
partially lost.
INK AND PAPER.
Of great importance is the careful selection of the materials for the
printing proper—_i.e._, the printing inks and papers, and the sometimes
necessary admixtures of the former.
=Printing Ink.=—The most simple and suitable of all greasy or oily
printing ink is the best chalk ink, as used in lithography. It is
composed of finest lamp black thoroughly incorporated with boiled
linseed oil (varnish). The collotypist must, in addition to different
coloured ink for mixing with the black, be provided with thin boiled
oil (middle varnish), and also turpentine and olive oil, the latter
being used to thin or reduce the printing inks, which, as usually
supplied to the trade, are too stiff for immediate use. When in
printing it is necessary or intended to use an admixture of several
colours, they must be first well ground in turpentine and the latter
allowed to evaporate. A small addition to the chalk ink of indigo or
Prussian blue greatly improves its colour and printing power, but may
be considered more suitable for the reproduction of line or stipple
subjects. A dash of Venetian red or carmine imparts to the chalk ink
a warmer tone, resembling the colour of an ordinary silver print.
Whoever ventures on colour printing must be able to prepare all the
necessary combinations with the greatest exactness, and should use for
each a special inking slab and roller. In the chapter dealing with
the printing, further particulars will be given as to the mixing and
thinning of the colours.
=The Quality of Paper= suitable for collotype may be next considered.
Without doubt the best is that known as dull enamel, or chalk paper,
which is now supplied of a pink tone as well as of the ordinary white
quality. It should not be glossy or adhere too strongly to the printing
surface, as the enamel will, under such circumstances, be partially or
wholly torn from the paper, a difficulty equally observable whether
the paper be used damp or dry. As previously observed, the adhesive
power of the printing surface increases with the softness of the same.
This may arise from using a small percentage of bichromate or a short
exposure to light, in which cases it is always more apparent in such
parts, as the densest portion of the negative may have been entirely
protected during the copying in the printing frame, or it may arise
from the addition of too great a proportion of isinglass. On the other
hand, a gelatine film strongly chromated, proportionately longer
exposed, and in which there is no isinglass, will be less likely to
exhibit this annoying tendency.
Well-sized fine glazed papers generally adhere least, and are usually
laid on dry. When the impressions are to be issued without glaze very
thin fine-surfaced cardboard—not enamelled—is frequently used, and
the margin of the printing plate covered with a suitable mask. Any
well-sized paper possessing a fine surface may be used. Impressions on
such paper may be passed through a weak solution of gelatine, and after
drying, rolled, varnished, mounted and again rolled. Any retouching
must be done immediately after the sizing, and before the application
of the varnish. Enamelled papers require a special glaze, not a
lacquer, or are finished without glaze at all.