INVESTIGATIONS ON COLLOTYPE.
August Albert[L] is of opinion that it is by no means a matter of
indifference whether bichromate of potassium or bichromate of ammonia
is used in the preparation of the Collotype plates; on the contrary,
he believes that the latter salt materially conduces to interruptions
in printing, especially in unsettled weather. The results of his
experiments with the two salts in the Collotype process, and of their
behaviour in combination with different sorts of gelatine, are as
follows:—
To 1000 cc. of water are added ten drops of a saturated solution of
chrome alum; this mixture, after having stood for a quarter of an hour,
must, if shaken up, show an opalescent appearance, and a slight milky
turbidity; if too much chrome alum has been added, the water acquires
a greenish colour. The gelatine is now added, and, for about fifteen
minutes, kept at a temperature of 134°, when the chrome salt is added,
stirring all the while, and the temperature of the water bath raised
to 150° to 160°. The temperature and time must strictly be adhered
to. The jelly thus prepared should be kept at least ten hours in a
solidified state previous to using.
[L] This experimentalist has evidently been confused with the
late Joseph Albert, of Munich, by some writers. It may be
mentioned that Joseph Albert, whose business is still carried
on by his widow, was father of Dr. Eugene Albert, known in the
photographic world for his orthochromatic collodion emulsion.
August Albert, of Vienna, is of another family.
=Hard Gelatine.=—Plates prepared with this, show after drying, a shiny,
transparent film, more or less according to the quality. Such plates
give hard prints, and are soon used up in printing, the finest and even
the middle tints becoming weaker, and the shadows gradually coalescing,
specially in the case of short exposure and very dense negatives, and
they are generally useless after a few impressions have been taken. If
ammonium bichromate is used, together with hard gelatine, then, as a
general rule, it is impossible to obtain more than one or two prints
from each plate. By washing the gelatine, these inconveniences are not
at all removed, no matter which of the two salts has been used.
=Soft Gelatine.=—If this is prepared with potassium bichromate, then
the plates will have a dull but rough and untransparent film, which
will be found very unserviceable for printing, as the prints will
lack vigour; the high lights will tone, and the whole picture appear
blurred. If, however, ammonium bichromate is used in conjunction
with soft gelatine, the plates will also have a dull appearance, but
though they work somewhat better than those prepared with potassium
bichromate, they will not be of sufficient durability, the finer tones
soon disappearing during printing. Therefore, neither hard nor soft
gelatine are suitable for Collotype work, as the printing operations
are rendered more difficult. If both sorts are mixed, the results
will be somewhat better, but not like those obtainable with middle
hard gelatine, which must be accepted as the most suitable kind for
Collotype work.
=Middle Hard Gelatine=, manufactured by Ferd. Friedr. Creutz,
absorbs five times its own weight of water at 66°, without signs of
disintegration after standing therein for thirty-six hours. If prepared
with it and with chemically pure potassium bichromate, the plates
after drying have a beautiful matt surface, from which long numbers
can be printed without any falling off of the half tints, and, if the
printing is done carefully, even the details in the shadows will be
maintained to the last. This bichromated gelatine requires also only a
very small amount of re-damping, reproduces faithfully the character
of the negative, and the jelly can be used even after a period of four
days. If, however, the work in hand requires a harder bichromated
gelatine—as, for instance, reproductions of black and white—then the
addition of a small quantity of ammonium bichromate will have the
desired effect; in some cases equal quantities of the bichromates of
potassium and ammonium. If ammonium bichromate is used instead of
potassium in conjunction with middle hard gelatine, then the plates
will acquire a shining, transparent coating, the manipulation of
which is very difficult in printing, and which work too hard, being
serviceable, at any rate, for reproductions without half-tint. If,
however, the middle hard gelatine is washed before use, it will give,
when mixed with the chromic salt, transparent films of insufficient
durability. Middle hard gelatine should therefore be used unwashed for
Collotype work, and mixed with chemically pure potassium bichromate.
=The Production of Grain= in Collotype plates takes place in the
last instant of drying the plates in the drying-cupboard; it depends
entirely upon the larger or smaller quantity, and the consistency of
the bichromated gelatine used. If the latter is of good quality, all
artificial means for the production of grain are not only superfluous,
but also disadvantageous. A larger addition of chrome salt generally
gives rise to the crystallisation of the salt; chloride of sodium
produces plates highly sensitive to moisture, but no grain; a larger
addition of chrome alum gives plates which will lack vigour, and the
manipulation of which, during printing, will be difficult, as they
often will require an “after-etching,” so that even printing is made
impossible.
To test the suitability of gelatine for Collotype work, two small
portions of the gelatine to be tested may be made up, adding chemically
pure potassium bichromate to the one, and ammonia bichromate to
the other, and a plate prepared with each solution. If both plates
show a dull and untransparent coating, this indicates that it is a
soft gelatine; if, on the contrary, both plates appear glossy and
transparent, then the gelatine is hard. Should it, however, be middle
hard, then the dried film prepared with potassium will be dull, but not
rough; that with ammonia, however, shiny and transparent.