THE BICHROMATES IN CONJUNCTION WITH ORGANIC SUBSTANCES.
=Chromic Acid=, now usually known as chromium-trioxide—CrO_{3}—is found
in beautiful red needles, which are slightly deliquescent. According
to the present accepted theory, water containing such crystals in
solution contains the true chromic acid, H_{2}CrO_{4}, which cannot be
produced as crystals. Both the crystals and a concentrated solution
in water give up with facility a part of their oxygen, sometimes with
explosions accompanied by the appearance of fire, and green chromium
oxide, now called sesquioxide, Cr_{2}O_{3}, is formed. Bichromates
of alkalies act in a similar manner, but in a lesser degree. If, for
instance, a few drops of sulphuric acid be added to a solution of
potassium bichromate, and then a little alcohol, or, instead of that,
a piece of sugar be added and moderate heat applied, the solution
will be violently agitated, its yellow-red colour will disappear,
the bichromate be decomposed, and chromium oxide be formed, which,
combining with the sulphuric acid, forms a solution of a violet green
colour. If a sufficient quantity of sulphuric acid be present, the
chromium sulphate and the potassium sulphate combine, forming chrome
alum crystals, octahedron in form, similar to those of ordinary alum,
but of a beautiful dark violet colour. The same are used in both the
carbon and collotype processes. Bichromate of ammonium is easily
decomposed by fire by heating it in a dry state. It will be understood
that in consequence of the decomposition, or rather reduction, of
chromic acid by organic matter, the latter is also decomposed and
oxidised in a corresponding degree. The agent producing this effect may
be either heat or light, or both together; but of special interest to
the collotypist is the action of light upon different combinations of
chromic acid, in the shape of alkali bichromates in conjunction with
organic matters.
=Glue or Gelatine=, in a solution containing about five per cent.
of one of the bichromates spread upon glass plates, and exposed to
light, will, where exposed, become dark brown, at the same time become
insoluble in water, and also lose its power of absorbing the same.
According to the researches of Dr. Eder, this brown substance consists
of chromium chromate and unaltered gelatine. The oxidised portion of
the latter remains soluble and may be entirely removed by treating with
warm water. During the exposure to light formic acid is undoubtedly
formed. Ammonium bichromate, with gelatine, is in the dry state much
more sensitive than the potassium salt; it also decomposes more readily
when heated in the gelatine solution, a matter which must not be lost
sight of in the preparation of the collotype plate.
=Isinglass=—a tolerably pure glutin. In conjunction with the
bichromates it presents the same characteristics as gelatine.
=Albumen=, when mixed with a solution of a bichromate, is not thereby
coagulated, as occurs upon the addition of pure chromic acid. If
poured upon glass and dried it also becomes insoluble in water after
a sufficient exposure to light. This solution is used with great
advantage in photo-lithography, as the unexposed parts are readily
soluble in cold water. The older method, which consisted of using
gelatine in conjunction with bichromate, rendered hot water necessary
in the development of the fatty ink image upon the transfer paper, and
as a consequence of the rubbing at times necessary, the ink, already
softened by the warm water, became smeared, the resulting image
considerably impaired, and the transfer thereby rendered altogether
useless. In collotype a bichromated albumen substratum is still used
by many operators, this coating when dry being exposed _through_ the
glass plate, the latter placed coated side downwards upon a flat
black support, preferably of velvet. After this exposure the gelatine
layer—from which the prints are produced—is applied. This method was
first published by Albert, who originated it. The chromated albumen
solution will not keep long even in the dark,[B] as the albumen
coagulates, and on this account photo-lithographic papers coated with
this solution must be used within a few hours of their preparation.
Such papers are now conveniently prepared with the albumen,[C] and
sensitised as required by floating upon an aqueous solution of one
of the bichromates, to which has been added twenty-five per cent. of
methylated spirit.
[B] I have found this solution keep a considerable time—a month at
least.—TRANS.
[C] I believe Husnik has now discontinued to prepare his photo-litho
transfer paper with an albumen coating. Recent issues of his paper
have been simply coated with gelatine.—TRANS.
The chromated gelatine used in collotype may be kept as a jelly in the
dark room, but is better prepared fresh.
Of carbon hydrates only gum chromate is of interest to the collotypist,
its employment being restricted to the “dusting on” process, and the
occasional addition of titles, &c., to the otherwise finished plate.
As with all mixtures of the bichromates and organic substances, it
is only sensitive to light when in a dry condition, and solutions of
it may, without harm, be exposed for some time to diffused daylight.
Aqueous solutions of gum arabic soon become sour and mouldy, and in
this condition are useless for the “dusting on,” or, in fact, any other
photographic process. As a preventive a few grains of sulphate of
quinine may be added, which will preserve it effectually, and is quite
inert in photographic operations.
This solution applied to glass and dried is, in its dry condition,
extremely sensitive to light. If printed under a negative, and then
exposed to a damp atmosphere, it absorbs moisture, and becomes “tacky”
upon the portions fully protected by the densest portions of the
negative, while the parts exposed under the clear glass of the negative
will be free from any such tendency. The gradations of the negative
between these two extremes will be represented by the surface of the
gum being more or less adhesive, and it is this property which renders
it valuable in various processes. After exposure under a negative
finely-ground colour—for instance, graphite—is carefully dusted over
the surface of the plate by means of a camel-hair brush, and the
powder will adhere in exact proportion to the protection afforded by
the negative. A picture is thus produced in graphite powder possessing
all the gradations of the original. From a negative a second negative
is produced, while from an exposure under a positive a positive will
result, each, however, reversed as regards left and right when compared
with the original. Advantage is taken of this process to produce
reversed negatives, a subject more fully dealt with in another chapter.
=Bichromate and Sugar= are never used alone, but as an addition to
heighten the sensitiveness and increase the hygroscopic properties of
the gelatine or gum-chromate solutions. By the addition of sugar and a
little ammonia to chromated gelatine the solubility of the gelatine in
hot water is increased. The addition of acetic acid instead of ammonia
acts in a contrary direction. It is advisable to become acquainted with
the action of certain substances, the application of which to the
printed layer increases its solubility after exposure to light, and
of others having a contrary or hardening influence on the same, such
oft-times proving of service in the case of under or over exposure. To
the former class belong all alkalies, as chloride of lime and cyanide
of potassium; while among the latter, which have a tanning or hardening
effect on gelatine, may be particularly mentioned tannin, alum, chrome
alum, and chloride of zinc.
=To remove spots=, which print dark, from collotype plates a solution
of chloride of lime is used. To produce dark spots without exposure
to light a solution of tannin is used, which is also available for
adding descriptive titles or other matter to the plates. The ink will
“take” as well on the spots or parts touched with the tannin solution
as if exposed to light. A collotype plate which has been prepared for
printing from may also have writing executed upon it with a bichromated
gum solution, once more exposed to light, the writing will then readily
take the ink, and appear in the impression as a black inscription.
Gum chromated solutions reduced to proper proportions, and applied to
different textures and the same subsequently exposed to the light,
render them completely waterproof.