By CH SCOLIK.
Since the emulsion process has taken root, no improvement has awakened
such a lively, steadily increasing interest as photography of colored
objects in their correct tone proportions; a process which makes it
possible to reproduce the warmer color-tones, particularly yellow,
orange-red, and yellow-green, in their correct light value as they appear
to the eye.
In professional circles, as also among the public, the value of this
invention cannot possibly be underestimated; an invention with which a
new epoch in photography may begin, and by which the handsomest results,
particularly in reproductions of oil paintings, can be attained. But in
portraiture, as well as in landscape photography, recourse must also be
had to orthochromatic plates to obtain effective pictures, particularly
as plates can now be produced in which the relative sensitiveness closely
resembles that of the ordinary emulsion plate. Although a good deal has
been written about this subject, none of these sometimes excellent
treatises contains a complete and generally comprehensive formula for the
production of color-sensitive plates, and this circumstance causes me to
publish my own experiences.
The following coloring matters are particularly recommended in the
several publications as preferable:
Eosine yellow and eosine blue shade, iodine cyanin, erythrosine, methyl
violet, aniline violet, iodine green, azalein, Hoffmann's violet, acid
green, methyl green, rose bengal, pyrosine, chlorophyl, saffrosine,
coralline, saffranine, etc.
Particularly important is the correct concentration. The most excellent
color matters make the plates oftentimes quite useless by an incorrect
proportion of concentration. If this should be too strong, the total
sensitiveness will sink (decrease); but when too weak, the color
sensitiveness is much reduced.
This fault, particularly, cannot be corrected during washing, but I have
mentioned, at the end, how such overcolored emulsion can be made of use
before wetting (flowing).
By the addition of some coloring matter to the emulsion, the light
sensitiveness of the film toward some individual colored rays is
increased, but the sensitiveness for the stronger refractive rays is, as
a rule, generally reduced. The result is a loss of the total
sensitiveness for white light. Color-sensitive plates are therefore less
sensitive to light than ordinary plates of the same origin.
The action of the coloring matter depends also very essentially upon the
emulsion. If the emulsion contains iodide of silver, it has a greater
sensitiveness for light blue and blue-green light. At all events, the
iodide combination must not amount to more than one or two per cent., a
small quantity of iodine acting much better upon the total sensitiveness
of the plates than can be obtained by pure bromide of silver emulsion.
Methyl violet, rose bengal, and azalein act perceptibly in 1/10000 per
cent. upon yellow sensitiveness. Eosine and its varieties, eosine yellow
shade, or eosine J, pyrosine J, erythrosine yellowish, may all be noted
as very good sensitizers for green, yellow-green, and eventually for
yellow. The bluish shades of eosine colors, on the contrary, have an
absorption band further in the yellow. This is also the case with the
blue shade eosine (eosine B) and the most bluish of all eosines, the
bengal rosa. Of both eosines, yellow shade and blue shade, the latter
gives a little more intensity.
Although the eosine permits a large limit in the quantity, it will reduce
the sensitiveness greatly in larger quantity.
If eosine solution is mixed with bromide of silver emulsion, which is
entirely free from nitrate of silver, no eosine silver can form; it acts,
therefore, only as an optical sensitizer.
Of the several kinds of cyanin, chlorosulphate, nitrate, and iodide, the
latter acts best, as stated by Eder.
Schumann has already said that one drop of cyanin solution, 1 to 2,500 to
6½ c. c. emulsion, already acted as sensitizing in orange; five to ten
drops cyanin. 1 to 1,500 to 15 c. c. emulsion, even gave red action.
There are two ways to color the gelatine film with a suitable coloring
matter: by mixing the latter directly before filtering into the ready
made emulsion, to produce at once colored plates; or to bathe dry
emulsion plates for one to five minutes in a solution containing the
sensitizing coloring matter. The plates have previously to be soaked for
a few minutes, whereupon they are bathed in an aqueous alcoholic solution
(with eosine yellow shade and eosine blue shade, in a solution of 1 to
3,000; but with cyanin in a diluted solution of 1 to 5,000). A mixture of
1/10 cyanin and 9/10 eosine yellow shade (of above concentration) acts as
a very favorable sensitizer. Lohse recommended bathing of the gelatine
plates in a solution of 0.03 eosine and 10 c. c. ammonia in 100 parts of
water. He found that very diluted eosine solutions, 1 to 20,000, acted as
a yellow sensitizer.
After washing, the plates have to be rinsed and dried--colored plates, as
long as they remain moist, being less sensitive than dry ones, and very
seldom the reverse.
This bathing of the ready made plates may give good results, but pure and
faultless plates are very seldom obtained, wherefore the first mentioned
manner (direct addition of color to the emulsion) is to be preferred.
After the experiments made by me, eosine mixtures acted equally in the
yellow and blue shade; likewise mixtures of cyanin 1/10 and eosine yellow
shade 9/10 were the most favorable. The process with eosine underwent
first of all a thorough test, of which the following are the results.
The color, solution I made as follows:
I. 0.5 grm. eosine yellow shade in 750 c.c. alcohol (95 per cent.) is
dissolved under good shaking.