Many efforts have been made to render chromatic acid an active agent in
the production of photographs. M. Ponton used a paper saturated with
bichromate of potash, and this was one of the earliest photogenic
processes. M. Becquerel improved upon this process by sizing the paper
with starch previous to the application of the bichromate of potash
solution, which enabled him to convert the negative picture into a
positive one, by the use of a solution of iodine, which combined with
that portion of the starch on which the light had not acted. But by
neither of these processes could clear and distinct pictures be formed.
Mr. Hunt has, however, discovered a process which is so exceedingly
simple, and the resulting pictures of so pleasing a character, that,
although it is not sufficiently sensitive for use in the camera, it
will be found of the greatest value for copying botanical specimens,
engravings, or the like.
The paper to be prepared is washed over with a solution of sulphate of
copper--about one drachm to an ounce of water--and partially dried; it
is then washed with a moderately strong solution of bichromate of
potash, and dried at a little distance from the fire. Paper thus
prepared may be kept any length of time, in a portfolio, and are always
ready for use.
When exposed to the sunshine for a time, varying with the intensity of
the light, from five to fifteen or twenty minutes, the result is
generally a negative picture. It is now to be washed over with a
solution of nitrate of silver, which immediately produces a very
beautiful deep orange picture upon a light dim colored, or sometimes
perfectly white ground. This picture must be quickly fixed, by being
washed in pure water, and dried. With regard to the strength of the
solutions, it is a remarkable fact, that, if saturated solutions be
employed, a negative picture is first produced, but if the solutions be
three or four times their bulk of water, the first action of the sun's
rays darkens the picture, and then a very bleaching effect follows,
giving an exceedingly faint positive picture, which is brought out with
great delicacy by the silver solution.
It is necessary that pure water should be used for the fixing, as the
presence of any muriate damages the picture, and here arises another
pleasing variation of the Chromatype. If the positive picture be
placed in a very weak solution of common salt the image slowly fades
out, leaving a faint negative outline. If it now be removed from the
saline solution, dried, and again exposed to sunshine, a positive
picture of a lilac color will be produced by a few minutes exposure.
Several other of the chromates may be used in this process, but none is
so successful as the chromate of copper.