As has before been stated, to produce the finest results in positive
photography requires certain modifications of the collodion bath and
developer, which, while not unfitting them altogether for negative
work, yet would render them quite unsuitable for high class work.
The formula for positive collodion here given has many merits, and
among those is a certain quality of film, which might be termed
opacity, but not in the strict sense of the word; it is that quality
in the film which enables it to cover up any small scratch or water
mark on the surface of the plate that would infallibly spoil the
picture, if almost any other collodion were used.
This quality, with great clearness or transparency in the shadows,
and a beautiful gradation of light and shade, make it unique as a
positive collodion.
There may seem to be a contradiction between the terms opacity and
great transparency of shadows, which may need further explanation.
As the shadows of a positive collodion picture are produced by the
black surface of the plate showing through the collodion, any defect
on that portion of the surface would be expected to show very
plainly, and it does so with the use of the ordinary collodions sold
by the dealers, but not so with the collodion under consideration,
which may be because it is less permeable by the silver solution,
and that consequently the deposit of iodide of silver lies more on
the surface and less within the texture of the film, and is thus
kept from contact with organic matter or other defects on the
surface of the plate; in some such manner only, can I account for a
very valuable quality that has saved thousands of [p017] plates,
that would under other usage have been thrown away.
This formula has never before been published, and must be prepared
as directed.
Alcohol and ether equal parts; gun cotton sufficient to make a
moderately thick film, say 5 or 6 grains to the oz.; put the cotton
in the ether first; when it is well saturated, pour in the alcohol.
To which add:
Iodide of Ammonium 4 grains to oz.
Iodide of Cadmium 2 " " "
Bromide of Cadmium 1 " " "
Bromide of Copper 1 " " "
The latter ingredient can only be had in aqueous solution, I
believe, and requires close calculation to get at the right
quantity.
This formula gives 8 grains of salts to the ounce of collodion, and
would require a silver solution of 50 grains to the fluid ounce to
balance properly.
The silver solution may be prepared as per directions on page 29,
under the heading--The Negative Bath.