The progress that has been made since Mr. Archer's time in perfecting the adaptability of collodion to photographic uses could not be better illustrated than by comparing the formulæ for collodion first published with those in use at this time.
The practice of photography in the present day by the collodion process is divided into two branches, the positive and the negative.
In the first the object is to obtain in the camera a [p015] direct image, which is to be viewed by reflected light, and as it is desired that the pictures so produced should possess pure blacks and whites, certain modifications of the collodion, silver solution and developer are resorted to, which cause these preparations to differ somewhat from those prepared for the production of superior negatives, consequently the formulæ given hereafter will be designated as positive or negative.
These modifications, however, are not of such a character as to render either of the solutions prepared for one process totally unfit to be used for the other, for in fact, many photographers at this day use the same collodion bath and developer for making both positives and negatives.
In the first place, we will take up for consideration the positive process.
The first positives on glass were called ambrotypes, and were the successors to the daguerreotype, which they superseded and displaced by the superior facility of their production.
For the same reason the ambrotype was succeeded by the ferrotype, which was a positive collodion picture, made on a thin iron plate with a black japanned surface.
The ferrotype or tintype is now about the only product of the positive collodion process of considerable importance, and is the only one that will receive consideration in these pages.
Ferrotype plates are sold by all dealers in photographic materials; they are mostly manufactured by two large concerns in Worcester, Mass. [p016]
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.