THE COLLODION.
This being the principal material we have to use, we must exercise great care in the selection of a sample that possesses all the characteristics which fit it for a dry process. We must reject all samples that possess great tenacity and contractile power: the Collodion must not be too thick, and it must flow evenly over the plate, and not set in ridges. The best condition for the Iodized Collodion is that known as powdery, that is, being spread on the plate and partially dry, it cannot be removed as a film, but crumbles up on being pressed by the finger in its passage across the plate; in fact, such a condition as would arise from using gun cotton prepared with acids at a high temperature.
The following formula will be found to answer most admirably:
| Gun Cotton | 60 | grains. |
| Absolute Alcohol | 5 | ounces. |
| Sulphuric Æther, sp. gr. ·730 | 15 | ounces. |
The cotton is to be shaken up with the mixture of alcohol and æther, and when dissolved, the bottle containing it must be stood aside, in order that any undissolved particles of cotton may subside. The clear liquid may then be decanted into a clean bottle for use.
It will be as well to test the quality of the Collodion thus prepared before coating any number of plates with it, for, although the above proportions are very excellent, some little latitude must be allowed for the different degrees of solubility of the various samples of cotton used from time to time. The film, when spread, on the glass plate and partially dry, should not be capable of being removed in the form of a skin, but should give before the finger and crumble up on its being rubbed across the plate.
Should any difficulty, however, occur, it would be better to obtain a sample of the Collodion made by an experienced hand, in order that a fair trial may be given to the process. If the Collodion be too contractile it will give rise to blisters in the film, and will wash off the plate during development.
The Iodizing Solution that I have found to give the best results in this process is made in the manner following:—
| Absolute Alcohol | 8 | ounces. |
| Iodide of Cadmium | 64 | grains. |
| Iodide of Ammonium | 64 | grains. |
The iodides are to be dissolved by agitation in the alcohol, and the resulting solution is to be carefully filtered, and preserved in a well stoppered bottle.
The Iodized Collodion consists of—
| Iodizing Solution | 2 drams | } | 1 ounce |
| Plain Collodion | 6 drams |
The Collodion should always be iodized at least twelve hours before it is required; this interval allows any insoluble matters either from the iodizing solution or from the Collodion itself to fall to the bottom, and enables the operator to pour off the clear solution into a perfectly clean bottle for use.
Next in importance to the Iodized Collodion comes