3149. Collodion for Photography

3149. Collodion for Photography. Collodion is the vehicle by which the photographic chemicals are united upon the surface of the glass and the sensitive coating produced. It is made by dissolving in equal of nearly equal proportions of sulphuric ether and alcohol, gun cotton or pyroxyline together with certain salts of potassium, cadmium, ammonium, etc., in proportions named in the formulae. Many formulae are published for this article to which great value is attached, some supposing that to its peculiar composition belong the principal causes of failure or success. This is only in a degree true. Inferior or carelessly prepared chemicals used in any stage of the process impair results. The writer has fixed as a general principle in the preparation of collodion the proportion of 1 gram of the exciting salts (in each ounce of collodion), to every 10 grains of silver in the bath. To illustrate: If the silver bath solution is at 50°, or, more definitely, 50 grains of silver to each ounce of water, we would make the collodion so as to contain in each ounce of collodion 5 grains of the various salts of cadmium, ammonium, etc.; or another way of putting it, the bath should be ten times as strong as the collodion. The sensitizing salts should be selected with a special reference to the peculiarities of the light or subjects. It can be made under one formula to cover almost all emergencies; yet special kinds of work for extremes of light or shadow can be improved by varying the combinations of the exciting or sensitizing salts. For portraiture in a room of evenly diffused light the iodide of cadmium as the principal excitant gives softness and delicacy to the image. Thus:

I.  Take of sulphuric ether, 1 ounce; 95 per cent, alcohol, 1 ounce; gun cotton, 6 grains; iodide of cadmium, 4 grains; bromide of cadmium, 2 grains.

II.  Sulphuric ether, 1 ounce; alcohol, 1 ounce; gun cotton, 6 grains; iodide of cadmium, 31/2 grains; bromide of potassium. 21/2 grains.

These two formulae give the utmost delicacy and transparency to the shadows, and work with rapidity, when preserving their proper relations to the silver bath solution, of which we speak in the proper place. If more brightness is desired to the image, instead of the iodide of cadmium put the same quantity of iodide of ammonium. If still greater contrasts are required, use iodide of potassium in place of either the cadmium or potassium. The latter is favorable for copying engravings, maps, plans, etc., in which strong contrasts of white and black are desirable. It is well to prepare from all these formulas and then modify results by mixing them together as the subjects or light may demand. Farther combinations may be suggested; under a feeble light, or where there are large masses of shadow, reduce the amount of the iodide salt one grain and increase the bromide one grain.

In Combining the Ingredients, measure out the required quantity of alcohol, and to it add the gun cotton and such of the exciting salts as dissolve in alcohol, and lastly the ether. Shake until all are thoroughly dissolved, and put aside over night to settle. When clear, decant into the flowing or coating bottle for use. Such of the excitants as do not dissolve in alcohol should be dissolved in as small a quantity of water as is possible and added to the alcohol, etc., a little at a time, and quickly shaken.