Important: Safety Warning
This page describes practices involving highly flammable materials in ways that create a serious risk of fire or explosion.
In addition, this page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
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To Re-develop a Negative
This is done by pouring upon the plate about 1 ounce of the pyrogallic acid solution to which has been added 5 or 6 drops of the silver solution designated for that purpose. (See No. 3152.) the collodion so as to contain in each ounce of collodion 5 grains of the various salts of cadmium, ammonium, &c.; 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 II. Sulphuric ether, 1 ounce; alcohol, 1 ounce; gun cotton, 6 grains; iodide of cadmium, 34 grains; bromide of potassium. 24 grains.