The effect of the presence of water is to produce viscidity in the collodion, and to cause a rapid decomposition when treated with the iodizer. It is therefore necessary that the exact strength of the alcohol used be carefully noted. Many collodion workers prefer to use a strong spirit. Absolute alcohol of a specific gravity of .8o5 at a temperature of 6o deg. Fahr. is used by many.
With the employment of a horny kind of pyroxyline, a small quantity of water seems to be necessary. The following useful remarks concerning the photographic effects of excess of alcohol in the collodion are also taken from Hardwich's work previously mentioned. The addition of alcohol to collodion lessens the contractility of the film, and renders it soft and gelatinous. These conditions are favorable to sensitiveness, perhaps from the play of affinities being promoted by the loose manner in which the particles of iodide are held together. The extra sensitiveness obtained by the use of alcohol, however, does not increase after a certain point; on the contrary, it diminishes, for it appears to be necessary to extreme sensitiveness that the film should coagu late within a certain time after it has been coated, and, therefore, the addition of alcohol must be stopped when the film loses its ready setting qualities, and is not coherent under the finger. Hence a porous collodion is soon injured in sensitiveness on adding too much alcohol, but a strong and tough pyroxyline will bear equal bulks of the two solvents without loss in that respect. These observations as to the effect of excess of alcohol in diminishing the sensitiveness of the film to dark objects apply particularly when the atmosphere is cold and damp, and evaporation is retarded. At a very high temperature, and in a dry air, it does not apply, since the presence
of alcohol is then useful in preventing the film from becoming surface dry, especially when it is necessary to keep the sensitive plate for a long time between exposure and development.
To get a good textureless film a method of refining the pyroxyline should be adopted. This is accomplished by making a plain collodion with cheaper solvents and half the quantities given. This is then poured into cold water in a fine stream. The effect of this is that a portion of the pyroxyline remains in solution in the water, and a portion becomes precipitated, and is of a finer quality than the original. This should be dried and again made up into collodion.
Collodion should be kept in long narrow bottles, to allow of the collodion being drawn off when all the impurities have settled.
Plain collodion sold by dealers usually contains about five per cent. of pyroxyline. Methy lated alcohol and ether may be and are chiefly used in manufacturing collodion owing to their cheapness. It may be useful to point out that collodion is an excellent substance to cover over wounds with, and to stop loss of blood. Should the photographer or any member of his house hold be so unfortunate as to get cut or wounded, a little plain collodion should be poured on. When set it forms a strong skin, and prevents the emission of blood and the admission to the wound of injurious matter.