HAVING obtained a soluble gun-cotton by one of the preceding formulae, the next thing to be done is to dissolve a portion of it in ether, to form the solution called Collodion,
Collodion derives its name from the Greek word "COLLE," to stick, from its adhesive properties when applied to any substance; this property has rendered it valuable in surgery, to cover wounds, cuts, and burns; and its first application, previous to its use in Photography, was to this purpose.
Gun-cotton does not appear to lose its peculiar properties by solution in ether, for if after precipitation with water it is again well washed and dried, it will be found to retain its properties of explosiveness and solubility.
Collodion was first made known by Mr. Maynard, in 1848, in the American Journal of Medical Science, who there gave a formula for its preparation.
When it was first pointed out that gun-cotton was soluble in ether, some doubt was thrown upon the truth of the statement, from the difficulty at first experienced by many in proving it. This doubt appears to have been caused by the use of pure sulphuric ether, as it afterwards became known that gun-cotton was not soluble in pure ether, that is, ether free from alcohol; a portion of alcohol, about one-eighth of the volume of the ether, being necessary to give it this dissolving power.
When gun-cotton, prepared in the manner previously directed, is dissolved in ether containing a portion of alcohol, a solution is formed, which on the evaporation of the solvent leaves (if the alcohol be not in excess) a tough elastic film, easily removed from glass when wet. When made with pure ether and the strongest alcohol, it can be obtained in transparent and tough sheets of great strength ; by the addition of acetic ether to the solvent, the gun-cotton more readily dissolves, but its strength as a film is partly destroyed.
To prepare the solution, the fibres of gun-cotton should be well separated and added to the ether by small portions at a time, well shaking the bottle after each addition of cotton.
For ordinary use, collodion made with the following proportions will have sufficient strength to hold the chemicals employed.
The ether used may be the ordinary rectified ether, which always contains a portion of alcohol. If pure or washed ether is used, about one-eighth of its bulk of alcohol is added, to enable it to dissolve the gun-cotton.
When strong collodion is poured on to glass, and allowed to set , it will be found to assume a distinctly cellular texture, generally five-sided cells.
If this strong collodion is iodized, and immersed in a silver bath, and a picture developed upon it, the cellular texture will be more distinctly visible, and will interfere very much with the delicacy of the image, and appear to break up, in a great measure, the more minute details of the picture.
This texture in the collodion can be entirely destroyed by the addition of alcohol; and in the preparation of iodized collodion a very large proportion of alcohol is added to make the solution flow evenly, and to prevent the deposit of iodide of silver in the collodion being thrown into ridges as the film sets. The even flowing of the collodion is, however, obtained by the sacrifice of a considerable portion of the strength and elasticity of the film; but for common use on glass, for obtaining pictures, this is not a matter of much consequence, so long as the dilution with alcohol is not carried too far.
The strength of the film of collodion can be tried by pouring a small quantity on to a piece of glass, and when it has set, removing it as a thin and delicate skin ; this can be done by taking the edge of it between the fingers, and gently raising it from the glass. When the collodion is strong it will bear entire removal from the glass.
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