When iodide of cadmium is used instead of iodide of potas sium, the iodized collodion does not become gradually deteriorated in the same way, but the nitrate of cadmium which is formed and accumulates in the nitrate bath has an acid reaction, which like nitric acid, impairs the sensitiveness of the film, and the density of the negative. Iodide of cadmium also injures the fluidity of collodion.
Iodide of ammonium is a very unstable salt, and liable to be de composed into free iodine and ammonia. Collodion iodized with this iodide becomes rapidly discoloured (particularly in the light), although when first iodized it yields remarkably good negatives. The free iodine introduced into the nitrate bath, and the instability of the nitrate of ammonia which is formed by the double decom position of nitrate of silver with iodide of ammonium, both tend to overcharge the nitrate bath with free nitric acid. Iodide of ammo nium is therefore a bad iodizer to employ for negative collodion, although a very good one for positive collodion, to which the same objections do not apply.
The alcohol used for making negative collodion need not be so absolutely free from the common impurities of alcohol as that which is required for positives, for these impurities act rather beneficially than otherwise in increasing the density of a negative. In positives, however, where density is not required, but a pure white metallic precipitate, they act injuriously on the tone of the picture. Posi tive collodion should be made with chemicals of the utmost purity, and yet it is generally sold at a cheaper rate than negative collodion.
Photographic pyroxylin is perfectly soluble to a mixture con taining equal parts of absolute ether (S. G. 720), and absolute alcohol (S. G. 794); and certain iodides, such as the iodides of calcium, ammonium, or cadmium, which may be used as iodizers, re soluble in anhydrous alcohol. It is possible, therefore, to make )dized collodion which shall be absolutely free from the presence of rater. But the film thus obtained, is found to be so extremely epellant of water, that it is impossible either to excite it properly in the nitrate bath, or to get the developer to flow evenly upon it. Water is, therefore, a necessary ingredient in photographic collodion. On the other hand, when collodion contains too much water, the film cracks all over in drying, and the picture is destroyed. Great
nicety is therefore required in so regulating the specific gravities of the ether and alcohol as that the collodion shall contain the proper amount of water, and no more.
It is sometimes advisable (in hot weather for instance) to use more alcohol in the collodion, in order to keep the film moist longer, as well as to increase its sensitiveness by modifying its mechanical pro perties. When absolute alcohol (S. G. 794) is used, considerable latitude is allowable in the quantity which collodion may contain. For instance, if to an ounce of iodized collodion, an ounce of absolute alcohol, and the corresponding quantity of pyroxyline and an iodizer be added, the collodion so made will yield excellent negatives scarcely distinguishable in any respect from the original collodion containing only half the quantity of ether. As alcohol is much less volatile than ether, this seems to indicate that for certain purposes the usual proportions of ether and alcohol may be advantageously modified. In making this experiment the reader must, however, remember that by " absolute alcohol" is meant alcohol S. G. 794. The alcohol frequently sold as " absolute," and even called so by chemical writers who ought to know better, contains as much as 6 per cent. of water, and would, of course, ruin the collodion, if used in the quantity indicated above.
Plain collodion may be kept for a very long time without deterio ration, in a cool, dark, dry place. It should be kept in a tall, well stoppered glass bottle, which should be shaken as little as possible when decanting the collodion, so as not to disturb the particles of undissolved cotton, &c., which settle to the bottom.
When too much iodide is added to collodion the film cannot con tain the whole of the iodide of silver that is formed in it by the nitrate bath. It therefore lies loosely upon the surface, or is partially washed off. On the other hand, if the filin contains too little iodide, it ap pears thin and blue when looked at in the light, and does not give quite so intense an image, the sensitiveness being also somewhat fliminished, though not much. It is better to have too little iodide in the film than too much.