The Dry Collodion Process

One of the most unfortunate circumstances connected with the practice of photography on wet collodion, is the necessity, in travelling, for the photographer to employ a tent, which adds greatly to his luggage. This inconvenience is due to the fact that the operations of sensitising and developing must quickly follow each other, for, if too long a time elapses between them, the nitrate of silver crystallises on the surface of the glass, and causes an infinite number of small spots to be formed over the entire surface of the picture. If the glass plate, covered with its sensitised coating of collodion, be washed with distilled water when re. moved from the silver-bath, and left to dry, it loses nearly all its sensibility, and will give but very imperfect pictures. M. Desprats and M. Dubosq have partially remedied this fault: the former, by adding half per cent. of resin; the latter, some few drops of amber varnish to the collodion; in whichever of the two methods it is prepared, it must be used exactly as ordinary collodion, with the exception that when the glass is removed from the silver-bath, it is to be well. washed and dried in the dark. Plates thus prepared will preserve their sensibility for many days. The time of exposure in the camera ought to be tripled, and the development made with solution of gallic acid. M. Dupuis recommends the adoption of the following process. Ordinary iodised collodion is employed, and the glass plate after being sensitised, is well washed in distilled water. A solution of dextrine, in ten times its weight of water, is left to settle, and then decanted so as to be free from impurities. A requisite quantity of this solution is poured over the sensitised glass plate, recently washed, and while still moist, in such a manner, that it spreads evenly over its surface; it is then drained and dried. The plates thus prepared will keep good for several days; but the time of exposure in the camera must be tripled, that is to say, if with wet collodion, 30 seconds of exposure is required, it will be requisite to give 90 with a plate prepared with dextrine. Before developing the plate, it is washed with pure water. An apparatus, consisting of a flat-bottomed flask, &c., similar to that described page 57, is useful for this purpose. The picture is developed with pyrogallic acid. A great number of other dry processes have been proposed; but two, above all others, have obtained the preference of the photographic public, as well in England as in France and Germany, these are the collodio albumen process of M. Taupenot, and the tannin process of Major Russel. These two processes will, therefore, be described in all their details. By van Monckhoven, Désiré van

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