Volltext Seite (XML)
[July 11,1862. THE PHOTOGRAPHIC NEWS. 332 1 0-5 400 GOO 11 5 FORMULAE FOR COLLODION. BY M. DISDEnI. Extreme sensitiveness in collodion is an indispensable quality for a good representation of living objects, butis not of itself sufficient for the perfect execution of a portrait which requires that the artist have time to pose his model arrange the drapery, and modify the effect: he will lose all the benefit of the rapidity of the luminous impression it the sensitive film dries too quickly. With the collodions whose composition I give, this result need not bo feared: the! remain humid a long time after being removed from the silver bath. In winter, the film retains all its sensitiveness for nearly an hour; and, during the hottest summer weather for twenty minutes at least. For Winter Operations. 1st Formula. 2nd Formula Alcohol 42° Ether 62° Pyroxyline Iodide of ammonium . Iodide of potassium ... Bromide of ammonium Bromide of potasium . Iodine may deduce the quantity of nitrate of silver really contained in the nitrate tested. To this end, it suffices to take note that the proportion’ of nitrate of silver, and of chloride of sodium, indicated above, arc equivalents, and that, consequently, if the nitrate of silver be pure, the 20 grains of its solution will require 20 grains of the salt solution to be exactly decomposed. If, therefore, instead of employing 20 grains of this solution, we take only 15 grains, this shows that the nitrate contains 15 parts of pure nitrate of silver, in 20, or 75 per 100. Sometimes the nitrate of silver may contain a little acid and water without any detriment to photographic success; to test for these it is necessary to dry the nitrate completely. I do not speak in this place of the other substances employed in photography with collodion. Distilled waters readily procured pure, but it must be kept securely corked, to prevent access of dust and atmospheric impurities. Sul phate of iron is also easy to obtain pure; and the purity ot the other articles employed has no sensible influence upon the result.* I shall here conclude this preliminary, and proceed to the study of the causes of the accidents most frequently encoun tered by the inexperienced operator, and analyse each ot them in succession, concisely, and determine, as fully as pos sible, their causes, and the means of avoiding them. (To be continued.) Before commencing this vast study, it appears to me necessary to indicate briefly the simplest means, by the aid of which the operator may satisfy himself of the purity of the materials he employs. No one can be ignorant that this purity is an indispensable condition of success, and that most of the failures and imperfect results are due solely to the employment of adulterated or impure chemicals. This is particularly the case with the products employed in the preparation of collodion, and the sensitizing bath, to which the presence of foreign bodies, even in minute pro portions, is injurious. Among these products the pyroxyline, or gun-cotton, as it is improperly called, as regards the quality of the collo dion, has most influence on the results, I therefore begin with the examination of this substance. A powdery cotton, too much acted upon by the acid and salt employed in its fabrication, gives a brittle non-tenacious film, which offers but a poor resistance to the action of the liquids with which it is covered, and yields only mediocre pictures. We cannot, by simple inspection, judge if such a cotton will give an unsatisfactory result or not; therefore, to satisfy ourselves on this point, we must dissolve a small quantity in a mixture of alcohol and ether, in suitable pro portions. For example, 1 drachm of cotton in 20 of alcohol and 40 of ether; and, after allowing it to stand a sufficient time for the undissolved fibres to subside, cover a small piece of glass with it, allow it to set, and then, with the finger, break up the film : the aspect of the fracture will enable us to ascertain the quality of the cotton ; if this frac ture shows a film without cohesion, if the part separated does not adhere to the glass, the cotton is of bad quality ; if, on the contrary, the rupture takes place with difficulty, and if the part separated adheres strongly to the glass, if the film presents the aspect of thin parchment, then the cotton combines all the indispensable physical qualities. To satisfy ourselves that this substance fulfils all the con ditions necessary in the preparation of a good collodion, it only remains to discover if the cotton has been sufficiently washed to remove all traces of the acids employed in its fabrication, The simplest means to this end consist in putting a sample of the cotton to be tested in a bottle with a given quantity of water, shaking it well, and then by means of a piece of litmus paper, testing the water for acidity. Commercial rectified ether is generally sufficiently pure for photographic purposes. It almost always contains a little alcohol, which however, cannot naturally disturb the photo genic properties of the collodion. The presence of an acid, on the contrary, is a very serions defect, and will produce the most mischievous results. The proportion of water contained in the ether has also to be taken into con sideration. The most suitable density is that which corresponds to 62° of Cartier’s areometer. It is also necessary to examine the nitrate of silver em ployed in the sensitizing bath, as its good quality is indipsensable to success in this process. An excellent method of ascertaining whether the nitrate of silver contains foreign salts, consists in precipitating its aqueous solution by means of hydrochloric acid, (carefully avoiding leaving an excess of acid or of silver salt in the liquid), filtering and evaporating the solution, which should leave no residue. If the liquid leaves a residue it will be easy to recognise its nature, and determine its weight, and obtain the exact composition of the nitrate tested. A simpler method in most cases, consists in dissolving in 60 grains of distilled water, 122 grains of the nitrate to be tested, and then adding, drop by drop, an aqueous solution of chloride of sodium (pure), containing 18 grains of salt to 100 grains of distilled water. By briskly agitating the liquor after the addition of each drop of the salt solution, it is easy to determine the exact moment when the precipi tate of chloride of silver causes to be produced, and noting the volume of salt solution which has been employed, we Dissolve the iodide and bromide of potassium in A t00 drops of distilled water, to facilitate their solution: •* . much water were added it would produce stri in the " and on this account we must reject weak alcohol. . rous These two collodions are very sensitive, and give vigot em. pictures. We must not expect extreme delicacy from The silver bath must be of the strength of 10 perC * The acetic acid must, however, be weighed, in order toascertaln be of the requisite density. Alcohol of 42° ... 400 Ether of 62° ... ... 600 Pyroxyline ... 11 Iodide of ammonium . 6 Iodide of cadmium ... 4 Bromide of ammonium 0-6 Bromide of cadmium . 0-4 Iodine 0-5
