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Gelatine and

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529. Print-out Silver Emulsions. Print-out emulsions are made on the commercial scale by the precipitation in gelatine (gelatine P.O.P.' Abney, 1882) or in collodion (collodion papers, G. Wharton-Simpson, 1865) of silver chloride (or sometimes bromide or phosphate of silver), together with citrate, tartrate, or oxalate of silver. The unwashed emulsion contains an excess of silver nitrate in addition to the salts resulting from the double decomposition (nitrates of potassium, lithium, strontium, etc.).

These emulsions are coated mechanically on to baryta-coated paper.' The resulting paper should always be packed film to film, each successive pair being isolated with straw paper.

Collodion paper—and, still more so, the gelatine paper—requires negatives much less vigorous than those required for salted paper. Certain varieties of collodion paper, prepared with emulsions containing silver chromate (the film is yellow or orange, according to the quan tity of chromate present), are made for medium or weak negatives (contrasty papers).

Collodion papers give a much richer image and are more suitable for warm, black tones by successive tonings with gold and platinum. In warm climates these papers have the advantage that they can withstand baths and washing waters at a temperature which would cause the coating of the gelatine P.O.P. to inch.

The castor oil, which is often added to collodion emulsions to give suppleness and to avoid risk of cracking, is sometimes a cause of trouble through being present in excess; in these cir cumstances it is difficult to make the various baths wet the surface of the film. The same trouble sometimes arises when glycerine has been used to give suppleness, if the glycerine has, in the course of time, soaked out into the paper support. Immersion in denatured alcohol before attempting treatment in any of the baths will make the film penetrable to the solutions ( J. Gaedicke, 1911).

530. Use of Collodion and Gelatine P.O.P. With the exception of the proofs sent out, on these papers, from portrait studios,i the papers are either toned prior to fixing or toned and fixed simultaneously in one bath (§ 541). For separate gold toning the alkaline baths recommended for salted and albumen papers can be used for gelatine papers, although their actions are very slow. But they are almost without action on collodion papers ; these latter should be toned in a bath containing a solvent of silver chloride, e.g. sulphocyanide or thiocarbamide (§ 536 and 537), which can gradually make a way through the slightly permeable film of collodion.

As in the cases of the papers already con sidered, an economy in the toning solution can be effected by preliminary washing ; or, if it is intended to recover the silver residues, the soluble salts can be held in the film until fixation. by treating with a bath of salt followed by care ful washing.

Fixing, when carried out separately, is best done in an alkaline bath.

531. Print-out Papers from Old or Fogged. Development Papers. Gelatino-bromide or grin tino-chloride development papers, which, for one reason or another, have become unsuitable for their proper purpose, may be converted into print-out papers by soaking them in a weak solution of silver nitrate (about o.5 per cent) or in a solution of a reducing substance such as sodium or potassium nitrite (about 5 per cent), salts of hydrazine, various developers, sodium sulphite, stannous chloride, etc.

After soaking for some minutes, the paper is placed to dry in the dark without any prelim inary washing. The same treatment can be applied to negative plates and films.

The method of using is the same as for gelatine print-out papers.