FADING, CAUSES OF All silver images, whether negative or positive, are formed by metallic silver in an extremely fine state of division imbedded in a vehicle of albumen, gelatine, or collodion. Everyone knows how prone silver, even in the form of spoons and forks or ornaments, is to tarnish, and consequently it can be well understood how much more readily the metal in a finely divided state can be attacked. In many cases, particu larly in that of prints, the fading is undoubtedly due to imperfect fixation or removal of the last traces of " hypo " or the hyposulphite of silver. It must not be overlooked that gelatine is a hygroscopic substance, and that " hypo " in the presence of moisture is decomposed, giving rise to sulphur compounds which readily attack the image. Whilst fading is not so commonly met with in negatives, it can still be detected some times, and it is then advisable to bleach the nega tive with a chlorising mixture, such as hydro chloric acid and potassium bichromate, wash well, and redevelop.
There is but little hope of saving a fading print, and care should be taken to ensure perfect fixation and thorough washing, the former being as essential as the latter. It will often be found
that prints mounted on cards show fading more readily than those that are unmounted, and this may be due to the card containing " hypo " or some sulphur compound which is gradually decomposed by the mountant or moisture, and acts on the silver image. Frequently, too, prints--especially collodion prints—will fade in circular spots, and this can often be traced to small particles of metal, such as the bronze powder used for gilding the edges, etc., electro lytic action having been set up by the acid moisture in the air between the two metals. Varnishing negatives and prints is some pro tection. In the case of framed prints care should be taken to see that the backboard of the frame fits well, that the prints and mount are thoroughly dry before being framed, and that all round the edges, and over any cracks in the backboard, good stout brown paper is pasted or glued.