Not Fade Away
Unless one has done something dramatic to a black & white image, such as toning or even inking it up as in Bromoil, it remains as the same metallic silver as was first developed. The only reason it doesn’t appear as shiny silver metal is that it is in the form on microscopic particles that block light, either transmitted through it or reflected off it, and hence appears dark to some degree.
However, silver is a reactive metal, & even when properly processed and washed is liable to be under attack, just as a silver teapot needs a regular shine-up, the same tarnishing applies to photo film given long enough. This old plate is maybe an extreme example – it’s about a century old, and was stored in a stack in it’s original box, interleaved with paper. The silver image has been attacked from the exposed edges, travelling inwards.
The same happens to prints. This one printed on ‘Barnet’ bromide paper in 1905 shows the tarnishing typical of a print of this age – it looks fine viewed normally and still has a good density range. But a proportion of the image silver has migrated to the surface of the print and plated out as a reflective silver film, look at it again but with light glancing off the surface, and the damage can be seen, and bear in mind this isn’t the end result, it’s still ongoing.
Similar effects can happen more quickly and dramatically – twice in the last year we have had newly produced prints returned supposedly with faults, loss of density in the shadows and little gold spots in the mid to low densities. In both cases it turned out the prints had been framed using low grade hardboard with no intermediary vapour barrier layer between the back of the print and the board. Similar rapid attack can happen in a display environment where there has been fresh painting, and higher than normal heat, light and moisture will always accelerate this oxidation process.
From the earliest days image fading was a known problem, and an immediate fix was toning, often with gold salts, but as papers became more robust, sulphide (sepia) toning became prevalent. In the 20’th century selenium toner arrived on the scene, and being espoused by luminaries such as Ansel Adams, came to be regarded as the ultimate finishing treatment for permanence. This was because selenium did not cause major colour shifts with most papers when used diluted, and as well as improving permanence tended to increase tonal range in a subtle way, and generally enhance the final print.
[incomplete, to finish]