Silver photographic images, even when well processed will always be prone to degradation through the ravages of time. The picture above, printed on ‘Barnet’ bromide paper in 1905 shows the tarnishing typical of a print of this age - still 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.
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 print 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. Selenium toner was also widely adopted as a permanising treatment for film records such as microfilm and astronomical plates.

The Strange Case of the Temperamental Toner
Lee, Wood, and Drago published a paper in 1984 dealing with the stability properties of a variety of toned images and found selenium acted very well as a protective treatment However when the Image Permanence Institute at Rochester Institute of Technology delved further into the action of selenium they found that although selenium worked well for high density areas (shadows), it did not convert the mid-tones and highlights that well, contrary to general opinion. When this was referred back to Kodak, they went back in their records for formulations and chemical sources. Prints produced by practical users in the field had highly stable prints, but this stability was not supported by lab tests, which showed a much lower level of resistance to oxidants. It was some time before they realised that there was a difference in the make-up of the chemistry; the consumers were using selenium toners made from GPR chemicals (general purpose reagents) while lab experiments were being done using analytical reagent grade chemicals (very high purity.) It turned out that sodium thiocyanate in the consumer toner was made from practical grade chemicals contaminated with a number of active sulphur compounds, while the high purity analytical grade chemicals contained no sulphur. Consequently, the action of selenium as a permanising agent was revealed to have been seriously over-rated, and those of sulphiding agents need to be re-assessed. This is indeed taking place, and most institutions involved in photographic conservation are now recommending some form of polysulphide toning rather than selenium.
Potassium and sodium polysulphides are not the most pleasant of chemicals in the raw state, corrosive and smelly and we do not normally stock or sell them. Fortunately there is a manufactured article, freshly restored to the marketplace.

Polysulphide Toner for Permanence Viradon is primarily sold as a brown toner, and when almost 100% of the silver has been converted to silver sulphide a warm brown image will result - no bleaching is required. It is particularly good for warm tones in combination with Ilford Warmtone Multigrade. Partial convertion will show little or no colour change, although the permanising effect is still active, and a strange property is that conversion is speeded up as dilution is increased, and the toner becomes more exhausted. Consequently, this type of highly active polysulphide toner can give complete protection even when used at 1+200 dilution. One authority recommends a time of 1 minute at a 1+50 dilution, which will convert approximately 75% of the silver to silver sulphide, leaving no colour change. Although there has been much research, and the evidence for the effectiveness of polysulphide is beyond doubt, it remains curiously unknown to the wider photographic public. There is much data on the web, and we will put the main references into a web page linked from the Viradon entry in our website. The ‘NEW’ bit refers to the fact that the previous version contained selenium, whereas this is now purely polysulphide based. The downside- evolves hydrogen sulphide (bad eggs) which is not good for film and paper, although when used at high dilutions this should not cause a problem. The price, a mere £5.88 for 125ml. And remember you read it here first, folks.