This thread has been fascinating to follow.
I have now looked in Tim Rudman book Toning and discovered he has a chapter on toning for permanence. Since copyright does not prohibit small snippets of text, i will type here. It seems to resolve some of the confusion about tone shifts or not, and how permanent toning and possibly "to completion", noted in this thread, and the knowledge and experiences that have likewise been spoken of here by the posters.
From c. 2003 Amphoto, pg.159
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Selenium Toning
This is still the commonest toner in use for archival treatment of prints. Its popularity stems from the mistaken belief that, like some of the much more expensive gold toners, selenium toning can provide excellent print protection with little or no colour shift, even on warm-tone papers when used at high dilutions. Although a number of myths about archival selenium toning have now been refuted, it does remain a valuable archival toner, but the following points should be borne in mind.
Selenium toner converts the image-silver into red/brown coloured silver selenide, which is a highly stable substance that resists attack by chemicals and pollutants very effectively.
Selenium does not tone evenly through all the image tones simultaneously. It works first in the the darkest tones - in the finer grains of the darkest tones to be precise - and then works up through the tonal range, eventually converting the lightest tones last. It only tones grains below a certain size - the largest, coarsest grains remain largely unaffected. These are the least vulnerable to deterioration as a result of attack from external agents.
On warm-tone papers, selenium toner produces a significant red-brown colour shift. On cold-tone papers the color shift is not seen. The explanation for this is due largely to the masking effect of the coarse cold-tone black grains that resist conversion. The apparent ability of the selenium to give archival protection to cold-tone papers without colour shift is due to the fact that it is the finer grains that are vulnerable to chemical attack. Although these are converted, the larger, more stable, black grains mask their colour. See also comments above regarding co-incidental sulphiding
1.
It has also been demonstrated that selenium toning is less effective for a given time of toning, when the selenium toner is used diluted with hypo-clearing agent (HCA) instead of water, and is more effective for a given toning time if these steps are undertaken separately. The role of HCA, when used prior to selenium is to prevent staining, not to enhance toning efficiency. The only advantage of combining HCA with selenium n one solution is to shorten the process. A toner stores well and can be kept almost indefinitely for future use. Dilute working strength HCA has a very short active life of a day or less in an open tray. It should then be discarded. Mixing the two wastes selenium toner.
All selenium toners (except the rare oderless variety) contain a significant amount of ammonium thiosulphate - rapid fixer. Treatment with HCA is therefore advised before and after selenium toning, followed by a full wash.
Advice that selenium toning at high dilutions of 1+20 or more will give good image protection wihtout colour shift on chlorobromide (warm) papers should no longer be followed. Toning for permanence should be at 1+9 or stronger for at least three minutes at 68F (20C) and colour change on these papers is usually inevitable, if full protection is to be achieved. An alternative approach sometimes employed to give protection without significant colour shift is to follow dilute selenium toning with Sistan treatment (see below)
2. I am not aware of any hard data on the effectiveness of this combination
Notes:
1. Selenium and gold toning only provided protection in proportion to the conversion (selenium) or coating (gold) that took place, and even at high levels of toning protection was incomplete. Some of the protection given by selenium and gold toning actually came from the incidental sulphiding from other constituents.
2. Sistan is a silver image stabilizer made by Agfa. Made principally from potassium thiocyanate, and also containing a substance known as 'POP' - polyoxyethylated octyl phenol - which is probably added as a wetting agent.
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I have printed a series of charts from
@Nicholas Lindan for Ilford MGIV FB WT in D-72 and Ansco 130, without and with selenium toning 1+9 for 20 minutes. Not that this addresses the change in color, but it seems the reflectance density is reaching the same top levels. It does provide one toning time example in practice. In his support files at
http://www.darkroomautomation.com/support/index.htm .