Bob, I'd be interested in what you have to say about the following (and the rest of you too, of course).
My experience with selenium toning has led me to believe that using time/temp/dilution to determine proper toning wastes time and produces inconsistent results. Visual evaluation, independent of time/temp/dilution seems to be the only way to get desired results for me. Now, I'm not saying that using a higher dilution does not give you results you like better, just that there is no practical way of really being consistent using time/temperature/dilution parameters given the variability mentioned below. Here's my reasoning:
First, no matter what dilution you start with, after the first couple of prints start weakening it, toning times to produce the same result will increase. The more prints you run through the toner, the longer the time you will need for the same degree of toning (and this may not even be true, see below). That 45 seconds is only valid for the first print or so, and unless you discard your toner WAY before you need to (thereby putting way too much toxic heavy metal into the environment or the municipal sewer system), there is no reliable way to compensate for this. Related to this is the effect temperature has on toning: colder = slower and less toning, warmer = faster and more toning.
Second, different papers tone different ways. Some tone very quickly in very weak solutions, some take what seems forever to tone in very strong solutions. Furthermore, the change in image tone varies from paper to paper, from almost not at all to a range of tones from red through reddish-brown to eggplant-purple. 45 seconds in a given dilution will produce different results on different papers.
Third, although I have not tested it definitively, the degree of dilution also seems to have an effect on the final image tone. I.e., it seems to me that toning for a long time in a weak solution produces not quite the same result in image-tone change as toning quickly in a rather strong solution (this latter seems to produce more pronounced image tone). Disclaimer: this may not be true, as I have not really tested it, this is just a casual observation, but if it is, then dilution has more effect than on just the speed of toning.
Fourth, different images seem to need different amounts of toning. A particularly heavy image with lots of blacks and shadows will seem to tone differently than a high-key one. Also, for purely expressive reasons, different subjects "want" more or less change in image tone. And, the type of lighting can make toning more or less apparent. Under tungsten light, toned prints seem to have more image change than under lighting sources with higher color temperatures (mixed daylight, north light, florescent, etc.).
The upshot of this all is that visual evaluation, using an untoned print in the same lighting for comparison, is the only practical way to achieve the desired toning when all the constantly varying parameters are taken into account (not to mention the subjective ones).
Finally, there is the question of when to discard a potentially dangerous heavy metal, and how. If you need a rather strong solution all the time, and toss it before it is completely exhausted, but merely weakened, then a lot of selenium is wasted and goes unnecessarily into the environment.
To minimize this, selenium toners should be used to near-exhaustion before being discarded or, as I advocate, replenished and re-used indefinitely. I have a couple of gallon jugs of KRST at different dilutions (labeled "strong" and "weak") which have been going for years. When toning times become uncomfortably long, I just add a small amount of stock to the working solution. I used to keep track, but this seems unnecessary. For me, it is only important that I achieve the desired visual effect. Replenished toner loses its ammonia odor, but not its activity. Storing toner like this results in a black precipitate that needs to be filtered out before use (I use coffee filters). This is, I believe, toned silver and silver-thiosulfate compounds falling out of solution. And, for those of you who are worried about this method building up contaminants, I should mention that I routinely test my prints for residual silver and thiosulfates and have never had any indication of contamination whatsoever (undetectable levels of stain with HT-2).
Best,
Doremus
