I regret that Luis characterized my post on silvering as "well
meaning." It's a backhanded comment that appears to miss the point of
my answer to the original question. At the risk of sounding defensive,
I'd like to make the following points:
First, as far as my resources indicate, my answer to the original
question (what is silvering?) was correct. Certainly, no one has posted
otherwise.
Second, I attached the thiourea/phosphoric acid formula as a matter of
interest (and stated so). I also was clear about the source (and date)
of the formula. I'm glad to know that photo-conservation has progressed
since the 1970's, but I think we could stop with having made that
point.
Third, and this is important to all of us going through the time and
expense of making large negatives, I have used the formula I posted and
it does remove silvering on gelatin/silver emulsions quite well. While
I personally would never have a reason to use it on an historically
significant image, I wouldn't hesitate to use it (or recommend its use)
if it made the difference between a negative I could print and one that
I couldn't. It might well cause long term harm, but as I have stated
before, with the processes we are using, there are far greater risks to
our negatives.
As an aside to those who wrote advising me of the toxic nature of
Thiourea. It is actually measurably less toxic than either Hydroquinone
or Metol. According to Shaw ("Overexposure") it is a suspected
carcinogen based on laboratory rat studies. Her advice is to avoid
frequent direct exposure, which I think is good advice for handling just
about any photo chemical.
Sorry to come down on you Luis. I am glad to know from you and Claude
and others that the formula is no longer recommended for conservation,
but I just wanted to reestablish the perspective on this thread.
Carson Graves
carson@ileaf.com