Nitrate negatives

Heida Shoemaker <hshoemaker [at] sfmoma__org> writes

>In examining
>the negatives this week, it was noticed that approximately 10 of the
>more-deteriorated negatives now have the words of the watermark
>("Buffered Apollo...") "written" on them in silver mirroring. In
>most cases, the mirroring is greater in the letters of the
>watermark, in a few it is the other way around. It seems as though a
>general pattern of overall silver mirroring concentrated in the
>center of each neg. was already present on these negatives, as well
>as a number of others.

Heida's message about nitrate negatives and mirroring occurring
around the watermark was brought to my attention by a few people.

Heida called me directly and mentioned that she would also be
posting to the DistList. She had told me that the mirroring was
largely confined to the center of the negative as was the watermark.
The most likely cause is that the negative is continuing to out-gas
nitrogen oxides causing further oxidation/migration of the silver to
form more mirroring. In the area of the watermark, the paper is
cleanly not in contact (i.e. with a distinct area in contact and not
in contact with negative.) Areas in contact may be neutralizing
enough of the most aggressive nitrogen oxides (nitrogen dioxides,
for example is both strongly oxidizing and very acidic) while in the
area of the watermark where the paper is not in contact, there is a
greater amount of mirroring occurring. Had it been the other way
around, I would've suggested that the greater alkaline environment
will promote reduction of silver ion back to silver (making whatever
reducing agents were present to be stronger al la the Nernst
equation.)

I must add that I've never seen this phenomenon myself.

The problem amounts to electrochemistry and diffusion and therefore
low temperature and RH should prevent this problem from occurring.
In fact, environmental control would be a better response (from the
point of view of the film) than using unbuffered enclosures.  There
appears to be a noticeable difference between buffered paper in good
contact and not in good contact with the film and therefore one
might conclude that a non-buffered envelope will simply allow more
even mirroring to occur. What one would like to do is to prevent the
mirroring and therefore low temperature/low RH storage is probably a
more important approach to take.

-Doug

                                  ***
                  Conservation DistList Instance 12:27
               Distributed: Wednesday, September 16, 1998
                       Message Id: cdl-12-27-001
                                  ***