We recently ran across this paper from Spanish conservators on a new approach to taking off mirroring on gelatin developing-out paper (DOP) historical photographs and black-and-white films. The three photo researchers that published the paper are Jordi Mestre i Vergés (original idea and research), Dr. Josep Maria Vergés i Bosch (Electronic microscopy and traceology), and Rita Udina i Armengol (coordination).
This mirroring is a by-product composed of silver that originally formed part of the image and eventually surfaced, generating an unwelcome sheen that makes the proper viewing and reproduction of a photograph difficult. It is a rather common problem for early silver gelatin prints, depending on their environment, although it is often only a minor distraction. As the three authors of the paper note, however, over 90% of the black and white silver gelatin prints can ultimately be affected by this mirroring issue—virtually all current chemically processed black and white prints.
In the past the problem with alternative procedures was that they involved immersing the gelatin prints in a solution of some sort and long washing periods, which can affect the gelatin emulsion. The new approach involves a cotton swab impregnated with tetrachloroethylene and a small amount of calcium carbonate, which is applied to the image layer in circular motions. The researchers further claim that "the treatment of silver mirroring through dry alkaline neutralization enhances even the state of conservation of the photograph."
For more details in a short article, click here: https://ritaudina.com/en/2018/10/14/silver-mirroring-removal-from-historical-photographs/.