Silver mirroring is the most common type of damage that appears in historical photographs on gelatin developing-out paper (DOP) and black-and-white films. Addressing the problem is potentially so problematic that it is often left untreated.
A new method for it, developed by Jordi Mestre and explained in detail in an article by J. Mestre, J.M. Vergès and R. Udina, will make this extraordinarily delicate and little-used procedure a common one within the photographic conservation community.
Mestre’s approach to this problem is based on two considerations:
The hazards of applying wet treatments on a gelatine layer, including the potential detachment of the layer from its support.
The understanding of the cause of silver mirroring, namely electron movement, which is triggered by an increase in acidity.
The new procedure uses calcium carbonate, which reacts only with the acid and not with the other components of the photograph. Calcium carbonate residue can be easily wiped away after treatment, and any remaining particles become a buffer against future acidic exposure, preventing further formation of silver mirroring.
Solvent is used to apply the calcium carbonate, which helps to remove grime, rubber-based tapes or fatty remnants, without altering the gelatine or any commonly found labels, masks, retouches and in-paints. A non-polar solvent is recommended, and since it is not meant to react with the calcium but rather be a vehicle for it, any solvent that addresses a particular need regarding solubility of the aforementioned additions will suffice, as long as it does not swell the gelatine.
Because calcium carbonate is even softer than cotton fibers, it does not cause abrasion. SEM photographs on cross-section samples show that even the thinnest gelatine layer that is partially detached from its support remains unaltered after being rubbed with this chemical compound.
Calcium carbonate removes the silver that has migrated from the emulsion to the top of it (and is no longer part of the image), but not the silver embedded in the emulsion layer; so there is no image loss. The only matter that is wiped away is the one on top of the emulsion, namely the silver mirroring, that covers the image surface and prevents its proper viewing.
The video below demonstrates the technique. A more detailed explanation of the chemistry and technical issues, along with SEM photographs, can be found on both Jordi Mestre’s and Rita Udina’s blogs.