Silver mirroring is, according to the American Institute of Conservation, "a natural chemical process that affects photographic materials containing silver over time. It results in a metallic sheen over the surface of the photograph, typically affecting the darker areas of a photograph most." As can been seen in the silver gelatin print above, image details are obscured and overall spatial continuity is disrupted from highlight to shadow.
Colloquially known as silvering or silvering out, the phenomenon is commonly understood to be caused by silver in the print's binder initially oxidizing over time to silver ions. These ions can then migrate upward from the gelatin layer to the print's surface and subsequently transform to silver sulfide. A number of possible conservation treatments have been explored, including the use of saliva as a mild enzymatic solution applied by swab to the photograph's affected areas.
Digitizing prints of this type can be challenging. When illuminated, metals produce almost nothing but randomly-polarized (aka unpolarized) direct reflections. These type of direct reflections can cause the fogging effect associated with silver mirroring. However, because this property is consistent among metals certain studio lighting and filtering techniques can be employed to consistently improve the image quality of captured prints. One such method is cross polarization.
Cross polarization begins with managing the linearity of a polarized light source. In the case of copy photography, that normally begins with two lights on either side of a table that contains a scene to be shot.
Here, mounted sheets of neutral gray linear polarizer film are positioned in front of two strobes that are part of the UConn Digital Imaging Lab's large automated X-Y table system. Once fired, the flash light that will eventually fall on the scene will be polarized according to the linearity of the film sheet in front of each head.
Both sheets are positioned so that their linearities are in the same direction. As a result, the large panoramic photo print on the table can now be illuminated with polarized light in a manipulated way.
Remember metals primarily produce direct reflections. However, through controlled polarization of the light source, those direct reflections can also be polarized with the same linearity. If we then use a circular polarizer filter on the camera lens and orient its linearity 90 degrees to that of the polarized lights this will produce a nearly complete blockage of directly reflected light known as extinction. In this way, the metallic sheen from silver mirroring can be mitigated during capture.
Here are a few examples of the benefits of the technique as illustrated through two panoramic photos that we recently shot in the lab. The first set comes from a 1909 photo of Hartford's Bushnell Park looking towards the Connecticut State Capitol building in the distance...
The version on the left was shot with no filtering of either the lights or camera. The cross-polarized version on the right only hints at technique's effect. In turn, here are a couple of 100% zooms that better exemplify the original silver mirroring and its mitigation...
The following group portrait from the UConn Archives' Grabowski Collection also contains areas affected by silver mirroring, though to a lesser degree. Note that the phenomenon's presence is mainly observable in the photo's shadows and darker areas, which are symptomatic traits. Yet even in this somewhat mild case, cross-polarization can clearly help reveal obscured contrast and detail.
If we return for a moment to the mechanism of cross-polarization and the notion of extinction, questions still remain. What are we seeing in these shots on the right? Why aren't they mostly black?
Up until this point, we've focused on silver mirroring and its ties to the direct reflection properties of metals. However, light reflected from a print is mainly a combination of two components: direct reflection and diffuse reflection. Diffuse reflection is basically light scattered at many angles. Another one of its characteristics is that it is randomly polarized (non-polarized). As a result, cross-polarization has little effect on diffuse reflections.
This variance between the two reflection types allows us to systematically separate them. In essence, cross-polarization can selectively reduce problematic direct reflections (e.g. silver metal) while leaving diffuse reflections mostly alone (non-metallic print surface). As illustrated above, this allows us to digitally capture historical photographs that exhibit silver mirroring with newfound detail.
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