Silver mirroring

Owned by Fixer Faults, Washing Faults and the Stains They Leave, which is precise about a distinction this entry has to carry with it: the oxidative attack comes from the environment rather than from the darkroom, so a badly washed print in a good enclosure and a well washed print in a bad one can arrive at the same place. It draws the same discriminator — mirroring shows in reflection, processing stains in transmission — from the conservation catalogue for gelatin dry plates, and repeats the handling warning that a mirrored area is extremely susceptible to abrasion. This entry was written earlier from permanence and image deterioration.

What you see

A bluish metallic sheen, visible under reflected light and not by transmission, starting from the edges and sometimes covering the whole surface.

Three details make it identifiable rather than merely nameable, and the first two are the diagnosis.

It is seen by reflection, not transmission. Improperly washed processing chemistry does the opposite - it appears as yellow or brown stains in transmitted light. Looking at a plate and then through it separates the two.

It starts at the edges, because that is where the atmosphere reaches first.

The affected areas are extremely susceptible to abrasion, which is a handling instruction rather than a diagnostic, and an important one.

Likely causes

1. Oxidising gases from the atmosphere — ozone, nitrogen oxides, peroxides.
2. Poor-quality mount, mat or framing materials in contact with the print. Lignin is named as a cause of fading, mirroring and severe yellowing.
3. An enclosure or a climate that concentrates pollutants where the print’s edges are.

What is happening: the chemistry and physics

The Image Permanence Institute states the sequence plainly for a framed print — the reactions from poor framing materials, “like those we see from air pollution, are often oxidation reactions that result in image fading. In a black-and-white image the faded silver can migrate to the surface of the print and be converted back into metallic silver by other pollutants … forming a mirror-like sheen on the print’s surface.”

So it is three steps. An oxidant takes an electron from an image grain. The silver becomes a mobile ion and migrates through the gelatin towards the surface. At the surface something reduces it again, and it deposits as a thin metallic layer.

Ag⁺ + e⁻ → Ag

Silver mirroring - the mobile ion is reduced back to metal, at the surface

The image silver is lost as image silver either way. What distinguishes mirroring from a general fade is only where the silver ends up, and that is decided by whether there is something at the surface to reduce it.

📓IPI's own control experiment, found rather than designed

The framing guide prints the perfect demonstration — a print whose edges mirrored under a poor-quality mat while the oval the mat window left uncovered stayed clear. The mat was the source, and the geometry of the damage says so without any test at all.

If your own print mirrors in a pattern that matches something it was in contact with, you have the same evidence.

Diagnostic questions

• Look at it, then look through it. Reflected-light sheen is mirroring. Transmitted-light yellow or brown staining is residual thiosulfate or retained silver.
• Does it start at the edges, or follow the outline of something the print was in contact with? Either points at the enclosure and the atmosphere rather than at the processing.
• What has the print been stored in, mounted on, or framed with?
• Do the enclosure materials claim to pass the Photographic Activity Test (ISO 18916)? That test uses a detector screening for the oxidation and reduction reactions that cause image fade, mirroring and red or gold spots. “Acid-free”, “archival”, “museum-quality” and “conservation board” are marketing terms that are neither standardised nor legal, so the claim to look for is the standard, not the adjective.

Corrective action

None that this course will give. Removing mirroring is conservation treatment on an original object, and the mirrored areas are extremely susceptible to abrasion, so an attempt to clean one is likely to take the image with it.

The correct action is to remove the cause and stabilise the storage — take the print out of the enclosure that produced it, and keep it somewhere with a better climate.

Prevention

Choose enclosures by the standard rather than by the adjective, and prefer materials that print a claim to meet ISO 18902 and to pass the Photographic Activity Test.

Improve the climate before you improve the enclosure. IPI are explicit that enclosures cannot overcome deficiencies in the storage climate and that improving the climate is more effective overall. Their recommendation for gelatin glass plate negatives is below 18 °C and 30 to 40 per cent relative humidity.

Toning converts the image silver to a more stable compound — silver selenide with a selenium toner, silver sulfide with a brown toner — and Kodak’s toning manual states that all its toners protect the image whether or not they shift the colour. It is a preventive treatment applied at the time of processing, not a remedy afterwards.

Handle a mirrored surface as little as possible, and support it rather than sliding it.