PMG Silver Mirroring

Conservation Specialties

Conservation Specialties Analysis and Imaging Preventive Care Education and Professional Development Workplace Practices About Conservation Lexicon

• Photographic Material

Group

• Photographic Materials Group

Section

• Deterioration Issues
• Treatment Techniques and Procedures

Page Information
Date initiated	March 2010
Compiler	Lyzanne Gann
Contributors	Thomas M. Edmondson, Lyzanne Gann, Toshiaki Koseki, John McElhone, Douglas Nishimura, Sylvie Penichon, Stephanie Watkins

Purpose

• To physically and chemically stabilize photographic material
• To restore the image closer to original appearance before silver mirroring
• To uncover hidden information obscured by silver mirroring
  

Definition of Silver Mirroring

Formation

Silver is a common component of most historical photographic processes. Silver mirroring is a natural deterioration, inherent within silver-containing photographic material. Silver is affected by oxidizing agents and silver ions are produced. Once these ions are produced, they can migrate through the gelatin layer, towards the support (Weyde 1954). Maestre, Vergés, and Udina (2018) also attribute acidity and humidity to silver mirroring formation, with ion migration to the surface as the result. Hendriks (1991) found that silver ions can also migrate to the surface, and through reduction process at the surface, transform to silver sulfide.

Silver Mirroring is a result of a physical alteration of the colloidial surface of a photograph caused by aging. Over time, the image bearing colloid layer shrinks and conforms to the underlying structure of the substrate or image particles. These physical alterations cause a change in the optical properties and, therefore, in the appearance of the photograph (Barger and Hill 1988).

Description of appearance

Silver mirroring appears as a bluish-metallic deposit or sheen. It can appear iridescent, which may change in reflective light. If not severe, silver mirroring may not be evident in certain lighting conditions. When very severe, silver mirroring can appear bronze in color. On negatives seen in transmitted light, affected areas appear more dense or yellowed depending on the degree of damage (Barger and Hill 1988).

Most Common Occurrences

Silver mirroring can occur on any photograph or negative containing silver as the final image material. It is most commonly seen in silver gelatin developing-out-prints and negatives, albumen, and silver gelatin printing-out-processes. In small, unvarnished areas of wet-collodion glass plate negatives, silver mirroring is also occasionally observed (Hendriks 1988). Likewise, when negatives have been filed or stacked in storage, silver appears to begin along the edges of plates or along seams of storage envelopes and moves from these areas toward the center of the plate (Barger and Hill 1988, di Pietro 2000). Areas of silver mirroring on photographs sometimes correspond to areas where adhesive has been applied on the back for mounting (Penichon 2000). Swelling of the binder from high relative humidity (RH) conditions, the presence of oxidative gases, and direct contact of the emulsion with poor quality materials, such as adhesives and paper products, contribute to the formation of silver mirroring.

Other Descriptive Terms Used (besides "silver mirroring")

The terms that follow have frequently been used incorrectly to describe the presence and appearance of metallic silver that is present at the surface of a photograph.

Silver Stain, silvering, silvering out

Oxidation A broad term referring to the oxidation of the image silver to ionic silver which results in the appearance of image fading and discoloration. This can be part of a chain of reactions such as the oxidation-reduction mechanism involved in the formation of silver mirroring.

Sulfiding Refers to the conversion of silver to silver sulfide resulting in a reddish-brown or yellow-greenish discoloration that can be uneven or overall (as in sepia-toned photographs) depending in the source of the sulfur among other things. Silver is deposited around silver sulfide nuclei. The source of sulfur can be from processing or from an exterior source such as poor quality housing materials, adhesives, or airborne pollutants.

Dichroic fog The non-image forming density produced when some of the silver halide in an emulsion is dissolved during the developing process, migrates to a different location, and is subsequently reduced. The resulting finely divided silver deposit often appears to change color with changes in viewing conditions (Ray 1994). Dichroic fog occurs when hypo (sodium hyposulfite, sodium thiosulfate, or ammonium thiosulfite), ammonia, or excess sulfite contaminates the developer. It is more commonly formed if the film that is saturated with developer is passed into a neutral hypo bath without immediate rinsing or a stop bath. In this case, the development may continue and cause brown staining by brown resinous deposits that are immediately visible, not the result of aging.

Bronzing A bronze or green surface sheen on the surface in high-density areas as a result of overprinting or after gold-toning and typically seen in the albumen and printing-out processes.

Factors to Consider Prior to Treatment

• Type of photograph, such as historical, archival, art object, and its intended use in relation to exhibition and storage, as well as its relationship to the collection and institution in which it is housed.
• What is the integrity of the photograph and the artist's original intent? Is it suitable to permanently remove some of the original silver image material?
• What is the overall condition of the photograph? Can the photograph withstand mechanical, aqueous, or solvent-based treatment?
• Is the location of mirroring localized, limited to the edges, or overall? Does the location of the silver mirroring inhibit artistic or historic research or display appreciation?
• How extensive is the silver mirroring? Is the item significantly disfigured? Does the extent of the silver mirroring inhibit artistic or historic research or display appreciation?
• Will treatment procedure(s) potentially produce a worse condition than the silver mirroring? Aqueous treatment may cause distortion, and specifically, curling. Aqueous treatment or strong eraser pressure could exacerbate micro-chip cracking. Uneven reduction of silver mirroring may result from treatment method(s) used.
• Silver mirroring may return after treatment (Luzeckyj and Bruckle 1999).
• Conservator's treatment must be governed by informed respect for the aesthetic, conceptual, historic, and physical integrity of the photograph. This respect is based on thorough understanding of the object that comes from careful examination, curatorial consultation, and historical research.
  

Effects of Damage and Consequences of Treatment

Arguments in Favor of Treating Silver Mirroring

• Silver mirroring can be very disfiguring. Treatment provides aesthetic enhancement and addresses the artist's original intent.
• Perception of detail can be regained in the maximum density (D-max) areas.
• Removal of silver mirroring enables retrieval of historically valuable information.
• Silver mirror layer can accelerate deterioration of remaining silver particles.

Arguments Against Treating Silver Mirroring

• The particles comprising the silver mirror are part of the original material.
• Removal of the silver mirror creates an active surface susceptible to oxidation.
• Image density may be lost during chemical treatment because the extent of treatment is not easily controllable. The image may lighten overall or become uneven in appearance.
• Treatment may be damaging to components of the photograph such as causing microcracking in the gelatin binder or removing matting agents held in the binder layer.
• Depending on the coating material used and surface characteristics of the photograph, residual material may remain behind on a microscopic level causing future differential aging.
• Treatment can result in a change in surface gloss.
• Treatment is not reversible.
• Silver mirror layer may not be more harmful than wet cleaning with cotton swab or non-aqueous surface cleaning with soft eraser.

Treatment Strategies

Please note that the extent of success of a treatment to reduce mirroring is a function of the condition of the object, including the extent of mirroring, the surface texture, the condition of the binder, and the skill and experience of the practitioner. "When non-intervention best serves to promote the preservation of the cultural property, it may be appropriate to recommend that no treatment be performed" (Guidelines for Practice of the American Institute for Conservation). Many of the strategies listed below are either from student investigations in the mid1980-1990's, or research on mock-ups or expendable materials. Please use your best professional judgement when applying these techniques.

Physical Abrasion or Manipulation to Reduce Silver Mirroring

Physical treatments may prove to be as effective and ultimately far less destructive than chemical methods (Baas 1985). The disadvantage is the risk of visible and microscopic abrasion to the emulsion surface.

Erasure

Materials tested include poly (vinyl) erasers by brand: Mars Plastic 526 50 by Staedtler Mars; Magic Rub by Faber Castell; and Pentel ZS-11 by Pentel

• Advantages
  • Solvents are not involved for moisture or solvent sensitive items.
  • Physical erasure is an effective removal of mirroring depending on the individual photograph being treated.
  • Physical erasure is an easily controllable, localized treatment.
  • While not a "dry" technique, uneven results can be corrected with a 50% solution of water:ethanol.
• Disadvantages
  • Surface of the photograph, whether glossy or matte, affects the results.
  • Matte surfaced photographs seem significantly altered from removal of matting agent particles (Luzeckyj and Bruckle 1999).
  • The image can be abraded during treatment procedure.
  • Loss of density is irreversible.
  • Uneven cleaning results in an irregular surface.
  • Residual particles of eraser may chemically interact with photographic components.
  • Residual deposition of eraser components, such as sulfur and vinyl eraser plasticizers, might be harmful in the future (Bernier 1997).
    • Staedtler Mars eraser: Staedtler Mars erasers can remove mirroring irregularly.

Other Abrasives

Materials tested include dry swab; Fuller's Earth (clay form of diatomaceous earth) applied on a swab, followed by water:ethanol rinse; anhydrous micro-alumina powder (variety of grits) and ethanol applied in a slurry. Quantity of the ethanol can vary the abrasiveness of the alumina.

• Advantages
  • Abrasive materials reduce silver mirroring, although results vary from good to marginal.
  • Like erasers, the use of abrasives is fairly controllable.
  • Work can be done very slowly under the microscope.
  • Ethanol can be used to reduce any residual powder.
    • Microalumina powder (0.03 microns) seems to work well on photographs with slight texture, without increased gloss (Baas 1985).
    • Microalumina powder (0.05 microns) mixed with ethanol can remove mirroring from very smooth gelatin photographs, leaving no apparent residue (Baas 1985).
• Disadvantages
  • Abrasion of the surface beyond the intended treatment area is possible.
  • Residues of the abrasive used can be difficult to remove completely.
  • Abrasives used on textured photographic surfaces polishes the raised surface areas to a higher glossy reflection and may not completely reduce the silver mirroring in the "valleys" of the paper texture.
  • Deterioration of binder and supports can occur from unknown additives, such as acids, in proprietary product mixtures.
    • Dry Swabs. Tests on mock-ups resulted in mainly uneven appearance.
    • Microalumina powder (0.05 microns) The microalumina powder does not remove silver mirroring on photographs with slight or pronounced texture, but rather increases the gloss (Baas 1985).

Surface Applications and Alterations

Water (moisture, vapor)

Water is applied by rolling a swab gently across the intended treatment area.

• Advantages. Moisture swells gelatin creating a more expedient removal of the mirroring.
• Disadvantages
  • Use of water is an irreversible chemical and physical interaction of the photographic material. Swelling of the binder and paper can result in a tendency of "frilling"" and "chipping" at the edges of the photograph (Baas 1985).
  • The introduction of moisture can also lead to delamination of the photograph from the mount, or distortion of the object.
  • Water or water vapor may not remove the mirroring as well as saliva or acetone.
  • More physical pressure on the photograph is required to gain the same removal result as with water:ethanol solution or saliva.

Saliva (mild enzymatic solution) followed by water:ethanol rinsing

Saliva is applied by gently rolling a swab across the intended treatment area.

• Advantages. Saliva can more effectively remove mirroring than water, especially when followed by an application of water:ethanol to even out the surface. Saliva swells the gelatin efficiently for an expedient removal of the mirroring. Saliva can work more effectively on glossy surfaces than matte surfaces.
• Disadvantages. Although less than with water alone, the gelatin swells on application of saliva and may initially appear to be increasing the mirroring. After the mirroring is reduced, the surface can appear iridescent. Even if sufficiently rinsed with water or ethanol, saliva (mild enzymes) can remain within the gelatin structure and on the surface.

Water, Alcohols, and Organic solvents

During a student workshop, tests of solvents and water:solvent combinations were tried: These included water:ethanol; ethanol; acetone; xylenes; heptanes; and water:Kodak Photo-Flo (from SDS sheet: combination of 60-70% water; 25-30% Propylene glycol; and 5-10% p-tert-octylphenoxy polyethoxyethyl alcohol) applied with a swab. Tricholorethylene (TCE) and tetracholorethylene (PERC) with calcium carbonate were tested prior to 2018 (Maestre, Vergés, and Udina 2018). Please consult current SDS for health and safety data. Use of a fume hood and personal protection equipment is advised when using organic solvents

• Advantages
  • Acetone, xylenes, and heptanes provide non-aqueous treatment options for water-sensitive materials.
  • Water:solvent combinations, including Kodak Photo-Flo, reduces the water exposure for water-sensitive materials during treatment.
  • Water:ethanol mixture The addition of ethanol lessened the swelling of the gelatin during the treatment procedure. The water:ethanol mixture worked well for reducing tide lines when applied after saliva procedure. The combination of saliva first, then clearing with water:ethanol mixture produced the most notable change for mirroring (Knipe 1997). When applied first, no surface change was observed. Glossy samples appeared unaltered by the procedure, whereas matte surfaces appeared more altered.
  • Heptanes Results can vary from little or no reduction to good results depending on the photograph.
  • Xylenes Results can vary from little or no reduction to good results depending on the photograph.
  • Water:Kodak Photo-Flo Notable disturbance of mirroring (Knipe 1997).
  • Trichloroethylene (TCE) or tetracholorethylene (PERC) with calcium carbonate: silver mirrored gelatin on glass test samples (Maestre, Vergés, and Udina 2018).
    • Gelatin is not hydrated with these two solvents and gelatin dried out less than alcohol use.
    • Silver gelatin removed, along with superficial scratches.
    • Mold (biologic materials) and fingerprints were also diminished with this process.
• Disadvantages. Overall, these methods are less effective at reducing mirroring than using water alone.
  • Water:ethanol mixture. Voids corresponding to the shape of matting agents, such as starch and silica, were possibly dislodged by swelling the gelatin and abrasive action of the swab (Luzeckyj and Bruckle 1999).
  • Ethanol Ethanol used alone had little to no effect in clearing the silver mirroring. In addition, surface analysis under scanning electron microscope indicated that ethanol may dehydrate photographic surfaces (Knipe 1997).
  • Heptanes Results can vary from little or no reduction to good results depending on the photograph.
  • Xylenes Results can vary from little or no reduction to good results depending on the photograph.
  • Trichloroethylene (TCE) or tetracholorethelyne (PERC) with calcium carbonate: silver mirrored gelatin on glass test samples (Maestre, Vergés, and Udina 2018).
    • TCE is banned in the European Union specifically for vapor degreasing, although allowed for other purposes.
    • PERC is banned in California, USA for use in dry cleaning because of toxicity and cancer risks.
    • Visual, "superficial scratches" can be removed with this process, even though calcium carbonate was considered softer than the gelatin used in the testing.
    • Scratches of less than 2 microns can occur. This was attributed to the use of cotton swabs. Potentially, alternative non-cotton swabs, such as rayon, may be a safer tool option.

Coatings

Coatings applied to the surface of the photograph change the index of refraction thereby reducing the appearance of the silver mirroring.

Waxes

Materials tested include microcrystalline wax, Renaissance wax (proprietary mixture of microcrystalline wax with solvents), Carnuba wax, and paraffin (historical use).
Application to the surface can be done with swab application, within a solvent slurry, and direct placement of small shavings or pieces under a sheet of silicon release Mylar, that are then heated with a tacking iron. After application, the surface can be buffed with a soft cloth or cheesecloth to the desired sheen of the surrounding surface reflectance.

• Advantages
  • Adding a coating over the silver means that silver is not removed from the photograph. The wax coating saturates the surface of the photograph. It is easy to apply an even coating layer. The coating may inhibit or slow the recurrence of mirroring to some degree (Luzeckyj and Bruckle 1999).
    • Carnuba wax hardens nicely and has good aging characteristics. Commercial formulations are available and can be self-formulated as needed (Koseki).
• Disadvantages
  • The addition of wax is not practically reversible or completely removable. The addition of a wax coating has the potential to dramatically alter the surface character, especially with matte-surfaced photographs. Applying the coating to a mounted photograph requires a template protection of the mount margins or they will be waxed as well. Wax can attract and trap dust creating future abrasion and acidity problems for the photograph. The wax coating may also blanch or change in appearance over time.
    • Paraffin wax was used historically, and is now considered inappropriate for conservation treatment use because of its tendency to yellow with age. Other waxes are better alternatives at this point in time. Synthetic resins are used as a varnish applied locally or overall, with either brushes or by spraying. Use of an airbrush allows greater control of thin applications of resin (Gann). The varnish should air-dry while the photograph is in a dust-free, protected environment.

Acrylic Resins

Materials used include:

Paraloid/Acryloid B-67 (isobutyl methacrylate polymer) and

Paraloid/Acryloid F-10 [poly (butyl methacrylate polymer) with matting agent;

Paraloid/Acryloid B-72 [poly(ethylene methyl acrylate), methyl methacrylate, P(EMA/MA)]

• Advantages
  • The resins chosen have good aging characteristics that are well documented within the paintings conservation literature. Gamblin, Magna colors, or other dry pigment with resin binder combinations may be used in visual compensation of the design layer when necessary.
    • B-67 and F-10 mix (Liquitex Matte Soluvar). The resins can be mixed directly or purchased in a spray can ready for use. Airbrush use allows more control with an overall application and more even results (Gann).
    • B-72 in solvents Volatility rates will change with different solvents allowing some control for the operator. Using a slower evaporating solvent, such as toluene or xylenes, may increase application and manipulation time. Use of dilute mixtures in slow evaporating solvents applied with an airbrush allows for more control of application and better results. Brush application of B-72 may saturate the photographic support throughout, yet, it may be the easiest method to apply an even coat to matte surfaces (Penichon).
• Disadvantages
  • Inpainting medium choices are restricted to those that will adhere evenly to resins. For example, watercolors cannot be used. The solution can become thick quickly when using a fast evaporating solvent. Application results can be uneven when using faster evaporating solvents, such as acetone.

Cellulose Ethers

Cellulose ethers are applied by brushing a 1-2% solution or by spraying a solution not exceeding 0.5% with an ultrasonic humidifier, nebulizer, or other equivalent device. The surface is protected from dust during the drying stage. Materials used include:

Ethylhydroxyethylcellulose (Ethulose) in water or ethanol

Hydroxypropyl cellulose (Hercules Klucel G) in ethanol

Hydroxypropylmethyl cellulose (1%) in ethanol

Methylcellulose (1%) in water

• Advantages
  • Cellulose ethers offer the best results of the tested material for application and reversiblity with non-polar solvents, such as xylenes, instead of ethanol to reduce swelling of the binder and achieve clear surface removal. Cellulose ethers are non-toxic, easy to apply with a soft brush, and readily available in most conservation studios and labs. Applied in very low concentrations, cellulose ethers can sufficiently and evenly saturate photographic surfaces. Cellulose ethers seem to inhibit silver mirroring recurrence to a certain extent (Luzeckyj and Bruckle 1999).
  • Ethylhydroxyethylcellulose (Ethulose). Ethulose delivers a cleaner, more homogeneous coating than methylcellulose, hydroxypropyl cellulose and hydroxypropylmethyl cellulose.
• Disadvantages
  • Some skill and finesse is needed to apply cellulose ether layers evenly.
  • Cellulose ethers can be difficult to remove or reverse depending on the product chosen and the deterioration state of the photographic material.
  • Cellulose ethers may not saturate enough to conceal mirroring. A reduction, not a complete masking, of mirroring can result.
  • Cellulose ethers often dry "shiny" (reflective surface) that may be different than the original surface quality, or may result in iridescence.
  • Use of cellulose ethers can restrict the choices of inpainting media to those that can adhere to the surface (e.g. Gamblin, Magma colors, pencil).
  • A cellulose ether layer can attract and trap dust and dirt causing future abrasion and acidity problems for the photograph.
  • Feller and Wilt (1990) raised questions about the long term stability and yellowing potential for some of the cellulose ethers. However, mirroring often occurs in darker areas of the photographic image where the color shift to a more yellow tone may be of negligible consequence in exchange for an integrated image (Watkins).

Gelatin

Liquid gelatin applied with a brush.

• Advantages
  • Gelatin can be hardened with formaldehyde mixed in or exposed to formaldehyde vapor.
  • It is easy to inpaint on a gelatin sized surface and the dried surface can be less shiny (lower reflectance) than wax.
• Disadvantages
  • The inherent strength of gelatin can cause distortion of the photograph and "bubble" and delaminate from a mount during drying.
  • If the gelatin is not hardened, it can adhere to adjacent materials if damp in the future.
  • Being hygroscopic, gelatin may attract and trap dust causing abrasion and acidic deterioration for the photograph in the future.
  • Subtle changes to the surface appearance can occur with the application of gelatin. Glossy photographs tend to develop an iridescent surface appearance.
  • Gelatin may not coat evenly depending on the surface and current condition of the binder.
  • Local treatments are easily evident in different lighting conditions. Therefore, gelatin requires very thin, multiple applications over the entire surface for the best results, yet, very thin applications can result in an iridescent surface appearance.
  • Unhardened gelatin is reversible in water, so removing the gelatin layer may also result in removing the mirroring.
  • If hardened with formaldehyde, the gelatin can be difficult to remove or reverse.

Chemical Treatments

As noted by Hendriks, et.al. in 1991 (p. 357), "The chemical treatment of a deteriorated photographic image is a controversial issue amongst conservators, photographers, dealers, and collectors." Into the twenty-first century, the treatments described herein remain controversial.
EXTENSIVE DAMAGE AND COMPLETE LOSS CAN BE THE UNINTENDED RESULT FROM THE PROCEDURES DESCRIBED IN THIS SECTION.
PLEASE USE INFORMED, CAUTIOUS, PROFESSIONAL JUDGMENT.

Please consult the latest version of the appropriate Safety Data Sheets (SDS) for current chemical, health, and safety information before performing any of the treatments described herein. Many links to free SDS databases exist online.

Iodine-Alcohol (Weyde 1955; Hendriks, et.al. 1991)

The procedure begins with mixing powdered iodine in an anhydrous alcohol. Weyde's original recipe called for a 1% solution, however, practitioners have found that a 0.1% solution of iodine to alcohol is preferred (Albright, Bisi, Edmondson). The 1% solution can eliminate your image and some wonder if it wasn't a typographical error in the original manuscript. Once your iodine is in solution, the photograph is immersed with constant agitation up to 3 minutes. The photograph is then subjected to a typical chemical processing cycle following the developer stage. Next, the photograph is placed in fixer from 1 minute (Weyde) to 5 minutes (Hendriks), then rinsed in water for 1 minute. Immersion in a hypo clearing agent bath is next at 30 seconds (Weyde) or 3 minutes (Hendriks), followed by another water wash, 3 minutes (Weyde) and 20 minutes (Hendriks) recommended. The entire treatment is finished with a 30-second rinse in a wetting agent, such as Kodak Photo-Flo, then air dried in a dust-free environment.

• Advantages
  • The procedure is less aggressive than other chemical treatments.
  • The morphology of the image silver is not altered (Hendriks 1991).
  • The alcohol penetrates the gelatin very slowly so that the iodine has access only to the metallic silver on the surface (Hendriks 1991).
  • The iodine appears to have a stabilizing effect on the remaining silver image (Brandt 1984, Nielson 1993, Luzeckyj and Bruckle 1999).
  • The treatment does not require working under a safelight.
• Disadvantages
  • Iodine is toxic, a severe oxidizer, and a severe corrosive material (see a current SDS).
  • Extreme caution when handling and treatment performed within an exhaust fume hood is recommended.
  • The photograph must be immersed and the procedure does not work locally.
  • Immersing photographs can result in interlayer cleavage and distortions.
  • Delamintation of the gelatin emulsion from the baryta layer was observed in non-image areas of freshly processed samples (Luzeckyj and Bruckle 1999).
  • It is difficult to control the extent visually because of the color of the solution.
  • The procedure inhibits future re-fixing treatments.
  • The treatment may leave behind residual thiosulfate.
  • The treatment is irreversible: Silver is actually removed from the image (Hendriks 1991) although reaction is limited to the surface as long as the alcohol does not contain much water.
  • Alteration of the tonality of the print may occur (Luzeckyj and Bruckle 1999).

Iodine vapor chamber (Bisi, Albright, Smith 2009)

A variation on Weyde's concept, a photograph is placed in a sealed glass vapor chamber (e.g. glass desiccator) with an interior glass dish containing iodine crystals is placed at the bottom. The photograph is exposed to iodine vapor for 30 seconds to 5 minutes. Next place the photograph in a commercial fixer, agitating constantly for 10 minutes. A running good quality water rinse follows for 1 minute before placing in hypo clearing agent for 3 minutes. Another clearing running water bath for 30 seconds is followed by a rinse with a wetting agent, such as Kodak Photo-Flo, for 30 seconds. The final step is to air dry the photograph in a dust-free environment.

• Advantages
  • Slow application of vapor allows more control during treatment than Weyde's iodine-alcohol solution described above.
  • This procedure is less aggressive than other chemical treatments listed here.
  • Iodine creates a potential stabilizing effect on silver images.
• Disadvantages
  • Iodine is toxic, a severe oxidizer, and a severe corrosive material (see a current MSDS). Extreme caution when handling and treatment performed within an exhaust fume hood is recommended.
  • Construction of a vapor chamber is required. Glass desiccators are expensive and limited in size.
  • The photograph being treated must be able to withstand aqueous treatment.
  • Some changes were noted in the low-density regions of the photographs tested, requiring further investigative research.

Acidified Thiourea (C. Fischer, n.d.; Kodak; Lavedrine 1990)

To begin, the photograph is cleaned with good quality, purified water (e.g. distilled) for about 1 minute. The photograph is gently, lightly wiped with cotton dampened with alcohol. Again with cotton, an ammoniated water solution (90 mL:265 mL or approximately 1:3) is gently applied to both sides of the photograph. The photograph is then rinsed with water for 1 minute followed by a gentle wipe of the surface of the image using cotton with the following solution: water 450mL (475 mL in Kodak reference), thiourea 19 g, and phosphoric acid 10 mL. Another water rinse of 1 minute follows. The rinse is followed by a 1 minute agitated fixer bath, again rinsed in water for 1 minute. The next step, a hypo clearing agent bath of 30 seconds is followed by a three minute running water wash and a final 30 second rinse with a wetting agent, such as Kodak Photo-flo. The photograph is air dried after treatment.

• Advantages
  • Effective treatment that can be used on plates, paper prints, and negatives.
• Disadvantages
  • Wearing personal protection equipment and performing work in an exhaust fume hood is recommended.
  • Chemical components in this recipe pose serious health hazards. For example, thiourea is a known carcinogen.
  • If not acidified, thiourea tends to soften and dissolve gelatin. Treatment causes an overall smoothing of the colloid layer, which alters the surface structure (Barger and Hill 1988, similar observation in a Kent workshop).
  • There is a potential for residual chemicals to remain that could react with the silver in the future.
  • Treatment appears to have little effect on the transmission density of silvered negatives (Barger and Hill 1988).

Ammonium Thiosulfate (Kodak 1985; Hendriks and Ross 1988; Lavedrine 1990)

After the photograph is cleaned well of dust, debris, and all surface oils from handling, it is immersed in the following solution for a few minutes, then wiped gently with a cotton swab. The solution is 2 volumes of purified water; 1 volume of Kodafix hardener (containing ammonium thiosulfate); and 15 g/L of anhydrous citric acid. Kodafix is a proprietary Kodak company mixture of water, ammonium thiosulfate, sodium acetate, sodium bisulfite, acetic acid, boric acid, and aluminum sulfate (see a current MSDS). After immersion in the solution, rinse in good quality water for 1 minute, followed by a hypo clearing agent bath of 30 seconds, another good quality water rinse for 3 minutes and a final 30 second rinse with a wetting agent, such as Kodak Photo-flo.

• Advantages
  • The ammonium thiosulfate is a colorless solution, so action of the solution on the photograph can be easily observed during the treatment.
• Disadvantages
  • CAUTION: This treatment has a reducing effect on the silver, therefore details and density may be lost if object is left too long in the bath.
  • The photograph must be able to withstand full immersion and the progress is difficult to control.
  • The treatment should not be performed near light-sensitive photographic products.
  • Citric acid, when added to fixer can provoke sulfurization of thiosulfate (Kodak 1985).
  • The solution is toxic, and gives off a strong odor of sulfur dioxide.
  • Wearing appropriate personal protection equipment and doing the procedure in an exhaust fume hood are recommended.

Silver Polish

Commercially available silver polishes work through a combination of abrasion and chemical action (reduction reaction). Polishes are usually applied locally with a swab or soft cloth. Instead of using commercially available brands, McElhone recommends making your own mixture with jeweler's rouge, thereby eliminating any thiourea components.

• Advantages
  • Commercial silver polish is easily available, ready made, and easy to use.
• Disadvantages
  • Commercial silver polishes can be difficult to remove completely from the surface, sometimes leaving a pink residue remaining on the surface.
  • Proprietary mixtures typically contain thiourea and exact compositions are often unknown.
  • Commercial silver polish recipes may change without notification.

Ammonium Hydroxide

Ammonium hydroxide swells the gelatin making the silver-mirroring easier to remove from the surface.

• Advantages. Easy to prepare and use.
• Disadvantages. Ammonium hydroxide is physically aggressive, potentially interacting with the silver in the photograph beyond the mirroring.

Bibliography

Baas, Valerie 1985. Reduction of Silver Mirroring on the Photographic Surface by Other Than Chemical Means. Lecture Abstracts from the 5th Annual Winter Meeting, Philadelphia. Washington, D.C.: American Institute for Conservation's Photographic Materials Group, 1-2.

Baines, Harry. 1970. The Science of Photography. London, England: Fountain Press.

Barger, M. Susan. and Thomas T. Hill. 1988. Thiourea and Ammonium Thiosulfate Treatments for the Removal of "Silvering" from Aged Negative Materials. Journal of Imaging Technology 14 (2): 43-46.

Bernier, Brenda. M. 1997. A Study of Poly (vinyl chloride) Erasers Used in the Surface Cleaning of Photographs. Topics in Photographic Preservation 7. Washington, D.C.: AIC-PMG, 10-18.

Bisi, Sarah, Gary Albright, and Gregory Dale Smith. 2009. Purple Haze. Paper Presented at the American Institute for Conservation's Photographic Materials Group Annual Winter Meeting, January 23-25, 2009.

Brandt, E. S. 1984. Mechanistic Studies of Stability. 2: Iodide Adsorption on Silver in the Presence of Thiosulfate and the Influence of Adsorbed Iodide on the Catalytic Properties of Silver Toward Hydrogen Peroxide. Photographic Science and Engineering 28 (January-February) 13-19.

Crabtree, J.I. 1933. Stains on Negatives and Prints. Rochester, NY: Eastman Kodak Company.

Di Pietro, Giovanna. 2003. Silver Mirroring. In German and Italian.

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Udina, Rita. 2019 (March 14). Restauració D'obra gráfica, 2 minute video on research regarding the cleaning of silver mirroring from photographic materials.

Ware, Mike. 1994. Mechanisms of Image Deterioration in Early Photographs: The Sensitivity to Light of W. H. F. Talbot's Halide-Fixed Images 1834-1844. London, UK: Science Museum and National Museum of Photography, Film and Television.

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