PMG Section 1.3 Effects of Exhibition on Photographic Materials

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Exhibition Guidelines for Photographic Materials

Date: July 2004
Compiler: Stephanie Watkins, 1993-2004
Initiator: Douglas Severson, 1992-1993
Contributors (Alphabetical): Catherine Ackerman, Nancy Ash, Sarah Bertalan, Jean-Louis Bigourdan, Barbara N. Brown, Ed Buffaloe, Carol Crawford, Corinne Dune, Thomas M. Edmondson, Debra Evans, Julia Fenn, Betty Fiske, Gwenola Furic, Judy Greenfield, Doris Hamburg, Marc Harnly, Pamela Hatchfield, Cathy Henderson, Nancy Heugh, Ana Hofmann, Emily Klayman Jacobson, Martin Jürgens, Nora Kennedy, Daria Keynan, Lyn Koehnline, Barbara Lemmen, Holly Maxson, Constance McCabe, John McElhone, Cecile Mear, Jennifer Jae Mentzer, Jesse Munn, Rachel Mustalish, Douglas Nishimura, Leslie Paisley, Sylvie Pénichon, Hugh Phibbs, Dr. Boris Pretzel, Dr. Chandra Reedy, Nancy Reinhold, Andrew Robb, Grant Romer, Kimberly Schenck, Douglas Severson, Tracey Shields, Angela Thompson, Sarah Wagner, Clara von Waldthausen, Dr. Mike Ware, Stephanie Watkins, Dr. Paul Whitmore, Faith Zieske, Edward Zinn.

First edition copyright: 2004. The Photographic Materials Conservation Catalog is a publication of the Photographic Materials Group of the American Institute for Conservation of Historic and Artistic Works. The Photographic Materials Conservation Catalog is published as a convenience for the members of the Photographic Materials Group. Publication does not endorse nor recommend any treatments, methods, or techniques described herein.

Effects of Exhibition on Photographic Materials[edit | edit source]

Photographs are created to be viewed by present and future generations. Later generations have attached importance to particular photographs, giving rise for the need for protective measures that promote the longevity of the material. In any exhibition, especially multi­-venue exhibitions, there are unknown elements posing risks for which only a limited number of precautionary measures can be made. Irreversible chemical and physical damage that will alter the perception of a photographic item for future generations can result from exhibition duration or frequency and from movement of an item from one geographical location to another. Loss of aesthetic and historic information can result. It is advisable to monitor the changes and damages during exhibition in order to limit damage and reduce situations that have potential for damage in the future.

Effects of visible and ultraviolet light[edit | edit source]

Light damage is cumulative and permanent. Light exposure causes both fading and darkening. Photographic materials may be returned to dark storage for a period after exhibition. Although termed a "rest period" implying a restorative or recuperative effect on photographic materials after light exposure, restoration does not occur because light damage is not reversible. Periods of "rest" in between periods of display only ration the useful life of photographs between present and future generations.

By type of print[edit | edit source]

  • Very early photographic materials may not be suitable for exhibition. A photogenic drawing of a piece of linen fabric made by William Henry Fox Talbot, possibly around 1835, darkened by printing out within five weeks of exhibition (5 footcandles (fc), 53.8 lux (Ix) tungsten source; 8 hours per day for 35 days while under UP-4 Plexiglas®). The climate was 70F±2° (21°C) and 50%±5% relative humidity (Reinhold 1993a and 1993b, 91-92; Ware 1994,47). Even monitoring of these types of photographs with densitometers can cause localized "fogging" and darkening. "The light sources in commercial reflectance densitometers are very intense (in the order of [10,000-15, 000 fc] 100-500 kilolux) and may cause a significant darkening of the small, but visible area of illumination (usually about [3/8 inch] 5 mm in diameter) during the time of taking a measurement. The use of finely graduated stepped gray scales on which are recorded the steps corresponding to key areas of images: the estimate would be made simply by visual comparison" is further suggested by Ware (1994, 55-56).
  • The worse the condition of the photograph, the less it can tolerate further degradation.
  • Kodak's charting of light fading indicates that there is little change in the early life of a photograph, but that change increases with time. However, upon exposure, pristine albumen prints visibly fade more than already faded albumen prints. "The rate of image deterioration in [photographic prints] is closely related to their condition. Without exception, the prints in better condition showed more density change than those in poor condition. Platinum and palladium prints have a reputation for extreme stability, but [one palladium print that was exhibited for nine weeks] yellowed considerably in the middle tones and shadows. One might assume the change is a yellowing of the paper base due to the acidic nature of the process, but the absence of highlight yellowing tends to contradict that notion" (Severson 1987, 135). Yellowing might be the catalytic effect due to the action of light, in which palladium causes faster yellowing of the paper support in image areas than in non-image areas.
  • The iron compounds that constitute cyanotype images may be faded by light exposure. This fading may be partially reversible by dark storage (Reilly 1986, 43), but that is not always possible nor can reversibility be infinitely cyclical. Therefore, the phenomenon of reversal should not be assumed or used as a basis for increasing exhibition light levels (see section on light levels for extremely sensitive materials).
  • Properly processed image silver is not directly affected by ultraviolet light exposure. However, residual silver halide, such as that present in unfixed or partially fixed prints, remains light sensitive. Many of the silver-thiosulfate and other silver complexes that accumulate in poorly washed silver prints can be photo-oxidized, causing staining or further reactions. The factors (high relative humidity, high temperature, oxidizing chemicals, and light) affecting oxidative-reductive deterioration of image silver have been reviewed by Hendriks (1989, 645-50). Some products of photo-oxidation of organic components may oxidize metallic silver (Eastman Kodak 1985, 108; Reilly 1986, 103). Silver ions, produced by the action of moisture and oxidizing agents, may be reduced by light exposure (Eastman Kodak 1985,84).

Coatings[edit | edit source]

  • Organic coatings tend to deteriorate, craze, crystallize, darken and yellow from light exposure. 19th century photographs may have various types of organic coatings including polymers derived from cellulose (collodion), natural water-soluble polymers (polysac­charides such as gum arabic), proteins (gelatin, albumen, casein), natural resins (Canada balsam, copal, dammar, mastic, rosin, shellac), beeswax, drying oils (such as linseed), essential oils (such as lavender oil), and hydrocarbons (paraffin wax, rubber, caoutchouc). For example, early albumen prints with watercolor retouching may have first been coated with a base coating of ox gall, gum arabic, alum, alcohol, or other materials (von Waldthausen 2003b). In addition, uncolored ambrotypes, tintypes, and autochromes are manufactured with varnish and lacquer.
  • Synthetic coatings made from modern polymers may also be present in the construction of a photograph. Polymers may darken (becoming gray or yellow), cross-link, and craze caused by light exposure. Inkjet dye prints may be coated or laminated with an ultraviolet protectant layer to protect the light sensitivity of the colorants. Some black-and-white Polaroid® prints are stabilized with a proprietary coating. The light stability of synthetic coatings used on prints is not well characterized in photographic conservation literature. Polaroid Corporation (1983, 33) additionally recommended for its products the application of print lacquers, such as McDonald Photo Products, Incorporated's proprietary product, Pro-Tecta-Cote™, to protect against physical damage such as fingerprints and abrasion.

Color dyes and retouching[edit | edit source]

  • Color stability depends on the nature of the dye (von Waldhausen 2003a). Organic dyes comprising the photographic image, or present as filters or sensitizers, are subject to light ­induced changes in density (Giles et al. 1973; Wilhelm 1979); ultraviolet radiation is particularly active in this respect. Light exposure induces changes in the density of dyes that results in a color shift or fading of the image (Schwalberg et al. 1990). "Black-and­ white" chromogenic prints and CAl process black-and-white prints will fade because of their color dye construction. Dyes used to tint the paper supports or binders of some albumen ) prints are extremely fugitive (Burgi 1982; Reilly 1986). Optical brighteners introduced in photographic manufacture in the 1950s are also fugitive and can discolor upon exhaustion (Messier 2000; Mustalish 2000).
  • Monochrome and color prints have frequently been heightened, retouched, or painted overall with a variety of media that may constitute the most fugitive components of a print. The black color of a paint or ink is no guarantee of its permanence as it may be composed of several fugitive colorants.
  • Daguerreotypes, ambrotypes, and tintypes with applied color, even the most vividly colored images, have very little pigment in comparison to pastels, watercolors, or oils, which means there is the smallest margin before visible fading can be recorded. Uncolored daguerreotypes are not particularly light sensitive, but the materials of the components comprising the case and daguerreotype package may be far more sensitive than the image (see section
  • Receptor coatings can be titanium dioxide (Jurgens 2000, 63). Some cationic fixing agents applied to paper can be detrimental to magenta dye chromophores (Lavery, et al. 1998, 329). Inkjet dye-based images and chromogenic prints are generally among the most light sensitive color processes (von Waldthausen 2003). The light-fastness of inkjet dyes will be improved if the substrates on which the dyes are printed do not generate oxidants, reductants, or radicals inherently (Gregory 2000,50). Despite the complications, dyes used in inkjet printers are steadily improved, as the industry is aware of consumer and photographic artist's concerns for light fastness and product longevity.

Binders[edit | edit source]

  • Proteins are subject to deterioration from visible and ultraviolet light. The albumen binder offers some protection to the photographic image from the damages of light (Moor and Moor 1992, 195); however, albumen itself is subject to bond-breaking and photo-oxidation by ultraviolet light (Messier 1991, 134), causing yellowing. Albumen prints also yellow from an internal protein-sugar chemical reaction, known as the Maillard Reaction, that occurs when albumen is subjected to light radiation in the presence of moisture from humidity (Reilly 1986, 35). Gelatin, less sensitive than albumen, may be yellowed and embrittled by prolonged light exposure; however, this deterioration is unlikely to happen at museum light exposure levels (Reilly 1986, 103; Eastman Kodak 1985, 84). Lavédrine and Gann suggest that prolonged light exposure can deteriorate the collodion binder causing cracking (von Waldthausen 2003a, 3).

Supports[edit | edit source]

  • Paper supports may lighten or darken and become weakened by light exposure, principally from ultraviolet. Lignin-containing papers may discolor (Reilly 1986, 103; Michalski 1987, 10). The yellowing of paper can alter the perceived color of the photographic media. Associated photographic support materials like album pages or colored mounts, such as the Pictorialists used, may be constructed of poor-quality and extremely light-sensitive paper.
  • Gelatin emulsions will absorb most of the incident ultraviolet energy, and a baryta layer will block most incident light from reaching the paper support. Since the 1950s, the composition of photographic papers manufactured by Kodak and Ilford may contain a portion of wood pulp and optical brightening agents (Messier 2000). The optical brighteners are exhausted through light exposure. A print with an exhausted brightener will appear dull (darker) overall.
  • "Crack propagation" of the poly(ethylene) layer of resin coated papers is caused by active oxidants forming internally in the titanium dioxide layer (the opacifying and reflecting layer below the image) when the photograph is exposed to light with an ultraviolet content (Parsons et al. 1979, 111-112). Poly(ethylene) in early resin-coated papers is particularly subject to crack propagation. These oxidants also attack the silver image, resulting in reduction-oxidation (redox) blemishes, mirroring, and orange colloidal silver (Roth 1999; Ctein 1998a, 1998b). Kodak incorporated stabilizers into the paper core around 1977. "Over time the stabilizer diffuses into the adjacent poly(ethylene) layer, thereby significantly increasing their resistance to embrittlement and cracking" (Wilhelm 1993, 128). Early resin-coated prints should be exhibited with caution.
  • The support effects the light-sensitivity of each unique inkjet print (Lavédrine 1997). Commercially available inkjet printers originated in 1994 and early dye inks were very light fugitive as the machines were designed for short-term office use (Wilhelm 1999).
  • Irreversible yellowing of poly (methyl methacrylate) (PMMA) comprising face-mounted photographs may occur. Exhibition lighting sources should be free of ultraviolet light.

Cases (for cased photographs)[edit | edit source]

  • Daguerreotype, ambrotype (positive collodion), and tintype cases are covered with dyed leather or paper. There are dyed textiles inside, as well as glass. Some cases are made of a more durable thermoplastic ("union cases"), but some have a lacquer finish with coloring and inlaid mother-of-pearl, which can be heat and light sensitive. Light levels need to be high enough to make the daguerreotype image easily viewable, but low enough to pose no undue risk to the textile linings and to avoid heat buildup under the cover glasses.
  • Heat buildup should be a concern because of the enclosed nature of the daguerreotype package, which traditionally consists of a brass foil preserver, a paper tape seal, a cover glass, a brass mat, and the daguerreotype plate. Heat can drive mechanisms such as glass deterioration. Light can also drive chemical deterioration of residues left on the plate by some cleaning methods (Barger and White 1991; Barger and Edmondson 1993).

Effects of extremes or cycling of temperature and relative humidity[edit | edit source]

Rapidly changing temperatures can cause moisture to form on the interior of glazing in a frame. This situation is most likely to occur during transportation of photographic materials from venue to venue. Sand and salt from coastal area waters, and snow and ice suppression can also affect the humidity levels in a building environment.

By type of print[edit | edit source]

Salts are hygroscopic in nature and therefore sensitive to changes in the relative humidity (Moor and Moor 1992, 196-197). "Nineteenth century processes have shown considerable shifts in density as a direct result of [changes in] relative humidity and equilibrium moisture content [EMC] levels in both exhibition and dark storage environments" (Moor and Moor 1992,197).

Coatings[edit | edit source]

Coating materials craze, darken or yellow, and bloom from extremes or cycling in temperature and relative humidity. 19th century photographs may have various types of organic coatings including polymers derived from cellulose (collodion), natural water-­soluble polymers (polysaccharides such as gum arabic), proteins (gelatin, albumen, casein), natural resins (Canada balsam, copal, dammar, mastic, rosin, shellac), beeswax, and oils (such as linseed), and essential oils (such as lavender oil), and hydrocarbons (paraffin wax, rubber, caoutchouc) that are susceptible to damage from environmental conditions. For example, early albumen prints with watercolor retouching may have first been coated with a base coating (ox gall, gum arabic, alum, alcohol or other materials) (Waldthausen 2003b). In addition, uncolored ambrotypes, tintypes, and autochromes are manufactured with varnish and lacquer. Coatings of natural water-soluble polymers (polysaccharides such as gum arabic) can easily soften in high humidity conditions. Synthetic coatings made from modern polymers may also be present in the construction of a photograph. Polaroid Corporation (1983, 33) recommended for its products the application of print lacquers, such as McDonald Photo Products, Incorporated's proprietary product, Pro-Tecta-Cote™, to protect against physical damage such as fingerprints and abrasion. At present, the effects of temperature and humidity on synthetic coatings used on prints are not well characterized in the photographic conservation literature.

Color dyes and retouching[edit | edit source]

Dyes used to tint albumen prints can fade in environments with high relative humidity (Reilly 1986, 37-38). Dyes used in inkjet printers can be very soluble in water. Blurring may result from elevated humidity and heat. Research by Robb (2000) on Iris, continuous area­ modulated inkjet, samples indicate that the magenta dye is most sensitive to moisture. Begun in 1994, first generation inkjet dyes were anionic-water soluble dyes (Gregory 2000, 50). Second-generation inkjet dyes were novel dyes. Alkaline conditions made the ink water-soluble; acidic papers greatly improved water fastness. In the late 20th c. organic pigments were being substituted for dyes in inkjet printing. Environmental conditions should be at least 71°F / 21°C and 40-50% RH during exhibition.

Binders[edit | edit source]

High humidity drives many deleterious photochemical reactions including reduction­ oxidation (redox) reactions in silver gelatin systems (Lavédrine 2003, 160). Keeping "the relative humidity below 60% will discourage biological attack (fungus and mold) on gelatin materials" (Hendriks et a1. 1991, 404). Low relative humidity levels can cause cracking and flaking in gelatin-based emulsions and cracking in albumen photographs (Reilly 1986, 35).

Supports[edit | edit source]

Fluctuations in temperature and humidity can alter the dimensions or the flatness of photographic materials, "accelerate degradation, facilitate migration of oxidated acidic complexes to the surface, allow absorption of atmospheric pollutants and give rise to internal physical stress and distortion in an already fragile stratigraphy" (Moor and Moor 1992, 196 to 97). The poly (ethylene) layer in early resin-coated papers are susceptible to cracking from normal ranges of cycling humidity levels, especially if exposed to light with ultraviolet content or for long periods of time (Parsons et a1. 1979, 113). "High humidity levels accelerate chemical reactions of image silver with residual processing chemicals and enable the formation of acidic compounds in paper in the presence of oxides of sulfur or nitrogen" (Hendriks 1989, 646). High relative humidity levels can promote biological growth, and thus, staining, in paper supports (Lavedrine 2003, 160).

Cases[edit | edit source]

Components of a sealed daguerreotype package are glass and metal. Therefore it might be best to try to keep the general relative humidity no higher than 50%, and possibly as low as 40-45%. These levels will reduce the potential for glass deterioration or condensation inside the package but will be high enough not to desiccate the leather, wood, textile, and paper components of the case and daguerreotype package. Similar concerns exist for cased ambrotypes (positive collodions) and tintypes. External lighting sources (radiant heat) can create an imbalance between the internal and external temperature and relative humidity of photographs on display, in essence forming a greenhouse effect within cased and framed photographs (Ritzenthaler et a1. 1985; Reilly 1986; Moor and Moor 1992). Another opinion is that heating caused in this way (too much infrared absorption from an illuminant) will drive down the relative humidity in any air space contained within the package but will not significantly affect the equilibrium moisture content of the photographic materials. In a frame, since both the matboard and the photograph will be losing water to the air, the relative humidity will not change very much. However, the photographic materials will lose a small amount of water. A significant drop in temperature can lead to the formation of condensation in the package. Display cases will behave differently, depending on the amount of hygroscopic material within the enclosed space. For exposed photographic items in a display case, the change in equilibrium moisture content may be significant.

Effects of airborne pollutants (oxidants, organic acids, and volatile organic compounds) [edit | edit source]

Airborne pollutants include, but are not limited to carbon dioxide, hydrogen sulfide, nitrogen oxides, ozone, and sulfur dioxide. Sulfur oxides (SOx) and nitrogen oxides (NOx) from the combustion of fossil fuels (found in automobile exhaust and smoke from factories, for example) can cause fading and loss of contrast in silver-based photographic materials. Both form acidic gases and thus can damage various binder and support layers. Sulfur dioxide (S02) is a reducing agent, which in the presence of oxidants will contribute to the oxidation-reduction deterioration of silver image material. These forms of silver image deterioration can be observed as silver mirroring, yellowing of the binder, and fading of images, including loss of highlight details. Hydrogen sulfide (H2S), an industrial pollutant of outdoor air, is an acidic gas that can cause yellowing and fading of silver image materials, and, when oxidized, reacts with moisture vapor to form sulfuric acid, which can damage cellulose substrates and affect binder layers. Sources of sulfur off-gassing include deteriorating rubber that can be found in some adhesives, rubber bands, and gaskets on cases used for transport, exhibit, and storage. Commercially available cleaning solutions and floor waxes can contain various volatile organic compounds that can be corrosive to photographic images. Ozone (03), contributes to the formation of peroxides and free radicals, which cause fading and discoloration of photographic images and damage support and binder layers by oxidation attack. Sources of ozone include smog in external air, HVAC systems equipped with an ozone purifier, and office copying machines. Oil-based paints, as they cure and dry, give off peroxides as well as acidic by-products that contribute to fading and discoloration of photographic images; thus these paints should not be used in exhibit (or storage) areas. Some cardboards, woods, wood adhesives, cleaners, and bleaches can also generate peroxides. Ammonia (NH3), sometimes found in cleaning products and in adhesives used to construct matboard, can cause fading of photographic images. Organic acids, such as acetic (CH3COOH) and formic (HCOOH), from formaldehyde (HCHO) can cause darkening of cellulose substrates and corrosion of some metals. Acetic acid can be generated from wood, wood adhesives, and paint. Drying oils, oil-modified, and oil­-based paints give off acids, aldehydes, and peroxides. Alkyd and oil-based enamels will off­-gas organic acids, aldehydes, and carbon dioxide if the temperature and cooking time are insufficient for proper attachment to the substrate. Formaldehyde gases can be generated by wood, plywood, and particleboard adhesives, insulation, and carpeting. Acetic acid gas is also a deterioration by-product of cellulose acetate base film materials and can be detected by its vinegar smell often before physical deterioration is visible to the unaided eye. Deterioration from build up of organic acids is generally of greater concern during long-term storage than during short-term exhibition. Inorganic acids (referred to as "mineral acids" in NISO 1999) include nitric acid (HN03), sulfuric acid (H2S04), hydrochloric acid (HCl), and phosphoric acid (H3P04). These may not be air pollutants per se, but, as noted, nitric, sulfuric, and hydrochloric acids can be formed in reactions between atmospheric moisture and air pollutants and other off-gassing components within a gallery space or exhibit display case. Some acids, such as nitric and sulfuric, are also strong oxidizers. Particulate materials such as dust, grit, mold spores (conidia), and combustion products (smoke and soot) can scratch soft emulsions and discolor paper-based photographic images. A tobacco-free environment is recommended, as even small amounts of tar and nicotine stain coatings, binders, and supports. Wilhelm (1993, 577, 576-624) cautions against long-term framing of black-and-white prints with plastics (i.e., Plexiglas glazing) because of the possibility of trace amounts of peroxides and other substances. Peroxides are aggressive oxidizers, as noted previously. There is insufficient long-term data on oxidation generated by long-term framing, however; resin­ coated (RC) papers have the potential to generate peroxides during display.

Resin-coated (RC) prints are thought to be more susceptible to the effects of airborne pollutants because of what is referred to as the "sump effect"; that is, with RC papers, atmospheric pollutants can go in through the front, emulsion side, and have nowhere to go, as the polyethylene coating on the paper support effectively seals it off. However, with fiber­ base prints, the paper support can act as a "sump" to take up the pollutants, away from the emulsion and image layer (Ctein 1998).

Though not quantifiably measured, several silver-based prints (from between 1920-1945) that already had silver mirroring in the darks and around the edges, seemed to develop more iridescent sulfiding after returning to the Metropolitan Museum of Art from a traveling show lasting a year and a half. While there are many factors that might be contributing, it was intuitively felt by then staff conservator, Betty Fiske, that the change was a result of the closed microclimate over such a long period of time. Fiske believed the issue merited an alert until various factors affecting photographic materials are studied and more fully understood.

Color print materials, being more susceptible to light damage and the effects of temperature, may demonstrate less reaction (relatively speaking) to airborne pollutants. The reactions to airborne pollutants, including fading from exposure to oxidants, definitely occur but may not be as readily discernible because the deterioration reactions from the effects of light and of temperature occur more quickly by comparison (Zinn et al. 1997). Face-mounting a photograph to a rigid transparent support, such as to poly (methyl methacrylate) (PMMA), has become a popular mounting method for large, oversize photographic prints amongst contemporary photographers. In 2002, one brand, Diasec®, advertised on their website mounting with acrylic and poly (vinyl chloride) (PVC) materials of varying gloss. The mounting process and materials involved have been described in recent publications (Pénichon and Jürgens 2001b, 2002a), and the long-term stability of these composite laminates is being tested (Pénichon and Jürgens 2002b). Face-mounted photographs on PMMA sheets may eventually craze because of the solvents contained in the silicone rubber adhesive formulation or in proprietary PMMA cleaners.

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