The conservation and restoration of photographic plates is the practice of caring for and maintaining photographic plates to preserve their materials and content. The practice includes the measures that can be taken by conservators, curators, collection managers, and other professionals to conserve the material unique to photographic plate processes. This practice includes understanding the composition and agents of deterioration of photographic plates, as well as the preventive, and interventive conservational measures that can be taken to increase a photographic image's longevity.
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History
Composition In general, black and white photographic negatives are made up of fine silver particles (or color dyes for color negatives), which are embedded in a thin layer called a binder; the two together comprising the emulsion. This emulsion layer sits upon what is called the support, which can be paper, metal, film, or, as in the case of photographic plates, glass. Before exposure, a photographic plate consists of a photosensitive substance layered on a support medium. Glass plates emerged as a common support medium for photographic negatives from the mid-nineteenth century to the 1920s. Depending upon the period, there can be variants to the binder and, thus, the chemistry of the image. In the case of the Wet Plate Collodion, the image is run under a wash bath to stop the development of the image after exposure. An important part of the photographic process, "fixing", is then used to wash the silver particles that are not part of the image, which then produces a stable negative image. The fix bath will ensure that the remaining silver halide crystals are no longer sensitive to additional light exposure, removing all excess. This negative image can then be used over many years to produce paper positives. It is important for the conservator to understand the chemistry, in order to prevent further chemical reactions.
Processes
Collodion glass plate negative: This process was invented by the Englishman Frederick Scott Archer in 1851. While the first process to take advantage of glass plates was the albumen print method, it was quite laborious and was quickly surpassed by the collodion glass plate negative in common use. The collodion photographic process was a wet plate process, which meant that the glass plate itself had to be wet while it was exposed and throughout processing. This required a portable darkroom to be taken wherever a photographer went, in order to produce a negative image successfully. During the process, the collodion emulsion was poured onto a glass plate before being exposed. The glass plate was then developed, fixed, washed, and protected with a varnish. Collodion is cotton dissolved in nitric acid. Because collodion was both complex and dangerous to produce, it was often purchased by the photographer. Once dissolved, iodide was added. Over time, bromide was added to help the image be more sensitive to light. Sometimes albumen (made of egg whites) was used to help the collodion stick to the glass plate. Pyrogallic Acid or ferrous sulfate was often used to develop the latent image, then sodium thiosulfate (also known as hypo) or potassium cyanide was used to fix the image. Gelatin dry plate negative: This process was invented by Richard Leach Maddox in 1871, but it was not commonly used until 1879, when the process became commercially successful. Because of this process's advances in photography, it soon replaced the wet plate process in the 1880s. The collodion binder formerly used was replaced by gelatin, which already contained light-sensitive silver salts. This meant the emulsion was already present and did not have to be painted on the glass plate right before exposure – which now took less than one second. Because of this advancement, photographers did not have to carry a portable darkroom, as the plate could be developed later. To make the gelatin dry plate, the glass was cleaned, polished, and treated to ensure the gelatin would adhere to the glass plate. Treatments included applying thin layers of gelatin, albumen, or chemical etching. After 1879, when further improvements were made to the gelatin emulsion, gelatin glass plates began being mass-produced by companies such as Wratten & Wainright, Keystone Dry Plate Works, and notably the Eastman Dry Plate Company. This led to the advanced use and production of photographic glass plates until around 1925 and marked the start of the development of modern photography as an industry. Screen Plate: The Screen Plate process is also known as Autochrome Lumière and was invented in France in 1907 by Louis and Auguste Lumière. Screen plates were an additive color screen process considered the first successful color process for commercial photography. The Lumière brothers drew upon the color theories of James Clerk Maxwell and Louis Ducos de Hauron from the second half of the 19th century. Autochrome plates were "covered in microscopic red, green, and blue colored potato starch grains". These grains, before being placed on the glass plate, were sorted through sieves to break them down to "thousandths of a millimeter" in diameter. Once broken down in size, they were separated into groups, then dyed either red, violet, or green. The grains were mixed and then spread over a glass plate covered with a tacky varnish. A second varnish was then applied over the layer of starch grains. The second coating of varnish was a hydrophobic layer composed of castor oil, cellulose nitrate, and dammar resin. Ambrotype: The Ambrotype process resembled the Wet Plate Collodion considerably in composition and creation, and was considered to be a "Collodion Positive". In 1850, Louis Désiré Blanquart-Evrard realized that the negative appeared as a positive by underexposing the image and placing the image against a dark background. Dark backgrounds—created with paints, fabrics, and papers—were used to achieve this effect. In some instances, bleach was used, after the image was developed, to yield a softer appearance. In this process, the chemical composition and fix bath are critical elements to the lifespan of the image, but the material that backs the glass plate may also cause deterioration.
Agents of deterioration There are ten accepted agents of deterioration: dissociation, fire, incorrect relative humidity, incorrect temperature, light, pests, pollutants, physical forces, thieves, and water. Photographic plates face risks of damage from both external forces and from their own chemical composition. For a conservator to create an appropriate plan to protect against agents of deterioration, they must understand what might impact a photographic plate. The following list addresses how each agent of deterioration harms photographic glass plates.
Relative humidity and temperature Relative humidity (RH) and temperature are two of the most common threats to photographic plates. As with all material collections, high temperature in combination with high humidity can cause mold growth and attract pests.
Photographic plates face significant structural and chemical challenges unique to their makeup. There are two types of photographic glass plates: collodion wet plates and gelatin dry plates. Structurally, collodion wet plates are held together with a specific emulsion type, made using a silver halide mixture in gelatin. Fluctuations in RH can strain the adhesive emulsion, causing the gelatin to expand and contract. The strain from incorrect RH can also cause the emulsion to crack or separate along the plates' edges. With gelatin dry plates, high humidity can cause mold to grow on the emulsion. High levels of humidity can cause glass plates that have been stored incorrectly to stick together, compromising the image on the plate. Increasing RH can cause deterioration of other elements; these include the silver halide, varnish, and glass support. Decreasing the RH will cause deterioration by eventually leading to the flaking of the binder and dehydration of the glass.
Much like RH, temperatures must be precise and closely monitored for the correct storage of photographic glass plates. A safe temperature to keep glass plates is ; however, a fluctuation of ± would not cause a significant impact, making the safest range . Low temperatures aid in slowing a plate's inherent fragility by delaying the chemical reactions that cause decay of the plate's structure. Increasing temperatures or frequently high fluctuations will speed up the decay process.
Theft and dissociation Although theft and dissociation can occur separately, it is not uncommon for the two to go hand-and-hand. Dissociation typically results in overtime from an ordered system falling apart due to lack of routine maintenance updates or from a catastrophic event leading to data loss. If an inventory is not regularly updated it could become easy for a single, or several, glass plates to go missing. Regular inventory maintenance can also serve as a deterrent against theft. Ensuring glass plates are locked and stored where only designated museum staff can access them is the best preventative measure against theft.
Water and pests Deterioration in glass is often directly related to moisture, from humidity or direct contact. Enough moisture over time will result in the chemical composition of the image to change. In the 1990s, The United States National Archive began to notice that some glass plates featured in their collection, on the non-photo bearing side of the scale, a crystalline deposit, known as sick-glass, was present.
If a glass plate has been subject to large amounts of moisture, it could grow mold on the plate's emulsion. Mold will eat away at the emulsion and attract other living pests. Insects will be more likely to appear in areas already compromised by inappropriate storage conditions. Insects will produce waste materials that, like dust, build up over time, causing further damage. Pests eat glass plate storage materials such as paper envelopes or cardboard boxes.
Light Photographic plates and all photographic materials are susceptible to light. Extensive and ongoing exposure to light can cause significant and irreversible deterioration. Sunlight is the type of light most damaging to photographic plates. However, indoor lighting and other forms of UV lighting all pose a threat to photographic plates, causing fading and yellowing. Light is especially threatening to color photographic materials as it causes accelerated fading of the color dyes. Exposure to light could lead to deterioration and discoloration of the pigments present on the plate.
Pollutants and fire Air pollution can threaten photographic plates through poor air quality and dirt that can damage the materials. This can include dust and gaseous pollution in an urban environment. Air pollution can cause fading of photographic materials. If a plate is subject to poor air quality, debris removal must be done with care using a cotton cloth; if done incorrectly, the glass might be subject to abrasions. Other sources of air pollution include "photocopying machines, construction materials, paint fumes, cardboard, carpets, and janitorial supplies".
Fire can cause severe damage to photographic glass plates. The heat produced by a fire can aid in increasing the chemical decomposition rate of the plate's emulsion. Pollutants in the air produced by the fire—such as smoke and debris—can also attach to or rest upon plates. The same care should be taken removing trash from the aftermath of a fire that would be used to remove dust and other air pollutants.
Material and chemical