Conservation and restoration of photographic plates

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. • 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. 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. 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. 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. 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. 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. 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". Corrosion of the glass plate support can also damage the image layer by causing the lifting of the binder and varnish layers. The other chemical components of glass-plate negatives can also be threatening agents of deterioration. For instance, the silver image layer could undergo oxidative deterioration, leading to fading and discoloration. Additionally, the collodion binder itself is made up of cellulose nitrate, which is known to be a highly flammable compound. Most of these agents of deterioration are the result of poor chemical processing as a result of inherent frgility, but poor environmental and storage conditions usually accelerate them. Physical Glass plates are relatively stable dimensionally but also very fragile and brittle. Glass is highly susceptible to breakage, cracks, and fractures. This can be caused by human error, including dropping or bumping the glass plate, or it can be caused by failure of storage equipment, housing, shelves, etc., which may lead to an impact against the glass. Different breakage and stress states affect the image layer and binder differently. Types of breakage: • Impact Break: Point of impact and surrounding radiating arcs. • Cracks: Running perpendicular to applied stress. • Blind Cracks: Breaks do not carry through the whole thickness of the glass. ==Preventive conservation==

Environment Environmental controls are a crucial part of the preservation of photographic glass plates. Relative humidity (RH), temperature, and light play a significant role in keeping the multiple materials in photographic glass plates maintained. The following environmentally regulatory measures are taken for their preservation: • For photographic glass plates, the temperature is kept cool at approximately . • RH levels are generally kept at 30–40%. If RH drops below 30%, the image binder of the glass plate will dehydrate. If RH rises above 40%, the glass will begin hydrating. • Fluctuations, called "cycling", in RH and temperature should be avoided. Environmental fluctuations can contribute to mold growth, chemical deterioration including discoloration and yellowing, degradation of the silver halide crystals resulting in silver mirroring, and deterioration of the emulsion. Acceptable fluctuations include ± 2 degrees of temperature and ± 3% of relative humidity. • Photographic glass plates, especially negatives, are preserved in dark enclosures due to their risk of deterioration when exposed to light, particularly UV and sunlight. If displayed, spot-lighting and uneven heating of the photographic plate is avoided. Light levels are kept below 50 lux. Handling Photographic glass plates are handled carefully to avoid physical or chemical deterioration and damage – the following measures aid in their preservation through proper handling: • To prevent fingerprints, non-vinyl plastic gloves are worn when handling – either latex or nitrile. Cotton gloves are not recommended by conservators due to the possibility of glass easily slipping from the cotton material. Cotton gloves are also susceptible to snagging on the emulsion, if it is flaking, or on the edges of the glass support. • the glass plate on a flat surface is always placed with the emulsion side up. Storage Storage of photographic glass plates is important to their preservation. Museums and other cultural institutions take the following measures to ensure their glass plates are properly housed: • Photographic glass plates are housed in four-flap enclosures, emulsion side up. These four-flap buffered enclosures prevent a glass plate from being pulled in and out, which would cause further deterioration of the image from flaking and abrasions. The four-flap enclosure allows the glass plate to be accessed by unfolding the flaps without pulling the plate across any surface or material. Glass plates should not be packed tightly and should not rub against each other. Each plate should be separated with stiffeners made of acid-free folder stock or cardboard to support the plate. • Photographic glass plates stored in a partially filled box will have spacers, most likely acid-free corrugated paperboard, inserted to prevent significant bumping or moving. • Glass plates larger than 10" x 12" are stored in legal-size boxes that are partially filled to prevent a box that is too heavy. The extra space in the box is filled with board or spacers to avoid shifting when jostled. • It is considered best practice to use steel shelving to store photographic plates. It is not recommended to use wood cabinets or crates. Wood shelves are susceptible to termites and are more prone to trigger chemical reactions with the plates. Wood shelves tend to possess finishes, paints, and glues that cause off-gassing. Acetic acid and formaldehyde build-up are also more likely to occur. Lastly, given the weight of the photographic plates, it is more problematic that the relative weakness of wood shelving can hold the weight of the collection. Storage of broken photographic plates Broken or cracked glass plates are stored specially, separate from other photographic plates, and in the following ways: • Broken glass plates are stored flat, unlike intact plates stored vertically. Stacking broken plates only five plates high is recommended due to the plates' weight. This will prevent further breakage and damage. • Photographic glass plates with cracked or damaged binder are stored on sink-mats. Those with minor flaking are still housed in the four-flap enclosure that is labeled appropriately, describing the damage. Glass plates with extensive flaking are stored on sink-mats horizontally and placed in a storage box with a label that reads "Caution: Broken glass. Carry Horizontally." To dust the emulsion side, it is best to use an unused paint brush and, very gently, brush from the center to the outside of the plate. To clean the underside of the leaf (non-emulsion side), dip a cotton ball or cotton round into a cup of distilled water, and work from the middle of the plate to the outside. Water on the emulsion side will wash the emulsion away, causing the image to be lost forever, be careful to ensure this cleaning treatment is only used on the glass support underside and not the emulsion side of the plate. • Conservators should also keep the surrounding collections area clean of dust, pests, and any other debris that may attract pests. Food and drink should not be permitted in the storage area as they attract pests. To prevent deterioration from air pollutants, it is helpful to have the air entering the storage area filtered and purified, windows closed, obsolete/outdated media minimized, and enclosures and cabinets in use to protect collection objects. ==Conservation treatment==

Many broken or cracked glass plates may benefit from conservation treatment. There are various actions taken in reassembling and restoring these plates using the following materials and methods: Handling • Conservators tend to wear Neoprene gloves to help protect the emulsion from fingerprints that will cause deterioration over time. They avoid handling glass fragments to help prevent further breaking of the glass. A padded (foamed polyethylene) and tight weave tissue or Sintered Teflon lined box are preferred by conservators to store fragments, as they help prevent further breaking or cracking. Adhesives Conservators use various adhesives; each adhesive type has benefits and disadvantages for different situations. • Paraloid B-72 – A solution of 50–70% B-72 in a solvent with added silica is used to reassemble glass plate fragments. It takes 1–2 hours to dry. One issue with this adhesive is that it creates "snowflakes" in between pieces, making an invisible reassembly impossible. • Epoxy resin – This adhesive is powerful and has minimal shrinkage. An issue with this method is that it yellows over time and is not advisable to be used on glass plates with a collodion binder. This is due to the potential damage to the collodion binder of the reversibility method. • Cyanoacrylates – This adhesive bonds firmly with alkaline surfaces but is very brittle and is only used for temporary repairs. • Pressure-sensitive tape – This adhesive type is ubiquitous, easy to use, and completely removable but only provides minimal support. • Sticky wax – As the pieces are assembled, sticky resin, such as that used for lost wax casting in jewelry making, is handy for holding the shards in place. ==References==