Glass plate negative
Glass plate negative

The conservation and restoration of photographic plates is the process of caring for and maintaining photographic plates in order to preserve their materials and content. It covers the necessary 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 conservation and interventive conservation measures that can be taken to increase their longevity.

History

Composition

Photographic plates consist of an image layered on a glass support. Glass plates emerged as a common support medium for photographic negatives in the mid-nineteenth century. 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. Combined with the image substance, these two elements make-up what is called 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.[1] Depending upon the time period, there can be variants to the binder and thus, the chemistry of the image. This is extremely important with the conservation and restoration of photographic plates because it helps the conservator prevent further chemical reactions. 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. Glass plates as a form of support were popular in use between 1851 and the 1920s.[2]

Processes

Early wet plate collodion portrait of lady
Early wet plate collodion portrait of lady

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.[13] Photographic plates face risks of deterioration from both external forces and from a plates own chemical composition. In order 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 & Temperature

Relative humidity (RH) and temperature are two of the most common threats to photographic plates.[14] 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 make-up. 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 type of emulsion , 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.[15] With gelatin dry plates, high humidity can cause mold to grow on the emulsion.[16] 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.[17]

Temperatures, much like RH, need to be precise and closely monitored for correct storage of photographic glass plates. A safe temperature to keep glass plates is 65 °F, however a fluctuation of +/- 2 °F would not cause a significant impact. Making the safest range 63 °F to 67 °F.[18] Low temperatures aid in slowing a plate's own inherent vice by delaying the chemical reactions that cause decay of the plate's structure. Increasing temperatures or frequently high fluctuations will speed the decay process.

Theft and Dissociation

Although theft and dissociation can occur separate from each other, it is not uncommon the two go hand-and-hand and occur simultaneously. Dissociation typically results overtime from an ordered system falling apart due to lack of routine maintenance updates, or from a catastrophic event leading to data loss.[19] If 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 is the best preventative measure against theft.

Water and Pests

Deterioration in glass is often directly related to moisture, from humidity or direct contact.[20] 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 baring side of the plate, a crystalline deposit, known as sick-glass, was present.[21] If a glass plate has been subject to large amounts of moisture, it could result in the growth of 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 that are already compromised from inappropriate storage conditions. Insects will produce waste materials that, much like dust, and build up over time, causing further damage. Pests also eat glass plate storage materials such as paper envelopes or cardboard boxes.

Light

Photographic plates, along with all photographic materials, are very sensitive to light. Extensive and ongoing exposure to light can cause significant deterioration that is irreversible. Sunlight is the most damaging type of light to photographic plates. However, indoor lighting and other forms of UV lighting all pose a threat to photographic plates causing fading and yellowing.[22] Light is especially threatening to color photographic materials as it causes accelerated fading of the color dyes.[23] Exposure to light could deteriorate and lead to discoloration of the pigments present on the plate.

Pollutants and Fire

Air pollution can pose a threat to photographic plates through poor air quality and dirt that can damage the materials. This can include dust to gaseous pollution in an urban environment. Air pollution can cause fading of photographic materials. If a plate is subject to poor air quality, removal of the debris must be done with care using a cotton cloth, if done incorrectly the glass might be subject to abrasions.[24] Other sources of air pollution include "photocopying machines, construction materials, paint fumes, cardboard, carpets, and janitorial supplies," and other types of outdated media.[25]

Fire can cause serious damages to photographic glass plates. The heat produced by a fire can aid in increasing the rate of chemical decomposition of the plate's emulsion. Pollutants in the air that are produced by the fire, smoke and debris, can also attach or rest upon plates. The same care should be taken removing debris from a fire that would be used to remove dust and other air pollutants.

Material and Chemical

The glass composition of photographic plates can be a factor of deterioration. Due to poor quality or an inherent vice, "sick glass" can occur. Environmental conditions are usually linked to the increase or presence of this glass corrosion. The effect of "sick glass" can be weeping and crizzling which can be caused by excessive alkali present and a lack of stabilizers.[26] Weeping involves droplets forming on the glass that appear as small crystals. This type of deterioration is especially threatening for cased photographs because the cover glass could be corroded and damage the photographic image underneath.[27] Corrosion of the glass plate support can also damage the image layer by causing the lifting of the binder and varnish layers.[28]

The other chemical components of glass plate negatives can also be threatening agents of deterioration. For instance, the silver image layer could possibly undergo oxidative deterioration which can lead 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 result of inherent vice, but they are usually accelerated by poor environmental and storage conditions.[29]

Broken glass plate - Avdella, izgorena, 1905
Broken glass plate - Avdella, izgorena, 1905

Physical

Glass plates are rather stable dimensionally, but they are also very fragile and brittle.[30] Glass is brittle, it 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 to the glass. Certain types of breakage and stress states affect the image layer and binder in different ways.

Types of breakage:[31]

Preventive conservation

Environment

Environmental controls are a crucial part of the preservation of photographic glass plates. Relative humidity (RH), temperature, and light play a large role in keeping the multiple materials in photographic glass plates maintained. The following regulatory measures are taken for their preservation:

Handling

Photographic glass plates are handled carefully to avoid physical or chemical deterioration and damage – the following aids in their preservation through proper handling:

Storage

Storage and housing of photographic glass plates is an important part of their preservation. Museums and other cultural institutions take the following measures to ensure their glass plates are properly housed:

Storage of broken photographic plates

Broken or cracked glass plates are stored in a special manner, separate from other photographic plates and in the following ways:

Maintenance/housekeeping

Maintenance/housekeeping of photographic plates requires minimal intervention:

Conservation treatment

Broken or cracked glass plates compose a large portion of those in need of conservation treatment. There are various actions taken in reassembling and restoring these plates using the following materials and methods:

Handling

Adhesives

There is no one ideal adhesive used by conservators, each adhesive has benefits and disadvantages for different situations.

Backing material

Application

Repair methods & techniques

Glass Plate Conservation Projects

The Glass Plate Negative Project at the Heritage Conservation Centre

References

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Identification resources:

Conservation & restoration resources: