Conservation and restoration of photographic plates

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, collections managers, and other museum 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 bound to 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. An important part of the photographic process, "fixing," is used to wash the silver particles that are not part of the image, which then produces a stable negative image. 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.
Agents of deterioration.
Photographic plates face certain challenges to their preservation. In order to protect against deterioration or damage, conservators seek to identify the agents that threaten the various materials that make-up photographic glass plates. The primary agents of deterioration include the following:
Relative humidity & temperature.
Relative humidity (RH) and temperature are two of the most serious threats to photographic plates and can determine their longevity. As with other cultural collections, an overall high temperature in combination with high humidity can lead to mold growth as well as attract pests. Heat is also a danger to collections because it is an element that accelerates any type of chemical deterioration that occurs, specifically in photographic materials.
Photographic plates face significant structural and chemical challenges unique to their makeup - Fluctuations in RH can strain the adhesion of the gelatin to the glass plate as changes can cause the gelatin to expand and contract as well as cause emulsion to crack or separate along the edges. Also, on a gelatin dry plate, high humidity can cause mold to grow on the emulsion and cause the plates to stick together. Increasing the relative humidity can cause deterioration of other elements as well such as the silver, varnish, and glass support. Decreasing the RH will cause deterioration by leading to the flaking of the binder and dehydration of the glass.
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, but indoor lighting and any other UV lighting all pose a serious threat to photographic plates causing fading and yellowing.
Light is especially threatening to color photographic materials since it accelerates the fading of the color dyes. It was not uncommon for photographic plates to be hand-painted in order to produce color images. Exposure to light of colored glass slides could deteriorate and lead to discoloration of the pigments.
Air pollution.
Air pollution can pose a threat to photographic plates through poor air quality and dirt that can damage the materials. This can include dust, gaseous pollution from an urban environment, and dirt particulate. Air pollution can cause fading of photographic materials as well as damage the surface of the image by causing an abrasion. Other sources of air pollution include "photocopying machines, construction materials, paint fumes, cardboard, carpets, and janitorial supplies," as well as other types of outdated or obsolete media.
Material/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. 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. 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 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 flammable compound. While this is very dangerous as a film support, it is not thought to be as threatening in the binder. Most of these agents of deterioration are the result of poor chemical processing, but they are usually exacerbated by poor environmental and storage conditions.
Physical.
Glass plates are rather stable dimensionally, but they are also very fragile and brittle. Because 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.
Types of breakage:
Certain types of breakage and stress states affect the image layer and binder in different ways.
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 make up 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:
Adhesives: (There is no one ideal adhesive to be used)
Application:
Backing material:
Repair methods & techniques:
The Glass Plate Negative Project at the Heritage Conservation Centre
Resources.
Identification resources: 
Conservation & restoration resources:
External links/case studies.
Conservation of Glass Plate Negatives at the Smithsonian
Case Study: Repair of a Broken Glass Plate Negative
The Conservation of Glass Plates: Student Placement