Conservation and restoration of photographic plates

Composition

The term photographic plates refers to a type of photograph that consists 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. ^[1] 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.^[2]

Processes

Early wet plate collodion portrait of lady

• 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 an albumen, it was quite laborious and quickly surpassed by the collodion glass plate negative in common use.^[3] The collodion photographic process was a wet place 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 successfully produce a negative image. During the process, the collodion was the binder which was poured onto the glass plate before being exposed. The glass plate was then developed, fixed, washed, and protected with a varnish.^[4]

• Gelatin dry plate negative: This process was invented by Richard Leach Maddox in 1871, but it was not commonly used until 1878 when the process became commercially successful. Because of the advances this process made in photography, it soon replaced the wet plate process in the 1880s. The collodion binder formerly used was replaced by gelatin which already contained the light-sensitive silver salts. This meant that the emulsion was already present and did not have to be painted on the glass plate right before exposure - which now only took less than one second. Because of this advancement, photographers did not have to carry their portable darkroom, but could develop later on.^[5] After 1878 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.^[6] This led to the advanced use and production of photographic glass plates until around 1925 and the development of the modern photography industry.

Relative humidity & temperature

Relative humidity (RH) and temperature are two of the most serious threats to photographic plates and can determine their longevity. ^[7] 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.^[8] Also, on a gelatin dry plate, high humidity can cause mold to grow on the emulsion and cause the plates to stick together.^[9] 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.^[10]

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.^[11]

Light is especially threatening to color photographic materials since it accelerates the fading of the color dyes.^[12] 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.^[13]

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.^[14] 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.^[15] Corrosion of the glass plate support can also damage the image layer by causing the lifting of the binder and varnish layers.^[16]

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.^[17]

Broken glass plate - Avdella, izgorena, 1905

Physical

Glass plates are rather stable dimensionally, but they are also very fragile and brittle.^[18] 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:^[19]

• Impact break - point of impact and surrounding radiating arcs
• Cracks - run perpendicular to applied stress
• Blind cracks - breaks do not carry through the whole shard of glass

Certain types of breakage and stress states affect the image layer and binder in different ways.

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:

• For photographic glass plates, the temperature is kept cool at around 65°F approximately.^[20]
• RH levels are normally kept between 30-40%. If RH goes below 30%, the image binder of the glass plate will dehydrate. If RH goes above 40%, the glass will begin hydrating. ^[21] For cased glass plate photographs, such as ambrotypes, RH levels are kept between 40-50% and temperature between 65-68°F. These levels differ because of the case and its risk for embrittlement, brass mat, and glass deterioration.
• Fluctuations, also called “cycling,” in RH and temperature are sought to be avoided. Environmental fluctuations can contribute to mold growth, chemical deterioration including discoloration and yellowing, as well as deterioration of the emulsion. ^[22]
• 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, which is uneven heating to the photographic plate, is avoided. Light levels are kept below 50 lux. ^[23]

Handling

Photographic glass plates are handled carefully to avoid physical or chemical deterioration and damage – the following aids 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. ^[24]
• When handling, a glass plate is not held by one edge or corner, rather it is held by two opposite edges and always with two hands. ^[25]
• When laying the glass plate on a flat surface, it is always placed with the emulsion side up. ^[26]
• Glass plates are never stacked or have any pressure placed upon them. The sleeve or enclosure is labeled before placing the glass plate inside. ^[27]
• Since glass plates are fragile and brittle, duplicates are created if a glass plate is used often for printing. This helps to minimize the threat of breakage.^[28]

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:

• 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 and further deteriorating the image through flaking and abrasions. Rather, the four-flap enclosure allows the glass plate to be accessed by unfolding the flaps without having to pull the plate across any surface or material. ^[29]
• The photographic glass plates are stored vertically on the long side of the plate in acid- and lignin-free storage boxes. The glass plates are not packed too tightly and are sought not to rub against each other. Each one is usually separated individually with stiffeners made of acid-free folder stock or acid-free cardboard in order to support the plates. ^[30]
• Photographic glass plates stored in a partially filled box will have spacers, most likely acid-free corrugated paperboard, inserted in order to prevent significant bumping or moving.
• Glass plates that are larger than 10” x 12” are stored in legal size boxes that are partially filled to prevent a too heavy weight. The extra space in the box is filled with board or spacers to avoid shifting and jostling. ^[31]
• Storage boxes consisting of photographic glass plates are stored on a lower shelf, specifically below four feet. This helps to prevent someone from lifting them down from above his or her head. ^[32]
• Each storage box of photographic glass plates is labeled with words such as “Heavy,” “Handle with Care,” “Caution: Contains Glass Negatives,” so that everyone knows to be extra careful when lifting the box off a shelf. ^[33]

Maintenance/housekeeping

Maintenance/housekeeping of photographic plates requires minimal intervention:

• For their preservation, light cleaning is carried out occasionally by removing dust with a soft brush. ^[39]

• Conservators also keep the surrounding collections area clean of dust, pests, and any other debris that may attract pests. Also, food and drink is not permitted in the storage area.^[40] To prevent deterioration from air pollutants, it is helpful for conservators to have the air entering the storage area filtered and purified, windows closed, obsolete/outdated media minimized, and enclosures and cabinets used to protect collection objects.^[41]