Conservation and restoration of photographic plates

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The conservation and restoration of photographic plates is caring for and maintaining

History

Composition

• 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.^[4]
• Eastman Dry Plate Company.^[1]
  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.
• dammar resin.^[6]
• 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 such as 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.^[8]
  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.

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 65 °F (18 °C); however, a fluctuation of +/- 2 °F (−17 °C) would not cause a significant impact, making the safest range 63 to 67 °F (17 to 19 °C).[13] Low temperatures aid in slowing a plate's inherent vice 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 and co-occur. 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.^[14] 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.

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 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.[15] Light is especially threatening to color photographic materials as it causes accelerated fading of the color dyes.^[16] Exposure to light could deteriorate and lead to 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 to 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.^[17] Other sources of air pollution include "photocopying machines, construction materials, paint fumes, cardboard, carpets, and janitorial supplies", and other types of outdated media.^[15]

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, smoke and debris can also attach or rest upon plates. The same care should be taken removing trash 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 caused by excessive alkali and a lack of stabilizers.^[12] Weeping involves droplets forming on the glass that appear as tiny crystals. This deterioration is especially threatening for cased photographs because the cover glass could be corroded and damage the image underneath.^[18] Corrosion of the glass plate support can also damage the image layer by causing the lifting of the binder and varnish layers.^[13]

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 vice, but poor environmental and storage conditions usually accelerate them.^[13]

Physical

Glass plates are relatively stable dimensionally but also very fragile and

Types of breakage:[12]

• Impact Break: Point of impact and surrounding radiating arcs.
• Cracks: Running perpendicular to applied stress.
• Blind Cracks: Breaks do not carry through the whole shard of glass.

Preventive conservation

Environment

Environmental controls are a crucial part of the preservation of photographic glass plates.

• For photographic glass plates, the temperature is kept cool at approximately 65 °F (18 °C).^[19]
• RH levels are generally kept at 30–40%. If RH drops below 30%, the image binder of the glass plate will
  ambrotypes, RH levels are kept at 40–50% and temperature between 65 and 68 °F (18 and 20 °C). These levels differ because of the case and its risk for embrittlement, brass
  mat, and glass deterioration.
• Though cold storage is safe for photographic plates, with proper
  film photography, is not recommended.^[20]
• Fluctuations, called "cycling", in RH and temperature should be avoided. Environmental fluctuations can contribute to
  yellowing, degradation of the silver halide crystals resulting in silver mirroring,^[21] and deterioration of the emulsion.^[22] Acceptable fluctuations include +/- 2 degrees for temperature and +/- 3% relative humidity.^[23]
• Photographic glass plates, especially
  UV and sunlight. If displayed, spot-lighting and uneven heating to the photographic plate is avoided. Light levels are kept below 50 lux.^[12]

• 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.^[12]
• When handling, a glass plate is not held by one edge or corner but by two opposite edges and always with two hands.^[24]
• the glass plate on a flat surface is always placed with the emulsion side up.^[24]
• Glass plates are never stacked or have any pressure placed upon them. The sleeve or enclosure is labeled before placing the glass plate inside.^[24]
• 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.^[25]

• 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 to 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.^[12]
• The photographic glass plates are stored vertically on the long side of the plate in acid, lignin, polyvinyl chloride (PVC), dye, sulfur, and alum free storage boxes. The acidity of any paper storage used should be a pH between 7 and 8.5.^[26] Glass plates should not be packed tightly and should not rub against each other. Each plate should be separated individually with stiffeners made of acid-free folder stock or cardboard to support the plates.^[27]
• 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 too heavyweight. The extra space in the box is filled with board or spacers to avoid shifting and jostling.[12]
• Storage boxes of photographic glass plates are stored on a lower shelf, specifically below four feet. This helps prevent someone from lifting them down from above their head.^[12]
• Each storage box of photographic glass plates should be labeled with words such as "Heavy", "Handle with Care", and "Caution: Contains Glass Negatives", so all with access to the collection is known to be extra careful when lifting the box off a shelf.^[27]
• When there are concerns about the reactivity of housing materials, the Photographic Activity Test (PAT) by the Image Permanence Institute should be consulted. The use of the PAT is a standard in the preservation of photographic plates. The PAT "explores the possibility of chemical interactions between photographs and a given material after prolonged contact".^[28]
• 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
  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 difficult with wood shelving to ensure they can hold the weight of the collection.

• 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.[29]
• Photographic glass plates cracked or damaged binder are stored on sink-mats.^[30] 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."^[12]
• Broken glass plate shards are "sandwiched" in between two pieces of buffered board and placed inside the four-flap enclosure.^[24]
• wheat starch paste or 3M #415 double-stick tape. Placing each shard in between the form-fit support to help prevent further damage. These broken shards are then placed in individual four-flap enclosures and stored flat with appropriate labeling that warns of their broken condition.^[12]
• Another method of storing broken shards is to place them on sink mats. If this method is used, each piece is separated with paperboard spacers to prevent the pieces from touching. These paperboard spacers are sometimes attached with
  adhesives to the mat so that physical damage does not occur to the shards. They are stored horizontally and placed in a storage box with a label that reads "Caution: Broken glass. Carry Horizontally."^[12]

Maintenance/housekeeping of photographic plates requires minimal intervention:

• light cleaning is carried out occasionally by removing dust with a soft brush for their preservation.^[31] For the emulsion side, to dust, 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 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.^[32]
• obsolete/outdated media minimized, and enclosures and cabinets used to protect collection objects.^[15]

• 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.^[34]

Adhesives

Conservators use no ideal adhesive; each bond has benefits and disadvantages for different situations.

• 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.
• alkaline
  surfaces but is very brittle and only used for temporary repairs.
• Pressure-sensitive tape – Plastic video 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.^[24]

Backing material

• Silpat sheet – This is made of
  fibreglass; textured and provides air pockets to prevent damage from the capillary application; it does not create trauma for the emulsion
  side of the glass.
• Secondary support – This method is used for glass plates broken into many pieces or over 5 × 7 inches in size. A second piece of glass is used with silicone to be inserted as a barrier layer.

• Wicking – This is used by conservators to apply the adhesive to the glass with a wooden or glass applicator. A capillary tube or bottle puts the appropriate amount of glue on the glass shard without excess.
• Direct application – When repairing a broken plate on an inclined plane, conservators apply the adhesive to the fracture interface. The shard is placed directly next to its corresponding bit on the inclined plane.^[24]

Repair methods and techniques

• Photoshop Software assembly – Virtually assembling broken glass shards through Photoshop by scanning or photographing all pieces is used by conservators
  . Once all details are within Photoshop, conservators will reconstruct a copy of the glass plate by moving and rotating the parts until the glass plate is fully assembled. This allows conservators to understand how the glass plate should be reconstructed while avoiding further damage and deterioration of the glass plate photograph. This method allows for further research and study of the container is still possible without the risk of further injury through continual handling.
• Inclined assembly – This method involves applying an adhesive to the glass shard interfaces and assembling them on an inclined surface covered with
  Mylar
  or Silpat. The glass shards are reassembled by direct application, which involves applying the adhesive directly to the shard interface and attaching it to its corresponding piece or assembling through wicking.
• Vertical assembly – This method is used because the glass shards fit back together, which is the most accurate vertically. This also helps to protect the side of the binder layer. The adhesive is not applied until all the pieces are assembled, enabling us to recognize any misalignment before they are adhered together. As the last step, the adhesive is applied through wicking.
• Light-line – This is often used to ensure all pieces are aligned, allowing a conservator to see any misalignment by producing a crooked line. Once the details are aligned, the light bar will be straight again.

Projects

The vertical assembly method along with a light line is used in The Glass Plate Negative Project at the Heritage Conservation Centre as outlined in the case study. This study shows how conservators also deal with other conservation issues, including accretions and residue. For instance, while the plates were considered structurally stable, they may have needed surface cleaning. This was completed by using swabs dampened with water/ethanol solutions to reduce stains or do away with any left tape residue. Pressure-sensitive labels were removed mechanically. Conservators used Whatman lens tissues to wipe off any other residue marks.^[35]

References

External links

• Conservation of Glass Plate Negatives at the Smithsonian
• Case Study: Repair of a Broken Glass Plate Negative
• The Conservation of Glass Plates: Student Placement
• The Preservation of Glass Plate Negatives
• A Method of Rehousing Glass Plate Negatives
• Gaylord Archival – rehousing resources
• Guidelines for Exhibition Light Levels for Photographic Materials
• Digitizing glass plate negatives