Glass Plate Negatives in the Collection, by Kathleen Watkin, MAS Museum Advisor
Types of Glass Plate Negatives
Photographic glass plate negatives can be divided into two main categories: wet collodion process, and gelatin dry plates.
Wet collodion process negatives were popular from approximately the mid-1850s to the 1890s, and have a milky brown appearance. The wet collodion is created using a solution of cellulose nitrate mixed with ether and alcohol. The negative was usually varnished after processing. Since the collodion layer is soluble in alcohol and acetone, wet collodion plates are difficult to clean.
Gelatin dry plates began to replace wet collodion plates in the 1880s. They have crisp black, grey, or clear tones. They often have a blue metallic sheen to them which is visible in high-density areas. The gelatin layer on dry plates is impermeable to and insoluble in absolute ethanol.
When handling glass plate negatives, always wear protective, lintless gloves made of nylon, cotton, or latex. These negatives are susceptible to mechanical damage, such as breakage, fingerprints, and scratches, and should not be left unprotected. They should be handled by two opposite edges, never by one edge or a corner. When placing the glass plate onto a surface make sure it is clean, flat, dry and free of any debris. Always place the glass plate emulsion side up so it will lay flat and the image will not become scratched. Never place any pressure on the plate for the same reason.
Why Glass Plate Negatives Should Not Be Placed Display:
· Glass plate negatives require a stable environment, free of pollutants including those that might give off gas from surrounding display materials.
· The longer the glass plate negative is exposed to light, the quicker the image will fade, so short exposure is required.
· If glass plate negatives are exposed to fluctuations in relative humidity (RH), RH of over 50%, or corrosive chemicals (such as peroxides, hydrogen sulphide, or ozone), the image will separate and crack. This is irreversible.
· They are extremely sensitive and can be easily scratched.
· They are vulnerable to breakage due to the glass support.
· They can easily develop a network of cracks along the edges of the plate, which may lead to image loss.
· The glass has become brittle and thus is more likely to break as time processes.
Ideas for Displaying Glass Plate Negatives
Often, we would like to show or put on display our collection of glass plate negatives. Here is a list of ways to safely do so:
· Exhibit the prints pulled from the negatives instead. It is visually more appealing to see a print than a negative.
· Create a scan or photograph of the negatives, print the image onto transparency film and use a “mock” version of it with a photo of the real plate as a part of the display  .
· Place the negative in a dark, slightly backlit box where when the flap is open, the light comes on and the negative can be seen. However, even by using this method the negative should not left on display for more than a month
Preservation and Storage of Glass Plate Negatives
The two greatest factors that affect glass plate negatives are relative humidity (RH) and aggressive chemical reactants. Fluctuations in relative humidity and corrosive, aggressive chemicals can cause the gelatin to expand on contact and if left untreated, the gelatin will separate from the glass altogether.
Glass negatives should be kept in an environment with RH levels between 20%- 50%, but preferably below 40%. The recommended temperature is between 15°C and 25°C, but preferably below 20°C. Although the negatives will not suffer if exposed to light for a short amount of time, it is better for them to be stored in a dark filing enclosure or box to protect them.
Glass negatives should be stored in stable, uncoated polyethylene or polyester plastic sleeves, then placed in high alpha-cellulose, pH neutral paper sleeves. They should be stored vertically in tightly packed manuscript boxes or in a box equipped with grooves made from a rigid plastic or metal coated baked enamel.
For more Information:
“CCI Notes 16.2: Care of Black-and-White Photographic Glass Plate Negatives.” Website: http://canada.pch.gc.ca/eng/1439925171071
Graphic Atlas: “Collodion POP.” Website: http://www.graphicsatlas.org/identification/?process_id=222
Graphic Atlas: “Gelatin Dry Plate.” Website: http://www.graphicsatlas.org/identification/?process_id=303#overview
National Archives and Records Administration: “How Do I House Glass Plate Negatives?” Website: http://www.archives.gov/preservation/storage/glass-plate-negatives.html
New South Wales’s “Archives Outside: Conservation Tip #4: A Method of Rehousing Glass Plate Negatives.” Website: http://archivesoutside.records.nsw.gov.au/conservation-tip-no-4-a-method-of-rehousing-glass-plate-negatives/
The J. Paul Getty Trust. “Photography: The Wet Collodion Process.” Website: http://www.getty.edu/art/collection/video/399915/photography:-the-wet-collodion-process/
University of Illinois at Urbana-Champaign’s Preservation Self-Assessment Program (PSAP): “Negatives.” Website: https://psap.library.illinois.edu/format-id-guide/negative#glasscollodion
 How to Scan a Glass Negative if your Scanner does not have a Negatives Setting: (1) In thin card board, cut a hole a hole just slightly smaller than the negative, as the negative should not touch the scanner’s glass (2) Place the negative on the scanner bed. (2) Put a white sheet of paper over it. (3) Hold a lamp over the white paper and scan as you would normally. (4) Then in Photoshop reverse the image.
 How to Photograph a Glass Negative: (1) Place the negative on a clean light table, bigger then the negative. (2) Use a camera stand to mount a digital camera over the light box. (3) Photograph the negative. (4) Reverse the image in Photoshop.