Images in Reverse: A Look at Photographic Negatives

28, October 2016

The photographs of the St. Louis Public Schools Collection provide a new look at St. Louis history from the viewpoint of one of the region’s oldest public school systems. The negatives alone offer over a century’s worth of memories, dating from the early 1900s into the 2000s. As one of the archivists for this collection, I’m responsible for carefully identifying the types of negatives I’m working with to ensure their proper preservation and storage. Here’s a crash course on identifying and storing photographic negatives—it may even help you with negatives you have at home!

Glass Negatives

Photo of a glass negativeA gelatin dry-plate negative from 1935. St. Louis Public Schools Collection, Missouri History Museum.

Among the earliest and most recognizable types of negatives are glass negatives. Generally, glass negatives consist of two main forms: the collodion wet-plate negative and the gelatin dry-plate negative. Wet-plate negatives appeared as early as 1855 and persisted into the 1880s. Their distinctive grayish black and creamy white tones and rough, hand-cut edges distinguish them from their younger dry-plate siblings. Dry-plate negatives were popular between the late 1870s and 1920s. Developers manufactured them on thinner glass plates cut into standard sizes and inked with deeper black and white tones.

Cellulose Negatives

Photo of deteriorating nitrate and acetate negativesA nitrate negative from 1927 (left) and an acetate negative from 1938 (right) in their final stages of deterioration. St. Louis Public Schools Collection, Missouri History Museum.

Cellulose negatives come in two types: nitrate negatives, used between the late 1880s and early 1950s, and acetate negatives, popular from the 1920s to today. These negatives are among the most difficult to identify because they appear nearly identical. One way to tell them apart is by inspecting the negative’s edges: The manufacturer usually stamps (as opposed to copying into the image) the manufacturer’s name, identification code, and notch codes on one edge of the negative. The next best technique for telling them apart is identifying how they’re deteriorating:

Stages of Deterioration

Signs of Deterioration in Nitrate Negatives

Signs of Deterioration in Acetate Negatives

Stage 1

Turns yellow and silver (“mirroring”)

Curls and turns either blue or red

Stage 2

Is sticky and nitric acid develops (smells foul)

Acetic acid develops (smells like vinegar), shrinks, and becomes brittle

Stage 3

Turns amber and fades

Warps, distorting the base

Stage 4

Develops soft surfaces that adhere to nearby objects

Bubbles and crystallizes

Stage 5

Disintegrates into brownish powder

Develops channels that crisscross the image

If the negative’s identity remains uncertain, contact a local conservator.

Polyester Negatives

Photo of a polyester negativeA polyester negative from the St. Louis Public Schools Collection. Missouri History Museum.

Polyester negatives, introduced in 1960, are currently considered the most chemically stable of all photographic negatives. To identify polyester, check for the word “Ester” stamped along the negative’s edges. If it’s not there, the next step is to use the polarization test, which involves placing the negative between two cross-polarized filters and holding the filters and negative up to a light source. If colors appear, the film is polyester; if the negative remains dark, it’s either nitrate or acetate.

Negative Storage

Photo of acid-free storage containersTypical acid-free storage containers for photographic negatives.

Each negative in the St. Louis Public Schools Collection receives its own acid-free, alkaline-buffered envelope with a pH between 8.5 and 10. Then the negatives are divided into acid-free boxes based on their type, rate of deterioration, size, and coloring (black-and-white versus color negatives).

How the negatives are stored depends on their type:

  • Whole glass negatives are stored on their edges in boxes to prevent cracking. Broken glass negatives need to rest on their flat sides in separate housing containers.
  • Ideally, nitrate and acetate negatives are kept in a freezer or cold storage for safety reasons and to slow deterioration. (If they’re stored in an environment with high or changing levels of temperature and relative humidity, nitrate negatives can catch fire.)
  • Polyester negatives are stored in an environment with consistent humidity levels and a temperature that doesn’t go above 64 degrees Fahrenheit.

Stay tuned for when we start making the images available in our cross-collection search in the next year or two!

—Sabrina Gorse, St. Louis Public Schools Project Photograph Archivist

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