Friday 17 July 2015

Deterioration of X-rays

X-rays were originally produced on glass plates using a photographic emulsion. X-ray sheet film was first developed by Kodak in 1913 and used a thick nitrate base. This was followed with film made with two sides coated in nitrate in 1918. Nitrate film was found to be flammable, which led to the development of “safety” film, made from an acetate base, by Kodak in 1924. From the 1950s onwards polyester was mainly used to make X-rays as it is a more stable material. Today, X-rays are made and stored in a digital format and can be printed out on film or paper if needed. 

X-ray of a pair of feet
The X-rays found in LHSA’s collections are a valuable source of information, however, the nature of their material composition means that they will, inevitably, degrade over time. As cellulose nitrate deteriorates it emits a strong odour, discolours to an amber colour, and becomes sticky and brittle.
Degraded X-ray on Cellulose Nitrate
As acetate film degrades it emits a vinegar smell caused by the release of acetic acid. This is known as "vinegar syndrome", which also has the potential to cause damage to paper-based collections held in close proximity. Once deterioration begins, the chemical process becomes autocatalytic, meaning that degradation will take place at a faster and faster rate. When the film degrades, the base shrinks, and the emulsion starts to separate from the base and begins to crack. The film becomes brittle and eventually shrivels or buckles distorting the image beyond use. 
Degraded X-ray on Cellulose Acetate
Due to these problems, it was decided to separate the X-rays from the rest of the collection. A comprehensive digitisation programme was carried out to capture the information held, and these images were cross-referenced with case histories that accompany them. Because of the risk cellulose nitrate film poses in an emergency situation such as fire, the limited number of originals on this type of film base will be safely destroyed. The X-rays on cellulose acetate will be placed in to frozen storage to slow down the rate of deterioration. Read all about how we do this in the next LHSA conservation blog!

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