Archives can hold collections made up of all sorts of materials; everything from parchment, paper, wax or metal seals and photo-reprographics, to negatives made from glass, paper, cellulose nitrate, cellulose acetate and polyester. All these materials are affected by different environmental conditions and degrade in different ways, some of which can pose significant risks, including to human health. One of these in particular is cellulose nitrate film.
Cellulose nitrate film was used from around 1889 to around 1950. Before nitrate film came along, glass plate negatives had been used. These were very expensive and could be quite tricky to handle. Nitrate film helped to popularise amateur photography by making it more affordable for the masses. As a result, most archives have large numbers of photographic negatives, with many made from nitrate and acetate films.
Cellulose nitrate is very flammable and capable of burning even in the absence of oxygen, and even when underwater! Nitrate film is also chemically unstable; the film support gradually turns yellow, brittle and sticky. It emits harmful corrosive by-products with an acrid smell, which cause fading of the silver image, decomposition of the gelatine binder and, eventually, the complete destruction the negative.
Recently, a large number of cellulose nitrate negatives were discovered in the strongrooms of Glamorgan Archives.
On opening the boxes the negatives were found to be degrading and were giving off a slightly cheesy acrid smell. They were immediately removed to the conservation studio and placed in the fume cupboard to ensure that the nitric acid vapour being emitted was contained. Wearing personal protective equipment and using a portable extraction unit it was possible to vacuum pack the cellulose nitrate negatives and move them for permanent storage in the freezer.
The negatives are vacuum packed to protect them from the environment within the freezer, to prevent contamination from anything else that is stored in the freezer, and to prevent the gas that is emitted by the negatives from contaminating anything else in freezer. By storing the negatives in the freezer it is possible to prevent further degradation and prolong the life of the negatives. Nitrate film requires hot or warm temperatures to self-combust and so being held at cold temperatures will help prevent this.
This is not the end of the story; plans are in place to digitise these items, so that although the negatives may be in danger, the images they contain will be secured for the future.