How do we identify one transparent film support from another more dangerous one? This was a challenge posed to staff working with the Spencer Digby / Ronald D Woolf Collection of photographic negatives. Conservator Photography Caroline Garratt describes the issues they face and the technology that helps them.

The introduction of plastics to photography

For the majority of the twentieth century, photographers used photographic negatives to capture their images. These were composed of a transparent support (base) coated with an emulsion that contained the image. Negatives were used by photographers from around 1840 with initial transparent supports being made of waxed paper before glass plates were introduced in 1847.

The development of cellulose nitrate in the mid-19th century was the beginning of our modern plastics industry. Its application in photography as a transparent support from 1889 onwards would become a game-changer. Photographers were no longer burdened with heavy and fragile glass plates to capture images.

However, cellulose nitrate had one significant flaw – it is a highly flammable substance. From the mid-1920s cellulose acetate began to be used as an alternative photographic film support. Unlike nitrate, it isn’t highly flammable and its point of difference was noted with a referral to acetate as ‘safety film’. Cellulose nitrate film was finally phased out in the 1950s but in the intervening years, both types of film support were used.

Spencer Digby / Ronald D Woolf collection

Photographers at the Spencer Digby Studios (which later became Photography by Woolf) were renowned for their studio portraits and wedding photography. Throughout the 1930s to 1950s, they captured images using both cellulose nitrate and cellulose acetate film supports.

The difference between the two supports may not have been much of a consideration for photographers at the time but now identification between the two supports is important. Cellulose nitrate is categorised as a hazardous substance (due to its flammability) and negatives in Te Papa’s photography collections are safely stored in an off-site facility. Acidic gases that are produced as cellulose nitrate degrades are also known to accelerate the deterioration of acetate negatives when they are stored together.

Which is which?

In order to separate nitrate negatives from acetate negatives, staff need to be able to identify one transparent plastic support from the other. Visually both supports appear the same so how is identification possible?

Sometimes discerning between the two supports is easy as film manufacturers would often edge stamp film sheets with the words ‘nitrate’ or ‘safety’. But this is not always the case so Te Papa staff have turned to new scientific methods to make the distinction.

A safe and effective method of identification

Over recent years infrared (IR) spectroscopy has become increasingly used in the cultural heritage sector as a fast, safe and accurate way of distinguishing between the two film supports.

Unlike visible light, IR light – or radiation – can’t be seen but we encounter this radiation all the time. For example, our tv remotes use IR to change channels. An IR spectrometer measures the interaction of infrared radiation with a substance. When subjected to IR radiation every substance will react in a characteristic and unique way, based on its atomic structure. This interaction is recorded as a spectrum.

The IR spectrometer in our conservation laboratory has an internal reference library of spectra for hundreds of different substances. Once a reading from a substance has been taken, the spectrometer’s software will find a match to a spectrum in the reference library.

An example reading can be seen below where the spectrum for a photographic negative, ‘B083092’, has matched to the long downward peaks of a nitrate reference spectrum in the spectrometer’s library.

Just over a quarter of the negatives being catalogued and imaged for the Spencer Digby Project require testing. Analysis only takes seconds but has a long-term positive impact on the preservation of our photographic negatives. Acetate negatives are moved to our on-site cold storage facilities and the nitrate negatives are returned to their off-site location, vastly improving the conditions for the former.

Thanks to IR spectroscopy the difficult task of distinguishing between two identical-looking materials is made easy.

Further reading

Browse through more of our Digby/Woolf images on Collections Online.