Evaluating the stability of colour slides: 30 years of natural ageing at the National Museum of Denmark

In the 1990s, chromogenic reversal films (also known as colour slides) were a widespread medium to
document objects from museum collections. At the same time, there was a growing awareness about the poor
long-term stability of these materials due to the vulnerability of chromogenic dyes to relative humidity,
temperature and light, as well as to other factors such as the quality of processing.
In this context, a study collection was created in 1992 at the National Museum of Denmark, with the aim of
evaluating the stability of different brands and models of 35 mm colour slides used at that time. Six films were
selected: i) Kodak Kodachrome (ISO 64); ii) Kodak Ektachrome (ISO 100); iii) Fuji Fujichrome Velvia (ISO 50);
iv) Agfa Agfachrome RS 100 (ISO 100); v) Agfa Agfachrome 1000 RS (ISO 1000); vi) Polaroid Presentation
Chrome (ISO 100). Different colour and neutral patches were exposed in the test slides to allow a proper
understanding of the fading of the different dyes. After processing, the films were contaminated with three
concentrations of fixer to simulate improper processing. A set of films was maintained untreated for
comparison. The films (treated and untreated) were framed, stored in plastic folders, and placed in eight
different locations at room temperature. A reference set was kept in a freezer at -21°C. To assess the fading
and production of yellow stain in the samples, density measurements were carried out before ageing.
More than thirty years after the production of the study collection, the overall test slides were assessed once
again, following the same methodology. Also, the climate of the locations where the slides have been kept
was monitored. Despite the locations have presented some differences in temperature and relative humidity
conditions, only two led to notable distinctive results: the reference set, with samples showing almost no
changes, and location 6 (an office room) with high frequency fluctuations of relative humidity and slight
exposure to light, leading to serious changes in the samples. From all chromogenic reversal films tested,
Ektachrome was the most stable. No significant yellow stain was measured in any of the samples, and residual
fixer did not prove to be an alarming problem in most cases. In general, some discrepancies were found
between the present study (natural ageing test) and the literature (based on artificial ageing tests).