Photodegradation of iron gall ink affected by oxygen, humidity and visible radiation

The causes of the discolouration of iron gall ink have been debated for decades. As a contribution to the understanding of the photodegradation behaviour of iron gall ink, this paper discusses the effects of three environmental factors that are of primary concern – oxygen, moisture and light. Using a range of historical paper-based samples, a 23 full factorial experiment was designed to quantify the effects of oxygen concentration [O2], relative humidity (RH), illuminance (Ev) of broadband LED light, and their interactions. A change in diffuse reflectance with time was observed to mainly take place beyond 600 nm into the NIR range. The change followed similar patterns under all the experimental conditions, which was modelled by a logarithmic relationship with time of degradation. The rate constant was obtained and was found to be mainly affected by [O2], RH and their interaction. The role of Ev needs further investigation, however, its insignificant effect on the discolouration measured in this research suggested the possibility of effective degradation control with relaxed lighting controls. Furthermore, the wavelength sensitivity of iron gall ink's discolouration was investigated using narrowband radiation centred at three different wavelengths (Λ): 450 nm ([Λ450]), 525 nm ([Λ525]) and 625 nm ([Λ625]), with varying [O2] and RH. RH, Λ, the interaction between RH and Λ, and the interaction between RH and [O2] were found to have strong effect on the rate constant of discolouration. Among these factors, Λ showed the strongest effect, which decreased as Λ increased. Approximately 3x and more than 10x faster degradation by [Λ450] than [Λ525] and [Λ625] respectively was observed, which was likely to be associated with photon energy and quantum efficiency in absorption at different wavelength. •Iron gall ink mainly discolours at wavelength beyond 600 nm.•The discolouration is mainly affected by oxygen, relative humidity and their interaction.•Visible radiation in the blue region induces the fastest discolouration.