Losing colour: the discolouration of plants in spirit preserved collections

The Research unit of the Haute-École Arc Conservation-Restauration (UR-arc CR), Neuchâtel, Switzerland, is carrying out a research project that aims to understand the discolouration of botanical spirit preserved collections and strives to propose practical solutions to these problems. This article reports on the results of the initial phase of this project, which focused on creating experiment jars of representative plants specimens immersed in fluids and monitored using the following protocol: photography of the jars, colorimetric measurements, and analysis by UV spectroscopy of the fluid as well as closer observations of the specimens. Colorimetric measurements tracked the colour changes over time. UV spectroscopy was used to identify the pigments present in the fluid. Thereby, different behaviours could be observed, depending both on the type of pigment and on the fluid. Specimens containing pigments with strong dying properties tend to opacify the fluid while keeping their colour, whereas other specimens such as colourful flowers or leaves show clear discolouration problems. Depending on the preservative fluid, the leached pigments degrade at different rates. Moreover, the botanical specimens show other alterations: they may not only assume a lighter or darker colour, but can also shrink, stiffen or soften.