Variations of Relative Absorptivity of Geranium and Madder Lake Pigments in FT-IR Spectra during Degradation

Historical objects have long been the subject of extensive scientific investigation. This need for research comes to address issues related to objects’ conservation. Normally, their materials age as years go by and their vulnerability increases. Lake pigments - a complex made of organic dyes with inorganic substrates – are an example of materials whose conservation state declines with ageing, implicating changes in the physical appearance of painted objects. This work focuses on geranium and madder lakes, two types of red colour used in the past which have different stability while ageing. The decaying permanence of these lakes in complex paint matrices presents challenges to non-invasive analytical techniques. Therefore, this study aims to evaluate the advantages and limitations of Fourier Transform Infrared (FT-IR) spectroscopy for analysing lake pigments in Cultural Heritage (CH). Understanding FT-IR’s limitations helps identify potential errors affecting the accurate interpretation of results and recognise its strengths, ensures its efficient use and provides information on when it should be combined with other techniques. This discussion is organised with the following structure. Chapter 1 provides an introductory understanding of lake pigments, including their historical context and applications in Cultural Heritage. Chapter 2 offers an overview of FT-IR spectroscopy and the analytical strategy adopted in the experiment. Chapter 3 presents a detailed description of the study goals and the analytical strategy adopted in the study. Chapter 4 details the experimental procedure performed at the A-Sense lab at the University of Antwerp (Belgium). It covers the synthesis of geranium and madder lake pigments and their accelerated ageing, considering the environmental conditions; The description of the performed FT-IR analysis on both, fresh and aged madder and geranium lake samples to measure the absorptivity changes; Complementary information from optical microscopy (OM), fluorescence microscopy (FM), and colourimetric analysis. This chapter also includes the analysis of the relative absorptivity of 20 materials usually found in CH objects to compare the changes in the relative absorptivity of the lakes. Chapter 5 presents the results, including all data collected during the research period and their interpretation. These results are fundamental for drawing adequate conclusions. Chapter 6 highlights research results, addressing potential misinterpretations and evaluating the suitability of FT-IR spectroscopy in identifying and characterising lake pigments, despite the challenges posed by photo ageing. It also discusses the influence of the experimental methodology influence on the accuracy of conclusions. In summary, the study demonstrates that FT-IR is a powerful non-invasive technique suitable for analysing both fresh and aged lake pigments.