Fig. 2

ATR-FTIR spectra (the relevant section is shown) of filter paper (a), modern rag paper (b), and historical rag paper (c) immersed in different concentrations of gellan; the peaks at the wavenumbers 1606 cm⁻¹ and 1412 cm.⁻¹ (indicated by vertical lines) show the presence of gellan, most clearly for 1% gellan compared to the untreated control. The spectra are stacked to make the regions of interest more visible. Full spectra are provided in supplementary information (Additional file 1: Fig. S1)