Fluorescence labeling of gelatin and methylcellulose: monitoring their penetration behavior into paper

For the conservation of artworks on paper, powdery paint layers on the paper matrix are stabilized with dilute solutions of adhesives (0.25-1 %), commonly gelatin or methylcellulose, which are applied as aerosols. This technique allows non-contact application. The distribution of the adhesives must be carefully controlled: they have to be delivered to unstable paint layers in the right quantity to avoid visual alterations of the artwork during the stabilization treatment. To visualize the distribution of aerosol-misted adhesives in porous substrates, gelatin and methylcellulose were labeled with fluorescent dyes, purified from excess label, and applied on sample specimen featuring powdery pigment layers on handmade rag paper. As blank comparisons, sample papers without pigment layers were included to verify whether aerosol-misted adhesives are a suitable method to stabilize fragile papers. Penetration of the adhesive-label-conjugates was observed at thin sections of the samples by fluorescence microscopy. The fluorescence labeling of gelatin with Texas Red (TM) allowed an excellent visualization of aerosol-misted adhesive (0.5-1 %) in all sample types. Methylcellulose (Methocel (TM) A4C) labeled with Texas Red (TM) C2-dichlorotriazine enabled fluorescence tracing if applied in 0.5 % solutions by immersion. Aerosol application permitted local adhesive application, making it a suitable technique for stabilizing fragile papers. If applied to samples with low porosity, aerosol-misted gelatin was mainly deposited at the surface, whereas in porous filter paper, penetration dominated over surface deposition. Intermediate drying between repetitive applications apparently limited the penetration of aerosol-misted gelatin.