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Flaking of paint layers is a widespread deterioration phenomenon that conservators face on a regular basis.
Traditionally, lack of adhesion in wall paintings is addressed by treatment with adhesive materials, applied behind or above the lifted paint layers. In order to ensure the success of the intervention, adhesives must be deposited at the interface between the lifted paint and underlying support, providing adequate contact.
Penetration and deposition of the adhesives is therefore paramount and given it is applied behind an opaque layer, particularly difficult to assess. A number of factors will determine how a solution will travel through porous substrates and where it will ultimately deposit, not least its viscosity and surface tension, as well as the prevailing environmental conditions.
Traditionally, conservators rely on experience to determine the success of adhesion of paint flakes, tweaking material properties based on empirical assessment. By methodically characterizing material properties and observing penetration and deposition of selected adhesives, the research aims at providing tools for the practical conservator to better inform adhesion interventions of paint layers on porous substrates.
The study interrogates invasive and non-invasive methods to determine penetration and deposition of adhesive systems coupling non-invasive unilateral NMR techniques and fluorescent labelling of polysaccharide and proteinaceous materials. Two commonly used materials, high Bloom gelatine, and Jun Funoriâ, a purified polysaccharide extracted from the red algae genus Gloiopeltis Furcata, are investigated for injection between organic bound paint layer and lime-based plaster support. Dynamic unilateral NMR experiments are used to gather real-time information on the penetration of the system, evaporation of the carrier and ultimately deposition of the adhesive in the stratigraphy as well as tracking solvent penetration and evaporation. Depth profiles of surrogate wall painting samples simulating flaking paint are assessed before, during and after the application of the two adhesives in a range of concentrations. Deposition of the adhesive is confirmed through fluorescence imaging, fluorochrome labelling of the two selected adhesives allowing for clear identification of the additive materials within the stratigraphy. For this research, gelatine is tagged with 5(6)-FAM SE, while 5- DTAF is used for JunFunoriâ. Successful grafting of the fluorophore on the polymers and absence of unreacted dye is assessed through thin layer chromatography. Viscosity and surface tension of the adhesives are characterized and correlated to their penetration and deposition in the stratigraphy.
Results show that concentration and temperature affect the adhesive’s rheological and surface tension properties and can play a crucial role in system penetration and deposition. Generally, adhesives lower the surface tension of the solvent, improving the system’s wetting properties and conformance to the substrates. At higher temperatures the solutions show a Newtonian behaviour, however with decrease in concentration a shear thinning effect is observed. Jun Funoriâ has a slight shear thinning behaviour and rise in viscosity as concentration increases and temperature decreases. Concentration also affects the gel point of gelatine solutions with a substantial impact on the penetration of the adhesive through the porous substrates. This is directly affected by the prevailing environmental conditions, setting the threshold gelling temperature for application. Material changes after tagging of the adhesives are investigated with unilateral NMR and FTIR. Whilst relaxation properties are slightly affected by the fluorochrome, no change is observed on the FTIR spectra. Unilateral NMR dynamic analysis successfully identified the movement of the adhesive system through the stratigraphy. Interestingly when correlating the NMR data to the actual deposition observed with the adhesive tagging, some discrepancies are observed. For the lower gelatine concentrations, the adhesive tagging shows deposition deep in the stratigraphy, however, this is not visible in the unilateral NMR analysis.
This highlights some of the limitations of the NMR technique and the benefits of using the two complementary analytical methods.
The study shows how the analytical protocol can successfully be employed to correlate material characteristics of the adhesives to assess their deposition in the stratigraphy. By better understanding rheological characteristics of adhesives, the conservator can tweak concentration and application temperature to better benefit from the material properties and support the design of adhesion interventions on paint layers.