Water dynamics during hydrogel and traditional poultice applications on multilayered wall paintings

This study addresses the intrusion of water from hydrogels and cellulose poultices into Danish medieval wall painting structures, aiming to improve the prediction and optimization of gel and poultice treatments in conservation practice. Absorption from gels based on agar and poly(vinyl alcohol), as well as cellulose poultices, was examined both in situ on medieval wall paintings and in laboratory replicas. To further assess the impact of the substrate, variations in plaster composition and salt contamination were incorporated in the replicas. Water absorption rates were assessed gravimetrically, while water distribution during treatments was monitored using nuclear magnetic resonance imaging (MRI), a non-invasive 3D technique. The results showed that water absorption was markedly reduced when using gels and poultices compared with unretained water: by up to 95% for PVA gels, 83% for agar gels, and 19% for cellulose poultices. Despite these substantial reductions, within 30 minutes, the agar gel and cellulose poultice allowed wetting of almost the entire plaster, whereas the PVA gel kept water confined primarily to the limewash and the upper plaster layer at that time. The wall painting stratigraphy produced three distinct absorption phases, reflecting water progression through the limewash, plaster, and brick. Higher binder content in the plaster increased the second absorption rate, while salt contamination also modified water intrusion from gels. For PVA gels, salt-induced deswelling increased absorption. The combined gravimetric and MRI measurements enabled the identification of these three absorption phases and demonstrated that the process constitutes a dynamic equilibrium between water uptake and further downward transport.