Not just simple water drips : the case of an early 1900s oil painting on stone

Abstract

Water drips marks are commonly found on wall paintings as a result of water infiltration, often perceived as stains. Water drips can cause material migration and deposition while potentially interacting with water-soluble constituents. Peculiar features of water marks noticed during a conservation campaign on a dome painting in a church in Malta, generated a series of investigations. This study presented aim at gaining insight into the process involved in the deposition of material on the painted surface and its potential implications for the conservation of the painting. Painted in 1903 by Giuseppe Calì (1846-1930), a leading local artist, the oil-based painting on stone is characterized by a complex stratigraphy and material composition. The painting was left in critical condition after a long history of recurring heavy rainwater infiltration triggering extensive salt deterioration and water drips. Visual observations indicated that the water drips emerged from paint craquelure and along their path they reacted with the painted surface. Semi-transparent white-brownish material deposited by the drips were examined by means of digital microscopy and were characterized into three distinctive morphologies: cubic, needle-shaped and amorphous. Sampled drips were further studied by a multianalytical approach, involving IR and Raman spectroscopies and GC-MS with pyrolysis. In particular, the organic fraction of the deposited material mainly consisted of fatty acids from the oil medium, with dicarboxylic acids being consistently detected. This could indicate that a portion of the dried lipidic crosslinked network, especially the more hydrophilic fraction, and formed metal soap were segregated and separated from the oil-based painting and partially leached onto the surface of the painting. These results provide useful insights into the interaction of liquid water with oil paints, and on its detrimental effects on the stability of the oil-based paint system.