Establishing the fracture properties of delaminating multilayered decorative coatings on wood and their changes after consolidation with polymer formulations

Abstract

In the field of cultural objects conservation efficient stabilisation of fragile and failing, multilayered decorative coatings is a complex and challenging task. This paper introduces a new application of the standardised double-cantilever beam (DCB) test method to improve the understanding of the mechanical properties of failing material and to determine the effect of polymeric agents (consolidants) added for their stabilisation. The adapted DCB method was used to measure the fracture energy, G Ic, and the fracture behaviour of brittle, protein-bound (gesso-type) foundation layers on wooden substrates that typically suffer from delamination and flaking. Wooden DCB specimens containing a brittle layer of protein glue mixed with finely ground clay powder were prepared, fractured, then consolidated with a range of commonly used polymer formulations and finally re-fractured to provide measurements for direct comparison. Consolidants tested included gelatine-based glues (bovine hide glue, isinglass; both pre-stained with Fast Green dye), acrylics (Lascaux Medium for Consolidation, Paraloid B-72/B-48N), poly(vinyl acetates) (Mowilith 50, Mowilith DMC2) and poly(vinyl alcohol) (Mowiol 3-83). Before second-phase fracture cross-sections were taken from the DCB specimens for determining penetration depth and gap-filling ability. For better visibility, the specimens containing acrylics were stained with Solvent blue G dye; iodine-potassium iodide was used for staining the other synthetic consolidants. The resulting data showed that the test method could determine measurable differences between initial G Ic (47 ± 22 J/m2) and post-consolidation G Ic values. Also, penetration behaviour could be well characterised and valuable, and detailed information on the type and location of crack path propagation was gained.