Background and summary
Testing the mechanical properties of original artworks is a daunting task. Paintings are often highly heterogeneous and have multiple paint layers, each of which can vary in pigment identity, size, concentration, dispersion. Each layer’s thickness plays a crucial role, too. For example, greater thicknesses require longer diffusion paths in order to intake oxygen in the early stages of curing, and then expel volatiles during the later curing stages. Each paint layer pigments, composition, thickness, etc. Another complication arises from the small sample volumes available to study. Fortunately, pre-existing cross-sections used for other techniques (SEM, Raman/IR, OTC, etc.) are sufficiently large to make nanoindentation feasible. Salvant et. al. performed nanoindentation tests on cross-sections of modern reconstructions– lead white and zinc white– created in their lab; as well as on cross-sections taken from original van Gogh artworks. For the VG samples, the exact composition (especially the organic binder) is unknown, though SEM-EDX gives a reasonable idea of the mineral content in each layer. The VG samples all had higher reduced elastic modulus, E*, as well as hardness, H, values compared to the modern LW and ZW reconstructions. The authors attribute this difference to the curing process, with atmospheric oxygen breaking double bonds in the organic binder and forming crosslinks between binder molecules. The increase in hardness is correlated with the increased crosslink density. Salvant et. al. also investigate creep in these samples. Viscoelastic polymers may reduce the internal stress (which can lead to rupture and cracking) imparted by the cure process by stress relaxation via creep. The authors note that the VG samples were more creep-resistant than the modern reconstructions, which is consistent with the premise that the VG samples are more crosslinked.
Salvant et. al. claim nanoindentation can offer layer-by-layer information about reduced elastic modulus, E*, and hardness, H. Do you think they are successful? Why or why not?
- When is it appropriate to test original artworks?
- What information can nanoindentation provide?
- What is creep? Is nanoindentation an appropriate technique to measure creep?
- How can we enhance this technique in the context of art conservation?
Cross-section: A small (on the order of microns) sample taken from a painting. Cross-sections often are taken from areas on the painting where there is already damage i.e. cracks. The paint sample is embedded in resin in a manner that shows the buildup of layers. The cross-section is then polished and ready for testing.
Creep: The time-dependent deformation response of a material to a constant applied load. For example, internal tensile stresses generated by the curing process are a load source. The plasticity of the paint allows it to slowly strain, or creep, to relax the stress. Creep experiments are typically performed over long lengths of time.
Elasticity: A time-independent deformation response obeying Hooke’s law: an applied force/stress is directly proportional to the resulting elastic deformation (and the material’s stiffness constant). Elasticity can be fully recovered upon load removal.
Hardness: The ratio of an applied pressure over a contact area. Hardness values
Plasticity: The time-dependent deformation response which has its onset at a material’s yield point. Plastic deformation is non-recoverable and has a nonlinear relation to the applied stress.
Reduced modulus E*: The reduced modulus is a function of the Poisson ratio, v, and the Young’s modulus, E. E* is used in nanoindentation literature because the removal of the indenter tip affects the indentation cavity geometry.