Paper #5 (Salvant et. al. 2011)

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?

More questions:

  • 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?

 

Key terms:

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.

Finding Neverland in the Netherlands: My Summer in an Art Heritage and Science Wonderland

Clockwise from top left: about to land at Schipol Airport; a scene taken during my daily bike commute to the lab; a private demonstration at De Kat, the only windmill still producing pigment; the Rijksmuseum and IAmsterdam sign

“There are some things you simply cannot prepare for,” I thought as the plane began the final descent over my home for the summer, the Netherlands. This first brush left me in awe– anxious to uncover the layers that make up this country’s unique character and rich past.

I had no idea that bike “traffic” existed, much less that there is a rush-hour for such traffic. I could not have anticipated being inches away from real-life actual Rembrandt paintings on our very first day. It is overwhelming to be in the presence of such revered masterpieces, and to contemplate how the artists of the Dutch Golden Age could achieve such strikingly realistic works. Even after seeing Girl with a Pearl Earring in person, I’m not sure I’ll ever understand how Vermeer was able to capture light in a bottle and apply it to canvas. Being in the center of this art hotspot feels like the luckiest day ever: as if you’d stumbled upon a whole field of four-leaf clovers, bathed overhead by a quadruple rainbow. Only in the Netherlands could I experience so much of the Dutch Golden Age. There have been so many once-in-a-lifetime experiences as I’ve learned about art conservation and Dutch cultural heritage.

The power of science to preserve these pieces is exhilarating. Admittedly, my background is in materials science and engineering, and so it has been eye-opening to be introduced to the art conservation field and its scientific, historical, cultural, and ethical components. I’ve learned it is a far cry from an easy task to preserve art; the endeavor seems quite chaotic when you stop to think of all the variables. What is the paint composition? What is the chemistry of the binders? How thick is each paint layer, and was it allowed to dry before the next layer was painted? What would the painting have looked like at the time of creation? This is just a tiny sample of questions that art conservators, art historians, and scientists must answer together.

As a student in the NU-IRES program, I conduct research at the University of Amsterdam (UvA) and make my own contribution to the art conservation community. My undergraduate research at Rutgers University heavily focused on characterization of polymer composites, and I became particularly interested in rheology. I am fortunate to continue studying rheology at Northwestern University as a graduate student in the Shull Group. Our group works extensively with the quartz crystal microbalance (QCM) to study polymer thin films. QCM is a powerful technique and—depending on film thickness– can be sensitive to mass changes and viscoelastic properties. At Northwestern, I have been using the QCM to investigate the effect of molecular weight on glass transition temperature in polystyrene thin films. A great advantage of the QCM is its portability. After my last final of the academic year, I packed the QCM equipment in a small cereal-sized box and took the lab on the road.

After landing in one of the art capitals of the world, it was time to shift to a material that art conservators and scientists really care about: linseed oil. I am investigating the QCM’s potential to study the early curing stages of this drying oil. My Dutch advisor, Dr. Piet Iedema, and his group have developed computational models to describe the mechanisms occurring in these early stages. I’m interested in developing a QCM experiment to study the changes in mass when a raw linseed oil film is applied to the QCM crystal. Linseed oil is composed of a hodgepodge of fatty acids; namely -linolenic, oleic, and linoleic acid. Atmospheric oxygen molecules react with the unsaturated bonds in these fatty acids and create highly reactive peroxide species, which then initiate a whole cascade of reactions. During these processes, the oxygen absorption corresponds to an increase in mass. The linseed oil also hardens during the cure process. I’m interested in using the QCM to monitor changes in both mass and viscoelasticity. Linseed oil has been more difficult than polystyrene to work with, and my equipment started malfunctioning. Fortunately, my best friend had planned to come visit in mid-July and was able to come with some replacement equipment from the Shull lab. But even with that stroke of luck, there’s always a risk of persistent problems. These various challenges provide great learning opportunities. So while I may not have been able to conduct as many experiments with linseed oil as I had hoped– I am still learning valuable information about QCM troubleshooting, circuits, mechanics, paintings, metal soaps, etc.

I am immensely grateful for the support of Dr. Shull, Dr. Iedema, Dr. Walton, and Dr. Gambardella this summer. Dr. Shull’s advising and troubleshooting help via email and weekly Skype calls has been a real lifeline. I thank him for his encouragement to apply for the NU-IRES program. The time here has been transformative in ways I could not have anticipated, but sometimes it’s fantastic to have your breath taken away.