In this issue: Color Darkening in Historical Paintings, Thickening When Stirred, How the Oldest Fossils Were Formed and More
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Color Darkening in Historical Paintings
X-ray studies of medieval murals explain why red pigments in historical paintings have degraded over time.
For over two millennia vermilion (mercury sulfide) has been the painter’s finest red. But in many old paintings the pigment has discolored to a drab brownish black. The reason for the change has been controversial, but a team of researchers from Belgium, Italy and France now claims to have figured out the cause of such degradation. The authors applied X-ray techniques to examine the composition of a discolored vermilion paint layer from a medieval mural in a Catalan monastery. Guided by simulations, they suggest the pigment darkens via a sequence of light-initiated chemical reactions that involve the formation of metallic mercury. The finding clarifies the connection between degradation and environmental conditions and may lead to better ways of protecting paintings through the proper choice of lighting and humidity conditions.
* Fabiana Da Pieve (contact author) et al, “Casting light on the darkening of colors in historical paintings”, Physical Review Letters (expected publication date: Nov 15)
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Thickening When Stirred
Researchers have developed a new model able to predict the behavior of fluids that thicken when stirred.
Certain liquid suspensions like cornstarch in water respond differently than normal liquids to shear: they thicken when they are stirred, becoming solid-like. The phenomenon may be detrimental in industrial applications (e.g. the processing of ink or slurries) but could also be exploited to engineer liquid body armors. Now a research team in New York has developed a numerical model that, for the first time, can accurately describe and predict the shear-thickening transition. The development is based on the recognition of the key role played by friction between particles suspended in the liquid.
* Ryohei Seto (contact author), R Mari, JF Morris, MM Denn, “Discontinuous shear thickening of frictional hard-sphere suspensions”, Physical Review Letters (expected publication date: Nov 18)
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How the Oldest Fossils Were Formed
The analysis of billion-year-old sedimentary rocks provides information on some of the earliest microorganisms on Earth.
Stromatolites are 3.5-billion-year-old sedimentary rocks that are regarded as the fossilized deposits of ancient biological layers. These formations may thus carry information on the oldest life forms on Earth. A team of researchers from MIT and the University of Johannesburg, in South Africa, has developed a model that explains the growth of conical stromatolites, the most simply structured forms of the fossils. They grow as ions of calcium or silica diffuse thorough a biofilm and precipitate as minerals around cells, causing them to fossilize. Over many generations, these structures can grow into meter-sized structures with conical symmetry. The model allowed the authors to estimate some of the properties of the microbial mats that led to the fossils we see today.
* Alexander P Petroff (contact author), NJ Beukes, DH Rothman, T Bosak, “Biofilm growth and fossil form”, Physical Review X (expected publication date: Nov 13)
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More from the APS Physics News Ticker:
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Journal articles and preprints are available to journalists on request.
Matteo Rini, PhD
Deputy Editor, Physics
