January 4, 2013 7:44 pm

Lights off for Van Gogh?

Some of the vivid yellow pigments in the painter’s works are unstable in art gallery lighting
Vincent van Gogh's 'Bank of the Seine'©Vincent van Gogh Museum

Van Gogh’s 'Bank of the Seine” (1887), featuring his trademark chrome yellow, which is gradually darkening

Art conservators worry about the effects of air pollution and light on their paintings. Now a scientific study shows that, when it comes to Vincent Van Gogh, there is real cause for concern.

A consortium from several European museums and universities has found that some – but not all – of the vivid yellow pigments that are an important feature of Van Gogh’s late works are unstable in the sort of lighting likely to be found in art galleries.

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“We started this investigation because a number of museum curators noticed some of the yellows becoming gradually darker,” says Koen Janssens, an analytical chemist at the University of Antwerp. “Some people thought the darkening came from the layer of varnish on the paint but others believed the paint itself was changing colour.”

The research focused on chrome yellow, a pigment introduced in the 19th century, which Van Gogh favoured because it gave intense colours. The difficulty faced by the investigators was that chrome yellow – lead chromate – came in several slightly different chemical and crystalline forms, offering variations such as “lemon yellow” and “primrose yellow” as well as the regular “middle yellow”.

Scientists used ultra-powerful X-ray beams at the European Synchrotron Radiation Facility (ESRF) in France and the Deutsches Elektronen-Synchrotron (DESY) in Germany to probe the structure of the materials.

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While “middle yellow” was quite stable, the other variants began to turn a brownish or olive-green colour after just a few days’ exposure to ordinary green-blue light at levels previously believed to be safe. The findings are published in the journal Analytical Chemistry.

“We found the unstable forms of chrome yellow in several well-known paintings, such as “Portrait of Gauguin” and the famous “Vase with Sunflowers” of the Van Gogh Museum in Amsterdam,” says Letizia Monico, a member of the study team.

Ella Hendriks, conservator at the Van Gogh Museum, which has the world’s largest collection of paintings by the Dutch master, says cautiously that more research will be required to discover to what extent her museum’s works have actually been damaged through light-induced degradation of unstable chrome yellow pigments. One complication is that, although Van Gogh did not varnish his paintings, they were generally varnished by subsequent owners in the 20th century, which may have protected them.

But Hendricks urges curators to take note of the findings. “Studies like this are very important to make museum curators aware that, even under ambient light conditions, the degradation of some sensitive materials proceeds continuously.”

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“Mr Darcy” effect helps mice find their way

The scent marks that male mice leave to define their territory contain a pheromone – sexual signalling molecule – with extraordinary powers to induce and fix spatial memories in other mice.

In a series of experiments, published in the journal Science, researchers at Liverpool University found that a pheromone in scent marks, called darcin, promotes rapid and durable learning of the spatial clues associated with its location.

“We have shown that a male sex pheromone in mice makes females and competitor males remember exactly where they encountered the pheromone and show a preference for this site for up to two weeks afterwards,” says Sarah Roberts of Liverpool. “Given the opportunity, they will find that same place again, even if they encountered the scent only once and the scent is no longer there.”

Darcin – named after Mr Darcy in Jane Austen’s Pride and Prejudice – works in a quite different way to the airborne sex pheromones that some animals use to attract mates over long distances. Darcin is a non-volatile protein that works by contact, when a mouse touches another’s scent mark with its nose. This contact activates it not only to remember the other chemicals in the scent – the marking mouse’s signature – but also to remember how to find it again.

Female mice remember the location because it holds the promise of a potential mate. Competing males are motivated by the desire to drive off rivals and countermark.

“Pheromones are used for sexual attraction and for advertising an animal’s location in many species, particularly among mammals,” says Jane Hurst, another member of the research team. “It will be interesting to discover whether such highly potent associative learning is induced by pheromones in other species and to understand the brain mechanisms involved.”

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A termite mound©Dreamstime

Golden opportunity? Australian prospectors turn to insects

Termite mounds worth weight in gold

Termite mounds and ant hills could give gold prospectors a good idea of where to start digging. Australian researchers have discovered that these insects concentrate gold from several metres below the ground in their nests.

“We’re using insects to help find new gold and other mineral deposits,” says Aaron Stewart, an entomologist working for CSIRO, Australia’s national research organisation. “These resources are becoming increasingly hard to find because much of the Australian landscape is covered by a layer of eroded material that masks what’s going on deeper underground.”

Although Australia’s tallest termite mounds are 5m tall and contain soil from as deep as 30m, smaller mounds and ant hills also contain useful indications of any mineral wealth beneath, the CSIRO researchers have found.

“The insects bring up small particles that contain gold from the deposit’s fingerprint, or halo, and effectively stockpile it in their mounds,” Stewart says. The metals then accumulate in the termites, and the insects get rid of them by excreting tiny stones rather like human kidney stones.

Although gold levels in termite mounds are higher than in the surrounding soil, they are not high enough for golden flecks to be visible to the naked eye or for it to be worth demolishing termites’ mounds in order to extract metal directly from them.

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Increased accuracy for home blood tests

New diagnostic technology to improve the accuracy of mass-market blood tests, such as for glucose levels in diabetics, has been developed by a UK company.

PA Consulting, a research and innovation group, came up with a simple but effective idea: to include a “control sample” of the substance being tested within the diagnostic kit. So, for a blood sugar test, the kit tests a known reference solution of glucose at the same time as measuring the glucose in the patient’s sample.

This “live calibration” reduces the chance of an erroneous reading, for example through the “haematocrit effect” caused by variations in the concentration of red blood cells between patients.

It also corrects automatically for temperature.

“We evaluated the performance of our live calibration in a clinical setting using blood samples from 160 patients with diabetes, with and without our correction technique,” says Michael Noble of Exacsys, a company spun out from PA Consulting to commercialise the technology. “The results showed that total error is reduced by about half, and outliers, where results are more than 20 per cent different from the reference value, are reduced by 90 per cent.”

The blood glucose test will need a couple more years of development before reaching the market. Another likely application of the technology will be in testing blood cholesterol levels; in that case a reference sample of cholesterol will be included in the kit.

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