The pictures in question were painted for the dining room of the Holyoke Centre, a modernist lump built by Harvard University in 1966. They did not hang there long, though. Rothko liked to mix his own paints, said Dr Stenger, and had no idea how his concoctions would react to the abundant sunlight the Holyoke was designed to admit.

The answer, it turned out, was not well. After just 15 years they had faded so badly that they were consigned to a darkened basement for their own protection. Worse, when Dr Stenger and his colleagues dug out photographs taken of them when they were new, the researchers were dismayed to find that the photographs were not light-fast either, and that they too had faded over the years.

Fortunately the emulsion used standard pigments. This meant a chemist could work out how it would have reacted to sunlight. That let the researchers work backwards to make a computer-generated image of the original photos, and thus of the original paintings. But what to do with this information?

Any restoration would have involved extensive repainting. A materially minded scientist might wonder why that should be a problem, as long as the result was faithful to the original. But the finer sensibilities of art historians are, apparently, offended by this approach. Such people regard simply slapping on a new coat of paint as unethical.

If you cannot change the paint, though, you can change the lighting instead. In 1986 Raymond Lafontaine, a Canadian art conserver, outlined how shining coloured light at a painting could counteract the effects of yellowish varnish overlying the image. Craft this optical illusion carefully and you can change the colours of a picture in a natural looking way.

In the case of the Holyoke Centre’s Rothkos this was not easy. Each had faded differently, depending on its original colours and how much sunlight it had seen. And various parts of individual paintings had faded at different rates, too. But modern technology allows optical illusions to be finely crafted indeed. The paintings are continuously observed by a high-resolution camera. Its images are compared, pixel by pixel, with the idealised versions provided by the restored photographs. A computer then works out, moment by moment, what mixture of light to shine back to make the faded originals match the vibrant reconstructions—with no messy repainting necessary. For now, the paintings remain under wraps while the museum at which they are stored is renovated. One day soon, though, they will be on display in all their illusory glory.