Physicists paint a clearer picture

Each layer of a canvas is also affected by humidity. In dry conditions, the size layer loses moisture and contracts, making the canvas tight. On more humid days, the layer gains moisture and expands. And since paint gets brittle with age, cracks can start to appear. Similar problems are caused by changes in temperature, as the layers of paint expand and contract at different rates.

The extent of cracking can be assessed visually by recording digital images of the painting using visible light or by taking an X-ray photograph. However, paintings decay so slowly that such images say little about what is actually causing the cracks to occur. What is needed is a map of the changes in the strain distribution in the painting, and it is here that electronic speckle pattern interferometry (ESPI) comes into its own. The technique allows curators to study how a painting responds to changes in environmental conditions, such as temperature and humidity.

So how does ESPI work? To prevent the artwork from being physically damaged, a “model” of the painting is first created – usually a small piece of canvas containing a copy of one section of the painting. The model is placed inside a small environmental chamber, which allows the temperature and humidity to be changed. The tension on the picture is then altered using a biaxial tensile tester, which consists of a set of clamps that stretch the painting in four directions. This mimics the loading on real paintings, which are pulled on all four sides by a stretcher. Load cells on each of the clamps measure the tension in the painting.