The ESPI optics, which consists of four diverging beams of laser light that cover the whole canvas, are mounted about one metre above the model. Each point on the surface of the painting acts as a point source for coherent scattering. A charge-coupled device (CCD) camera records an image of the interference fringes, or “speckle pattern”. The tension, temperature or humidity is then changed, and another image is taken. The final result is a map of the difference between the two interference patterns, which shows the deformation. The map is directly related to the strain on the canvas, and gives conservationists a clear and accurate picture of movement in the painting under different conditions.
According to Young, the tensile tests have shown that the loads in the paintings are not uniform, and the ESPI has revealed that there can be some very high strains on a canvas. She has also found some unexpected compression forces at the corners of paintings – just where many cracks occur.
But as with any new conservation technique, it will take a while for the art world to build up confidence in ESPI. As a general rule, most techniques are first tested out on model paintings or damaged pieces. More famous works of art are left alone until a technique has proved its worth. However, if a painting comes in for restoration and there is no other way of approaching it, a new method can suddenly find itself in the limelight. It is hoped that ESPI will soon be playing an important role in trying to slow down the deterioration of the Tate’s collection of William Blake paintings.
Dr Sharon Ann Holgatesharonannholgate.com