Detection of Delamination in Wall Paintings by Ground Penetrating Radar

Abstract

Wall painting is an important part of cultural heritage. Generally speaking, painting on the wall of buildings or rocks, and those on the wall of caves are called wall paintings. But painting on the rock face is called rock painting. Wall painting on the building can be approximately classified into drawing murals, relief frescoes, mosaic murals and etcetera material paintings. Chinese ancient wall paintings can be generally distinguished according to different drawing site, such as palace paintings, temple paintings, grotto frescoes, coffin chamber murals, residential paintings and so on. Most of the paintings, including grotto frescoes, palace paintings or temple paintings, have several hundred years, or even several thousand years of history. During this time, under the influence of environmental factors (light, temperature, humidity, wind, sand and so on), biotic factors (micro-organism, insect), painting support walls and materials, architectural composition and human factor, wall paintings have undergone various kinds of diseases and damage. The most common painting diseases are delamination, flaking, disruption, smoking, pollution, deep-loss, paintlosses, cracks-hatch, mechanical-damage and so on. Delamination is the loss of adhesion between layers in the support (wall, rock mass or others) and plaster stratigraphy, causing separation between plaster and suport. Delamination can occur between plaster layers, plaster and support. Generally, delamination causes painting surface crack and protrusion, even leads to painting losses because of gravity force from wall painting itself. Literally speaking, Tibet is a region with abundance of cultural relics. According to an incomplete statistics, there are more than 2,000 ancient architectures all over the region, among which 3 are included in the world heritage list, 27 are national key preservation units, 55 are provincial level ones and 96 are city or county level ones. A primary survey shows such cardinal ancient architectures, just like Potala Palace, Norbulingka and Sagya Monastery, and the wall paintings are in severe need to be conserved. The architecture deterioration mainly occurs in the forms of structural distortion, roof leakage, rafter mildew, moth-eaten, rat-bitten beams, while the wall painting deterioration displays in delaminated plaster, pigment flaking, plaster and wall crevice, plaster disruption, soot and contaminant, among which the most serious damage, taking up more than 75% areas in total seems to be delamination [1]. In this sense, the great task in the conservation of Tibetan cultural relics proves to be the combat against the delamination in wall paintings.