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Structural delamination in mural paintings is a complex phenomenon and is considered among the most frequent types of damage. In conservation practice, the most common technique to identify structural detachments in wall paintings is the percussion method, otherwise known as the ‘tap and listen’ approach. Full-field optical techniques based on interferometry, such as shearography, can potentially provide a more scientifically substantiated evaluation of the condition of heterogeneous structures of wall paintings. The empirical nature of the percussion method was observed during the condition assessment of two medieval wall paintings in Maria Church, Nisse, Netherlands. Consequently, the need to obtain more knowledge regarding the structural condition of these wall paintings was established. Furthermore, to allow the formulation of specific treatment needs for structural delamination in wall paintings, accurate defect mapping is needed. The application of shearography coupled with thermography was believed to provide an holistic representation of the structural condition of the wall painting depicting St. Christopher in Maria Church. A comparison of the practical execution and the interpretation of data between shearography and the percussion method was conducted to deliver a description of both techniques. Initial comparison of the two methods confirmed what was already hypothesised: I) the percussion method can provide an approximate indication of delaminated areas. Results, which are considered both non-qualitative and non-quantitative, are dependent on the experience of the conservator; II) shearography can produce qualitative and quantitative results regarding the presence of delamination in complex structures of wall paintings. Analysis of the shearography data together with the percussion method may result in a reliable way to map the defects for future conservation activities. The next steps include optimisation of the thermal excitation for shearography to maximise the defect detection and adapt the shearography output for onsite interpretation by conservators and professionals in the cultural heritage.
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