Comparison between thermographic techniques for frescoes NDT

The use of infrared thermography in the architectural restoration field is examined. Three samples, made of a support of marble, brick, or tuff, covered with a layer of plaster with inclusions to simulate detachments or cracks in frescoes, are considered. Different techniques: pulse thermography, lateral heating thermography, lock-in or modulated thermography and pulse phase thermography are employed to detect the flaws artificially created; advantages and disadvantages of each technique are discussed. It is found that pulse thermography is easy and fast to use for information about the state of the art treasures, but data may be affected by non-uniform heating and local variation of thermal emission; the lateral heating can help to overcome interference effects due to non-uniform heating but it is more troublesome to use. When the evaluation regards rare art treasures the lock-in technique seems to be the only response since it is able to operate within very low increase of surface temperature; this technique is also able to give information about the material composition. The pulse phase thermography may be used to detect more in depth flaws but it needs higher temperature increase with respect to the ambient temperature and so it is recommended to control, before testing, the temperature sensitivity of the artefact. (C) 2002 Elsevier Science Ltd. All rights reserved.