THz imaging tool for characterizing both visible and concealed metals in paint layers

The Terahertz (THz) radiation can penetrate optically opaque materials, and in combination with low photon energy makes possible imaging applications in a variety of fields such as biomedicine, material science, security, etc.. This has motivated a growing interest in THz techniques during recent years and, in turn, has resulted in significant technological improvements and the introduction of several commercial-type systems. In particular, THz technology has been demonstrated to be particularly useful for the study of multi-layered non-metallic structures such as those encountered in easel and mural paintings, as well as of glued paper layers and of other similar ones. This work focuses on metallic layers concealed beneath paint layers in which THz imaging can be very effective thanks to the high reflection of metals. THz imaging measurements have been carried out using a time-domain THz spectrometer equipped with an XY translation stage. Both mock-ups and original multilayer samples were imaged. The former were prepared by gluing metal foil polygonal shapes onto a plastic substrate and then covering the whole of it with paint layers. The THz imaging revealed the metal shape beneath the paint layers, thus making possible some assessments on the layer depth. Original pictorial samples from a Sienese painting of the XIII-XIV century were also investigated. In this particular case, the THz imaging was capable of distinguishing between the original gold leaf and the paint layers (overpaint).