Abstract
Dry and wet drawing materials were investigated by THz time-domain spectroscopy in transmission mode. Carbon-based and iron-gall inks have been studied, some prepared following ancient recipes and others using current synthetic materials; a commercial ink was studied as well. We measured the THz signals on the thin films of liquid inks deposited on polyethylene pellicles, comparing the results with the thick pellets of dried inks blended with polyethylene powder. This study required the implementation of an accurate experimental method and data analysis procedure able to provide a reliable extraction of the material transmission parameters from a structured sample composed of thin layers, down to a thickness of a few tens of micrometers. THz measurements on thin ink layers enabled the determination of both the absorption and the refractive index in an absolute scale in the 0.1-3 THz range, as well as the layer thickness. THz spectroscopic features of a paper sheet dyed by using one of the iron-gall inks were also investigated. Our results showed that THz time-domain spectroscopy enables the discrimination of various inks on different supports, including the application on paper, together with the proper determination of the absorption coefficients and indices of refraction.
Highlights
THz pulse imaging and spectroscopy[10] is an emerging noninvasive method for the characterization of cultural heritage artefacts that provides complementary information on traditional analytical tools
The THz-Time Domain Spectroscopy (THz-TDS) technique proves to be able to distinguish among the various black inks even when they are in the form of a thin film
THz timedomain spectroscopy was applied to investigate black inks, as iron-gall and carbon based inks commonly used in artworks
Summary
T-ray technologies are supported by a series of commercial off-the-shelf systems that enable THz spectroscopic investigations. Designed for few specific applications other than cultural heritage, these systems are not open to full control of signal detection and processing. We developed a specific experimental method and data analysis to disentangle multiple reflection signals. Our investigations are based on a well established technique, but the careful building of the THz-TDS experimental set-up and the acquisition procedure enables a very high signal to noise ratio that is not common in this type of experiment. The accurate analysis of the data based on the successive iterations in the fitting algorithm enables the calculation of the absolute absorption coefficient and the index of refraction of the materials, as well as the sample thickness down to tens of microns both in single layer and bilayer configurations
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