Thin layered drawing media probed by THz time-domain spectroscopy

TL;DR

This study demonstrates the use of THz time-domain spectroscopy to analyze thin layered drawing inks, including ancient recipes, providing detailed optical properties and enabling ink discrimination on various supports.

Contribution

It introduces a new experimental method and data analysis procedure for extracting optical parameters from thin ink layers using THz spectroscopy.

Findings

Able to determine absorption and refractive index of thin ink layers

Discriminates different inks and supports using THz spectral features

Successfully measures ink properties on paper and other substrates

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 thin film 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 a new 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 thickness of 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 one of the iron-gall inks were also investigated. Our results showed that THz time-domain spectroscopy enables to discriminate the various inks on different supports, including the application on paper, together with a proper determination of the absorption coefficients and indices of refraction.