  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.
