  THz time-domain spectroscopy in transmission mode was applied to study dry
and wet drawing inks. In specific, cochineal-, indigo- and iron-gall based inks
have been investigated; some prepared following ancient recipes and others by
using synthetic materials. The THz investigations have been realized on both
pellet samples, made by dried inks blended with polyethylene powder, and
layered inks, made by liquid deposition on polyethylene pellicles. We
implemented an improved THz spectroscopic technique that enabled the
measurement of the material optical parameters and thicknesses of the layered
ink samples on absolute scale. This experimental investigation shows that the
THz techniques have the potentiality to recognize drawing inks by their
spectroscopic features.
