Thin layered drawing media probed by THz time-domain spectroscopy

Area of Science:

• Physics
• Chemistry
• Materials Science
• Spectroscopy

Background:

• Characterizing inks, particularly historical ones, requires non-destructive analytical methods.
• Terahertz (THz) time-domain spectroscopy (TDS) offers unique capabilities for probing material properties in the sub-terahertz to terahertz frequency range (0.1-3 THz).
• Accurate extraction of optical parameters from thin, layered materials presents experimental challenges.

Purpose of the Study:

• To investigate the application of THz time-domain spectroscopy for analyzing carbon-based and iron-gall inks.
• To develop and implement an accurate experimental and data analysis method for extracting transmission parameters of thin ink layers.
• To determine the absorption coefficient, refractive index, and thickness of various ink formulations on different supports.

Main Methods:

• THz time-domain spectroscopy was employed in transmission mode.
• Measurements were performed on thin films of liquid inks and dried inks blended with polyethylene powder.
• A specialized experimental and data analysis procedure was developed for structured, thin-layer samples.

Main Results:

• THz-TDS successfully determined the absolute absorption and refractive indices of inks in the 0.1-3 THz range.
• Accurate layer thickness measurements were achieved for inks as thin as tens of micrometers.
• Spectroscopic features of iron-gall ink on paper were successfully investigated, demonstrating applicability to different supports.

Conclusions:

• THz time-domain spectroscopy is a powerful, non-destructive technique for characterizing diverse ink types (carbon-based, iron-gall).
• The developed method enables reliable discrimination of inks and determination of their optical properties on various substrates, including paper.
• This approach holds significant potential for art conservation, historical document analysis, and quality control of printing materials.