Terahertz Time-Domain Spectroscopy in Reflection Configuration for Inorganic and Mineral Pigment Identification

Area of Science:

• Spectroscopy
• Materials Science
• Archaeometry

Background:

• Terahertz time-domain spectroscopy (THz-TDS) is a non-destructive technique.
• Its application to inorganic and mineral pigments in reflection mode faces challenges like weak signal intensity and data scarcity.
• Analyzing ancient pictorial materials requires robust identification methods.

Purpose of the Study:

• To demonstrate and validate THz-TDS in reflection configuration for analyzing inorganic and mineral pigments.
• To establish a reliable experimental procedure for determining material absorption coefficients.
• To classify vibrational modes for material identification in cultural heritage contexts.

Main Methods:

• Utilized THz-TDS in reflection mode on 10 inorganic and mineral pigments used in ancient frescoes.
• Developed a method involving normalization with a reflective reference sample.
• Implemented filtering to remove secondary reflections and considered instrument dynamic range limitations.

Main Results:

• Successfully differentiated molecular phases of the same metal (e.g., azurite, Egyptian blue, minium, cinnabar).
• Identified specific pigments by classifying their molecular oxide and sulfide vibrational modes.
• Demonstrated reliable material identification up to 125 cm^-1 despite signal limitations.

Conclusions:

• THz-TDS in reflection mode is a viable technique for analyzing inorganic and mineral pigments.
• The established methodology overcomes common limitations, enabling precise material identification.
• This approach has broad applicability beyond cultural heritage, extending to other material studies.