Fig. 1. Schematic of THz-TDS

Fig. 2. Influence of pigment particle size on THz spectra^[37]. (a) Change of colors by varying particle size; (b) THz spectra of celestite with different particle sizes; (c) spectra of painted samples observed by FTIR

Fig. 3. THz spectra of five white pigments^[37]

Fig. 4. THz spectra of mineral pigments^[37]. (a) Natural/artificial lead (Pb) composites; (b) natural/artificial copper (Cu) composites; (c) natural calcium (Ca) composites; (d) natural earth pigment (Fe, Mn, Al)-based composites; (e) natural carbon black and ivory black pigments; (f) artificial cobalt (Co) composites; (g) artificial zinc (Zn) composites; (h) artificial cadmium (Cd) composites; (i) artificial iron (Fe) and manganese (Mn) composites; (j) artificial arsenic (As) composites

Fig. 5. THz spectra of organic dyes^[37]. (a) Natural dyes derived from insects and squid; (b) natural dyes derived from plants; (c) artificial dyes

Fig. 6. THz spectra of binders^[37]. (a) Oils; (b) natural resins; (c) various binders, beeswax, casein, gum arabic, and glues; (d) artificial polymeric binders; (e) artificial cellulose-type glues

Fig. 7. THz spectra of mixed binders^[37]. (a) Venetian turpentine or oleo-resin, a natural mixture of oil and resin; (b) Beva, a mixture of PVAc and natural resin; (c) black oil, a mixture of walnut oil and litharge

Fig. 8. THz spectrum of cobalt blue pigment^[37]. (a) Mixture with natural oil; (b) mixture with an acrylic resin

Fig. 9. THz transmission spectra and absorption spectra of three traditional Chinese pigments^[42]. (a) (d) Blue pigment azure; (b) (e) red pigment cinnabar; (c) (f) yellow pigment, arsenic yellow

Fig. 10. Seven traditional Chinese pigments^［43］. (a)‒(f) THz absorption spectra of seven pigments;（g）‒（l）absorption spectra of the FTIRS

Fig. 11. Schematic of THz wave scanning imaging^[26]

Fig. 12. Typical drawing model and penetration depth of each electromagnetic wave^[37]

Fig. 13. Simulated sample^[48]. (a) Optical photograph of graphite butterfly sketch under paint; (b) THz image of sketch under paint seen in Fig.13(a); (c) THz image of sketch through 4-mm thick plaster

Fig. 14. Wall paintings^[48]. (a) Optical photograph of the original mural; (b) optical photo of the new mural on the original mural; (c) underlying THz image of blurred murals

Fig. 15. Multilayer fabric sheet from canvas painting in 1850^[37]. (a) Multi-layer fabric sheet containing oil and/or resin content under the new linen sheet; (b) double layer fabric sheet with or without paint and its THz image

Fig. 16. THz image of a multi-layer fabric sheet from a mummy^[37]. (a) Sample and measurement area; (b) THz reflection image and cross section of embroidered fabric sheets; (c) THz reflection image and cross section of embroidered fabrics covered by more than two fabrics

Fig. 17. THz image of a wrapped mummy at the Metropolitan Museum of Art in New York^[37]. (a) Measurement device; (b) observation area; (c) images obtained by X-ray CT scanning system

Fig. 18. THz image of the mummy’s shoulder and center^[37]. (a) Reflection image of the shoulder; (b) cross-sectional image along dashed line aa'; (c) part of the output signal at point A; (d) THz region image of the central region

Fig. 19. Preparation layer model and THz cross-sectional image of dry process^[37]. (a) Egypt; (b) China; (c) Japan

Fig. 20. Nondestructive analysis results of fragments of Danzhao Temple murals in Inner Mongolia^[37]. (a) Test points of Raman spectrum; (b) pigment recognition based on Raman spectroscopy; (c) THz reflection image; (d) XRF element analysis result; (e) near infrared reflection image

Fig. 21. Results of data processing^[62]. (a)(b) THz images after data processing; (c) THz images without data processing; (d) (e) data processing processes

Fig. 22. Oil painting on iron foil with size of 15 cm × 11.5 cm, a crowned Virgin standing in the countryside^[37]

Fig. 23. Oil painting on tinfoil with size of 25 cm × 20 cm, San Geronimo holding a cross^[37]

Fig. 24. Three different types of lacquerware, including Kentai, Kiji, and Hikimono^[64]

Fig. 25. THz image of a ceramic can^[65]