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Terahertz time‑domain
spectro‑imaging and hyperspectral
imagery to investigate a historical
Longwy glazed ceramic
F. Fauquet
1, F. Galluzzi
3, Philip F. Taday
2, R. Chapoulie
3, A. Mounier
3, A. Ben Amara
3 &
P. Mounaix
1*
In this paper, we present the potential of Terahertz Time‑Domain Imaging (THz‑TDI) as a tool to
perform non‑invasive 3D analysis of an ancient enamel plate manufactured by Longwy Company in
France. The THz data collected in the reection mode were processed using noise ltering procedures
and an advanced imaging approach. The results validate the capability to identify glaze layers and
the thickness of ceramic materials. To characterize the nature of the pigments, we also use with
X‑ray images, visible near‑infrared hyperspectral imaging spectroscopy, and p‑XRF (portable X‑ray
uorescence) to qualitatively and quantitively identify the materials used. The obtained information
enables a better understanding of the decoration chromogens nature and, thus, to determine the
color palette of the artists who produced such decorative object. We also establish the eciency of a
focus, Z‑tracker, which enables to perform THz imaging on non‑at samples and to attenuate artifacts
obtained with a short focus lens. Then, 3D images are extracted and generated, providing a real vision.
We also report the evaluation of the internal damage state through the detection of fractures.
Keywords Earthenware, Polychrome glazes, Pigment, Ceramics, p-XRF, Hyperspectral imaging, Infrared
false color, Terahertz imaging, ickness, Fracture, Non-destructive analyses
e conservation of cultural heritage greatly benets from innovative scientic techniques, which have an impor-
tant impact. Given the value and exclusivity of art pieces, non-invasive diagnostic methods are highly esteemed
by conservators1. Terahertz (THz) waves have shown their capabilities to provide structural information about
materials, as well as data about their chemical nature in art science2,3. While the ultraviolet penetration length
is limited to varnish layers, visible light to paint layers, and infrared to imprimitura layers, THz waves can reach
the preparation layers and substrate. Terahertz time-domain imaging (THz-TDI) is used to analyze paintings4–6
because of its penetration depth. Reconstructing three-dimensional air gaps or material structures within those
objects was also demonstrated, which could be an invaluable tool for the evaluation of damage7 and restoration
of paintings, pottery, sculptures, buildings, etc. Moreover, measuring the varnish layer is possible and permits
the curator to restore such a layer without altering the paint layer underneath8. Similarly, assessing the physical
characteristics of the stratigraphy, not only across a painting but also in-depth, is one of the most important
procedures for gaining insight into its structure9. Using THz pulsed time-domain imaging, the internal structure
of a painting can be observed layer by layer by detecting the reection-pulses generated at the interfaces as the
THz pulse transmits, and a cross-sectional image can be reconstructed from the output data10,11. Terahertz is
rarely used as a diagnostic tool for ceramic fabrics but it can provide information about their layered structure
and spatial dielectric properties3,12–15. Moreover, information regarding internal features is crucial for determining
the state of conservation of a glazed object, highlighting detachments, inhomogeneities, or other defects under
the surface. Pottery is one of the oldest human inventions that originated before the Neolithic Period. e earli-
est known pottery vessels were discovered inJiangxi, China, dating back to 18,000 BC. e pottery techniques
evolved with knowledge and spread all over the world16, among which French porcelain is famous worldwide
thanks to an emblematic city: Limoges. French porcelain is a permanent tableware, characterized by renement,
OPEN
1Laboratoire IMS-UMR 5218 CNRS, Université Bordeaux, Bat A31, 351 cours de la libération, 33405 Talence,
France. 2TeraView Ltd., 1, Enterprise, Cambridge Research Park, Cambridge CB25 9PD, UK. 3Archéosciences–
Bordeaux: Matériaux, Temps, Images et Sociétés-UMR 6034, Domaine Universitaire, 33607 Pessac Cedex,
France. *email: patrick.mounaix@u-bordeaux.fr
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historical know-how, and extraordinary originality. Ceramic can be classied into three categories: porcelain,
earthenware, and stoneware. e main dierences between them are the composition, ring technique, and
temperature. is ring process produces vitrication, which provides strength, transparency, and a non-porous
nature to the porcelain3,12. In this paper, we analyze an ancient enamel plate made by the “Longwy Company” to
report on the 3D structures of the plate, the potential of separating the diverse coloring techniques, and evaluating
the thickness of this ceramic material. e paper is organized as follows: the investigated sample is described in
section“Enamel technique”; section“Method” explains THz data acquisition and processing analysis of time-
domain data, and the results of measurements made on an ancient enamel plate are presented; section“Results”
summarizes the results with imaging and reports on the detection of fractures in ceramics, while the characteri-
zation of pigments through hyperspectral visible near-infrared imaging spectroscopy and p-XRF are discussed
in section “Discussion”; “Conclusions” and future works are provided. e “Annex” includes the methods and
comprehensive results from spectroscopic and elemental analyses. e acquired information made it possible to
better apprehend the nature of the decoration chromogens and, thus, to establish the color palette of the industry
that produced this type of decorative object. We also demonstrate the eciency of a focus, Z-tracker and express
the possibility of performing THz imaging on non-at samples. en, 3D images are extracted and generated,
providing a real vision of the 3D object.
Enamel technique
Born in the Lorraine region of France, the foundation of ceramic arts, the “Manufacture des Emaux de Longwy”
1798, was the rst ceramic industry in France created in the eighteenth century. e enamel manufacturing pro-
cess involves numerous expert artisans who know how to create it. To create a single piece, at least seven dierent
techniques are required: modeling, casting, nishing, pressing, cooking, and cracking. is process can take up
to 100h. e transformation of crystal powder into enamel with magical luminosity is a precise task requiring a
high level of skill and expertise. For more than a century, Longwy has been producing enamels in earthenware.
e production of ceramic pieces decorated with relief polychrome glazes consists in printing the ornamentation
in black on the raw biscuit, then lling each cell, thus surrounded drop by drop with colored enamel.
To make ceramic objects by casting, we do not start from clay bread, but from clay in liquid form. ey used
dehydrated clay to liquefy water. is liquid clay was then poured into plaster molds to obtain a given shape. e
modeling workshop created a plaster model, which was then used to create a hollow plaster mold for pouring slip,
a mixture of kaolin, clay, and water. e plaster mold absorbs liquid from the slip, forming a crust along its walls
once it reaches a thickness of 7/8mm. e excess slip was emptied aer the mold was turned over, and the piece
was le to dry in the mold before being removed and air-dried. For plates, ancient and well-known techniques
consist of shaping one’s clay with the help of a machine called a Potter’s wheel. Recently, modern techniques have
used molds. en, the piece was red at 1050°C overnight.
e resulting white clay is calledbiscuit. e biscuit is printed with a line of black ink that repeats the decora-
tion of the piece and partitions the colors, thereby preventing them from mixing. Since the nineteenth century,
the printing process has been extensively developed to reduce costs. e procedure starts with a patterned metal
printing plate similar to those used for engravings or etchings on paper. e plate is used to print the pattern
on tissue paper, using mixes of special pigments that stand up to ring as the “ink.” e transfer is then placed
pigment-side down onto the piece of pottery so that the sticky ink is transferred to the ceramic surface. Before
transfer printing, ceramics were hand-painted, which is a laborious and costly process. Each cell created by the
line was then lled with colored enamel using a drop-by-drop technique17. As the drop of enamel dries almost
instantly on contact with the biscuit, it is impossible to "paint" this piece, hence the use of the technique which
consists of placing one drop of enamel next to another until the color cell is completely lled. Each alveolus cre-
ated by the line was then lled with colored enamel using the drip technique. Aer the object was completely
enameled, it was red at approximately 750°C overnight. e piece is then retouched with overlay, requiring a
second ring at 750°C; then, gold is deposited onto the decoration, and the piece is red again at 600°C.
Figure1 presents a central picture of plate with a colored “phoenix” in the center. e layer of colored enamel
is unusually thick and produces a volume and depth of color that is generally impossible to reproduce in deco-
rated ceramics. e external diameter was 45cm. As the designation suggests, relief glazes take their name from
the line that outlines the pattern and hold the colored glazes deposited in the various areas thus formed. e last
painter applied the colors (pigments with frit) in light touches on the enamels in relief to provide illumination
to enrich the décor, such as the pink color on the trees, the ramications on the leaves, and all the details inside
the phoenix bird.
Method
Let’s remind you that THz radiation lies between infrared and microwaves of the electromagnetic spectrum,
and has a frequency range between 0.1 and 10THz (or wavelengths between 3mm and 30μm). THz radiation
is non-ionizing and due to its nature penetrates most dielectric materials. It has dipole selection rules which
interacts with both the intramolecular and intermolecular motions of solid matter18,19. One of the interesting
features of THz radiation is that it can propagate several mms into a sample which allows for the characterization
of materials that are quasi-transparent to X-rays. is paper for rst time complementary information provided
by IR multispectral imaging, X-ray (Fein Focus nano focus, 10–160keV), and terahertz tomography20. THz
pulses are produced by focusing ultrafast (120fs) pulses of near-infrared light (780nm) onto the gap between
the electrodes deposited on LTG-GaAs (TeraPulse, Lx, TeraView, Cambridge UK). A signal-to-noise ratio (SNR)
of approximately 80–90dB was achieved. THz-TD imaging is based on the emission of a picosecond short pulse,
that is a THz electromagnetic pulse, which is reected in the sample under test and interacts with it. Reected
pulses formed by the sample surface and each further inner interface appearing between materials with dissimilar
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refraction indices are collected and studied as a function of the time-of-ight (ToF), that is, the total propaga-
tion time from the emitter to the material interface and returning back to the receiver. e rst reection is due
to the interface between air and the outer surface of the sample, and subsequent pulses correspond to internal
material interfaces, giving rise to a complex mixed-signal21. erefore, the THz waveform composed of a single
point contains the depth prole of the sample at that point, while a bi-dimensional cross-sectional image (also
called B-scan) is obtained by collecting data along a trajectory. Finally, the raster scan of an area generates a data
cube from which dierent pictures of the sample can be extracted by displaying dierent values obtained in the
time or frequency domains.
To obtain the best possible lateral resolution, a short focal length must be used to focus the THz beam onto a
target or sample, resulting in a depth-of-eld that is a few times the wavelength. Ideally, when imaging a planar
object, we want to keep the target surface within a few hundred micrometers of the focal plane. e standard Lx
PolyScan gantry unit features manual depth adjustment to set the proximity of the sensor relative to the target
surface (the focus). e Lx PolyScan Head classically consists of a terahertz emitter and receiver placed adjacent
to each other and focused with either a 7-mm or 18.5-mm focal length lens. In our experiment, these lenses were
made of high-resistivity silicon. For samples with irregular surfaces, we added and compared a third motion
axis to the Lx PolyScan unit to permit automatic adjustment of the focal plane with respect to the sample. e
details of the performance are given in “Annex”. To perform this measurement, we mounted a laser-range gauge
at a 30-degree angle near the terahertz focusing optics. To keep the terahertz focus at the object surface, the
instrument control system performs a raster scan with a laser range gauge of 10–20 scans before the terahertz
focal spot. is soware developed a topological map of the surface to be scanned. e position of the terahertz
optics is continuously updated using a topological map. is keeps the terahertz beam focused on the surface of
the object. e soware allows the user to either keep the focus on the surface of the object or to oset the focus
by up to 3mm to examine the subsurface structure.
Results
To evaluate the refractive index from a raw frequency image, a reference electric eld must be measured. Er(ω)
refers to the electric eld generated by the acquisition system. e reference, Er(ω), is obtained by placing a metal
plate that is precisely located where the surface sample was positioned for object imaging. From the reference
electric eld Er(ω), the experimental reection transfer function Ts(ω) = Es(ω)/Er(ω), can be calculated, Es(ω)
is the sample frequency-dependent electric eld. e experimental reection transfer function Ts(ω) = Es(ω)/
Eref(ω), can be calculated, Es(ω) is the sample frequency-dependent electric eld. e transfer function Ts (ω) is
a function of the refractive index n(ω) and extinction coecient κ(ω) of the sample under inspection22. Es(ω)
depends on Fresnel’s coecients for transmission Ts(ω) and reection Rs(ω). e complex refractive index n*(ω)
at each pixel location can be extracted from the experimental transfer function Ts (ω) by solving an inverse
electromagnetic problem23. is function denotes the disagreement between the experimental waveform Es(ω)
and waveforms24. We performed the extraction on a at area of the blue substrate near the main fracture. An
average value of around 2.5 ± 0.1 like a standard glass, is found, and the local thickness is very large for an enamel
layer around 650µm (conrmed by an optical thickness gauge near the fracture).
We show THz images based on raw THz signals obtained directly from the scan. e motorized stage allows
the scanning of a surface of 200 × 200 square millimeters which is less than the total surface area of the plate.
Figure1. Picture of the plate (a) center and (b) global view of the sample (diameter of the plate: 45 cm).
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THz C-scans (two-dimensional appearance of the data displayed as a top planar view of the decoration) are
shown in Fig.2.
For the THz C-scan in the time domain shown in Fig.2a, the selected imaging contrast mechanism was
the peak-to-peak amplitude of the reected THz signal (i.e., the peak-to-peak amplitude of the main temporal
signal). e THz C-scan in the frequency domain can also be displayed by taking the Fourier transform of the
temporal waveform at each pixel and showing the magnitude of the frequency components in a given frequency
window or at a specic frequency. THz C-scans mainly present the THz response of the surface material, and
they reveal the surface roughness or orientation with respect to the generated THz beam, as well as evidence
of subsurface features. In Fig.2a, the Fresnel coecient between the rst ceramic layer and air depends on the
refractive indices of the material; however, in our case, the image of the plate is mainly correlated to the atness
of the sample. A clear intensity gradient was observed from the center to the exterior, which was associated with
the 3D surface morphology of the plate. In the presence of surface irregularity, reection of THz radiation occurs,
and consequently, there is a weaker specular signal leading to a gradual decrease in the intensity. Nevertheless,
some identied zones present dierent reectivity, such as the pink painting overlaying small trees and some dark
green on leaves. A video permits the superposition of THz and visible images. Figure2b, which is an intensity
X-ray image, reveals identical features at the same location, while Fig.2c serves as a guide for the localization
of the dierent color areas of the plate. Because the pigment-colored enamel used a drop-by-drop technique, it
presented a slightly concave shape due to the molten procedure, the THz beam was partially reected out of the
sensor, and the outline of each enamel zone was detected to be larger than the real zone. is irregular surface
shape was also observed in the B-scan.
However, a THz C-scan is an X, Y image of an object, and a THz B-scan (two-dimensional presentation dis-
played as a cross-sectional view of the sample) provides depth information along a line in the X and Y planes14,25.
e optical delay can be converted into depth information by determining the refractive indices of the various
layer materials. e THz B-scans based on the raw data with the two cross-sections are plotted in Fig.3a, their
positions are displayed in Fig.3b. e dierent lines are due to the magnitude of the reected signal at a given
optical delay, and each linked identical interface encountered by the propagating beam. e rst large feature of
the optical delay is the reection of the rst ceramic layer. e complexity of the layered structure can be clearly
observed, corresponding to the time delays from 0 to 10ps aer the main peak. A pronounced horizontal concav-
ity was observed because the sample was evidently not at while the detector was scanning in a constant vertical
position. Based on these results, we can consider that only a 50mm circular zone maintains the main reected
peak in the Rayleigh range, and the data are reliable and easy to analyze. e rest of the data provides the relative
position of the interface with the sensor, but the amplitude information is incorrect because of reection losses.
To complete the 3D rendering of the plate and compensate for this drawback, we completed the acquisition with a
Z-tracker. Details of the procedure are provided in “Annex”. To obtain the best possible lateral resolution, a short
focal length must be used to focus the THz radiation on the target or sample. is results in a depth-of-eld of
few wavelengths. Ideally, when imaging a planar object, we want to keep the target surface within a few hundred
micrometers of the focal plane. With a nonplanar surface, it is impossible to maintain the surface at the focal
plane over a large imaging area. Figure3c displays a B-scan measured using the Z-tracker option. e time peak
position was kept almost constant within 1ps over the entire scan area.
Consequently, we can merge both the information and the 3D rendering the real shape of the plate. Figure4a
displays the ceramic object and expresses the concavity in the X- and Y-directions. e height dierence from
the center to the edge is approximately one centimeter which is the order of magnitude observed on the plate.
In Fig.4b, we slightly rotated the data to provide evidence of a “at zone” in the center of the ceramic. Figure4c
presents the 3D reconstruction of the central area of the plate.
Figure2. (a) Peak-to-peak raw data of the central area of the plate terahertz map, (b) X-ray intensity, (c) visible
picture of the Phoenix.
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A meshed network of tiny fractures or hairline ssures in the enamel is referred to as “crazing” or “ssuring,”
and it can result in the total loss of a clean appearance for food or drink. e only reason is that with time, dier-
ent tensions develop between the biscuit and the glaze underneath. e item was not designed to endure the heat
stress caused by pouring boiling drinks or rinsing at temperatures higher than 80°C. e body has less physical
mass because it is more porous than the glaze covering it. Dierent expansions occurred owing to the slower
rate of heating compared to the glazing layer. Masses exhibit distinct contractions under cold conditions and
expansions under hot conditions. Consequently, the link between these two materials fractures and creates cracks.
ese tiny ssures become larger with use and eventually create surfaces and ridges that allow foreign materi-
als to build. For example, tea and coee leave unsightly coloring in fractured glazes. In general, the likelihood of
cracking increases with a decrease in the ring temperature. Given that only ceramics may develop these types of
glaze cracks, this further suggests that there is no aw in porcelain. Hard porcelain has a physical density similar
to the glaze’s since it is red at a minimum temperature of 1350°C. As so, there is zero chance that it will crack.
Figure5a presents the THz C-scan in the time domain shown equivalent to Fig.2a, the peak-to-peak amplitude of
the reected THz signal is extracted and displayed using the z-tracker scan. e intensity is clearly homogenous
without artifacts and now the large crack with a lambda shape (λ) is now visible between the central-colored
zone and a golden decoration. An X-ray image presented in Fig.5b validates the rst observation and also small
fractures are detected and they are diverging from the main failure along three directions. With our apparatus,
X-ray imaging fails to reveal hairline cracks in ceramics due to several factors: e two main reasons are rst
resolution limitations: Standard X-ray imaging has limited spatial resolution, meaning it might not be able to
detect very ne details like hairline cracks, which are extremely narrow and small. e second origin is a contrast
issues: Hairline cracks may not create sucient contrast in X-ray images. X-rays work by detecting dierences
in material density and thickness. Hairline cracks are oen too narrow to cause a signicant dierence in X-ray
absorption, making them hard to distinguish from the surrounding material.
Figure6 presents a focus on the main λ fracture. A raster scan is performed with a spatial step of 100µm. e
main fracture is perfectly resolved and surprisingly, the hairy fracture network is resolved and clearly detected.
We also present the 3D reconstruction of the fracture. Figure7bis a B-scan with the temporal reected signal
obtained for position 1 near the fracture and position 2 inside the fracture (red square)reported on the C-scan
Fig.7a. e reected signal 1 is displaying three main peaks, the rst air-enamel, the interface between the
biscuit and the enamel and the last interface plate-air. is is corroborated by signal 2 where the second peak is
enhanced due to the changing Fresnel reection coecient between air and the biscuit. e depth resolution of
THz reectometry is sucient to resolve the two-layer stratigraphy of the ceramics.
Discussion
In our study, we employed three complementary analytical techniques—THz-TDS, p-XRF, and Vis–NIR HIS to
achieve a comprehensive understanding of the Longwy plate.
Time-domain terahertz-transmission-line-detector (THz-TDS) measurements oer insights into the temporal
delays and amplitude uctuations of terahertz (THz) pulses traversing or reected by a given material. ese
measurements can be employed to assess the material’s intrinsic properties, including its conductivity and
refractive index. ey also enable the detection of defects and inclusions within the material. However, it should
be noted that THz-TDS measurements are not sucient to provide a comprehensive understanding of the
object under study. p-XRF provides elemental composition, which complements the molecular and structural
a) b) c)
Figure3. Dierent terahertz B-scan presented at position A and B without Z Tracker (a) and with Z-Tracker
(c).
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information obtained from THz-TDS. e spectral data from Vis–NIR HSI can be used to validate the spatial
distribution and specic identication of pigments. By combining the elemental and spectral data from p-XRF
and Vis–NIR HSI with the structural and defect analysis from THz-TDS, we achieve a more comprehensive
understanding of the pigments and their application methods26. is integrated approach allows us to better
interpret the historical and technical aspects of the ceramic production processes.
en, we perform two non-invasive analytical techniques, portable X-ray uorescence (p-XRF) and visible
near-infrared hyperspectral imaging spectroscopy (Vis–NIR HSI), to characterize the glaze and the coloring
agents in the decorative ceramic plate from the manufacturing company Longwy. Details of the techniques
employed and the main results are presented in the “Annex”, while the obtained results are resumed in Table1.
Infrared false colors (IRFC) images facilitated the distinction of chromogen agents of similar color but dierent
compositions (such as copper-based and cobalt-based blue regions that appeared dark blue and red, respectively).
e data showed a great variety of chromophores and pigments used to achieve a wide range of hues. at
showcased a noteworthy palette and color prociency within the manufacturing industry. e procedure and
results for blue and green colors are detailed hereinaer as examples.
)b)a
c)
Flat Zone
Figure4. Biconvex real shape of the plate extracted from t-o-f and Z tracker acquisition, 3D representation
of the plate shape, (b) delimitation of the at area in the center, and a large crack is barely observed. (c) 3D
representation of the phoenix in the center of the plate; the dierence depth is 0.4 mm.
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rough hyperspectral spectroscopy analysis combined with Spectral Angle Mapper (SAM) classication for
data cube processing, it was determined that the pigment responsible for the bright blue hue surrounding the
central scene was also implemented in a portion of the bird’s tail and head (Fig.8a). e assumption is further
conrmed by the IRFC image, which displays consistent dark hues in these areas, also dismissing the possibil-
ity of the use of cobalt-based pigment, typically identiable by its red coloration in such imagery17 (Fig.8b).
In Fig.8c, the reectance spectrum of these regions (endmember1) showed a broad absorbance from 700 to
900nm, aiding in identifying the chromophore as Cu2+ in a 3d9 state with octahedral coordination (2Eg → 2T2g)27.
e detection of copper was conrmed by elemental analysis (p-XRF), which also revealed the presence of iron.
e lack of spectroscopic identication of the element can also be explained by the impossibility of detecting
the characteristic absorbance of Fe3+ ions in tetrahedral coordination (6A1 → 4T1(D)) at 380nm attributed to the
range limits of the implemented camera28. e turquoise-green hues observed in both the sky of the bird scene
(endmember2) and the back of the plate share a common composition, relying on copper and iron chromo-
phores. is conclusion has been drawn from the characteristic broad absorbance peaks of Cu2+ around 800nm
and one slight peak of Fe3+ at 440nm (6A1 → 4A1(G)28. e broader reectance peak of the spectrum, spanning
from 462 to 563nm, might be highlight that the light blue tone was achieved by incorporating a reduced amount
of copper compared to the deeper blue hue of endmember1. e detection of both copper and iron with p-XRF
strengthened their assumption.
Figure5. (a) Peak to Peak THz image with tracking. (b) X-ray image of the fracture.
Figure6. Detail of the “λ” plate fracture showing the base ceramic material with ssures (a) visible image, (b)
THz image, (c) 3D representation of the fracture.
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Cobalt was identied as the chromophore in both the bird wings and the reection in the water (endmem-
ber3), with both areas appearing bright red in the IRFC image. e three characteristic absorption bands of Co2+
in tetrahedral coordination (4A2(F) → 4T1(P)) are visible around 530nm, 595nm, and 645 nm29,30. Additionally,
one absorption peak of Fe3+ was observed at approximately 420nm (6A1 → 4E(G))3. Furthermore, the elemental
analysis revealed zinc, manganese, and antimony, most likely intentionally added to achieve the nal greyish hue.
Figure7. (a) C scan and (b) B -Scan through the main Fracture. e temporal position of the peaks delimits
the biscuit and the enamel layers without ambiguity. Interface are numbered 1 : Air-glaze-interface, 2 Enamel-
biscuit interface, 3 biscuit-back ceramic. , a schematic layer is in insert to describe the reected time signal at the
interfaces.
Table 1. Elemental characterization by portable X-ray uorescence (p-XRF), spectral characteristics, and the
chromophores identied for each color implementing Hyperspectral Imaging Spectroscopy. Silicon (Si), lead
(Pb), sulfur (S), potassium (K), and calcium (Ca) were identied in all areas, exhibiting the same proportions
in the colored regions and the white area on the back of the plate.
Endmembers Colors XRF results (Chromophores) HSI Results (Chromophores & Spectral Characteristics)
1 Bright blue Cu, Fe Cu2+ (broad absorption 700–900nm)
2 Turquoise green Cu, Fe Cu2+ (broad absorption 700–900nm)
Fe3+ (~ 440nm)
3 Grey blue (bird wings) Sn, Mn, Fe, Co, Zn Co2+ (530nm, 595nm, 645nm)
Fe3+ (420nm)
4 Light green Cr, Fe, Cu Cu2+ (broad absorption 700–900nm)
Cr3+/Fe3+ (445nm)
5Green Cr, Fe, Co, Cu, Zn Cu2+ (broad absorption 700–900nm)
Cr3+/Fe3+ (445nm)
Co2+ (595nm, 645nm)
6 Dark green Cr, Mn, Fe, Co, Cu, Ni Cu2+ (broad absorption 700–900nm)
Cr3+/Fe3+(445nm)
7 Bright red Cr, Fe Inection point at 650nm
8 Violet_Bird neck Cr, Fe, Sn (traces) Fe3+ (420nm and 440nm)
Inection point at 650nm
9 Red_bird chest Ba, Cd, Fe, Co, Se Inection point 598nm
10 Orange/red Ba, Cd, Fe, Se Inection point 570nm
11 Pink tree Fe, Cu SPR of Cu nanoparticles (555nm)
12 Yellow Cr, Fe PbCrO4 (inection point at 515nm
13 Gold leaf Au, Ag, Pd, Zr, Fe Inection point at 600nm
14 Black Cr, Mn, Fe, Co, Cu, Zn Cu2+ (broad absorption 700–900nm)
Cr3+/Co2+ (648nm)
15 Green label ink Ba, Fe, Cr, Cu Cu2+ (broad absorption 700–900nm)
Cr3+ (650nm)
Behind the plate Turquoise green Fe, Cu Cu2+ (broad absorption 700–900nm)
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e leaves and vegetation scene incorporates three distinct green hues, from lighter to darker. ese
variations were achieved by introducing coloring agents, such as chromium, iron, and copper, with higher
concentrations in the darker shades. A broad absorption around 800nm is detectable in all three reectance
spectra attributed to Cu2+ ion absorption (Fig.8d). Moreover, all spectra exhibit a peak around 445nm, which
may result from a shi in the rst peak of Cr3+ (commonly found at 450nm), possibly due to overlap with Fe3+
absorption. In the spectrum of light green (endmember 4), an absorption peak of Cr3+ at 690nm is also slightly
distinguishable31. e cobalt element is observable in the p-XRF of the two darker green hues, which is also well
distinguishable in the absorbance spectrum of green leaves (endmember5) by two absorbance peaks at 595nm
and 645nm. Conversely, the absorbance is too high in the darkest hue (endmember6), and the cobalt peaks
are indistinguishable. In this latter hue, nickel and manganese have also been detected in noticeable amounts.
e presence of Mn2+ ions might have contributed to the shi of the reectance peak to a lower wavelength
observed in endmember6 (480nm) compared to the other two spectra (520nm and 540nm, respectively in
endmember5 and 4), as manganese cations have an absorbance at around 430 nm32. e presence of manganese
might be explained by its common use in the glass and ceramic industry, where pyrolusite (Mn2O) is added to
iron chromate (Fe2(CrO4)3) to achieve dark colorations33. In contrast, the detected zinc in medium green leaves
may have been added as network former. Zn2+ ions could inuence cobalt in dierent manners: if present in
high amounts, the ion increases the speciation of cobalt and reduces the distortion of the tetrahedral complex,
providing, in turn, a bluish-green hue34.
Concerning the other colors, various red shades were attained using chromium-based red pigments for bright
reds and purples, while CdSxSe1-x nanocrystals were present in the red bird wings. Pink, on the other hand,
was obtained using copper nanoparticles. Yellow was achieved using chromium-based pigments, potentially
lead chromate. Black colors were obtained by mixing various oxides in high amounts (Cr, Mn, Fe, Co, Cu, and
Zn). Finally, the investigation into gilding revealed a gold alloy, and the green ink used for ceramic production
labels was found to be copper/chromium-based with traces of iron and barium. e comprehensive data from
spectroscopic and elemental analyses of all these pigments is described in the “Annex”.
In ceramic, the color is provided, at the level of the paste or the level of the enamel, by dyes, and pigments,
specic to ceramic, which must resist the temperatures of ceramic ring. Most oen, these are oxides or
combinations of oxides.
Our experience shows that certain metal oxides are ceramic dyes, e.g. their powder is mixed with a paste or
enamel provides color to the red product. Here are some examples of the most common oxides and the colors
Figure8. VNIR HSI results. (a) Infrared false colors (IRFC) image; (b) false color representation of the
pigment distribution determined by SAM classication; Reectance spectra (endmembers) of (c) blue and (d)
green hues.
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they give to an oxidized red enamel: cobalt (blue), copper (green, blue), manganese (brown, purple), iron
(yellowish, reddish, greenish), chrome (green, yellow, red), nickel (brown, greenish, gray), and there are others
(uranium, titanium, vanadium, etc.). e intensity of the color increases with the level of coloring oxide which
made the analysis very complex. e intensity of the color depends on the covering power of the oxide (for
example cobalt oxide, 0.1% of this oxide will give a very powerful blue, while the same level of iron oxide will
give a tint very pale). e achieved color by an oxide also depends on the characteristics of the product in (or
on) which it is introduced. Let us look at iron oxide which will give yellowish tints in a paste rich in alumina and
reddish tints when there is little alumina. While copper oxide gives blues in alkaline enamel and greens in lead
enamel. Antimony oxide gives yellows (Naples yellow) in lead enamel and is practically not coloring in others. By
mixing pure oxides, it is possible to obtain intermediate shades. When the quantity of oxide added to the enamel
is too high, new shades can be obtained, for example, metallic shades, by saturation. e cooking temperature
and atmosphere (reduction or oxidation) have an inuence on the color given by an oxide. e color of the
oxide mixed with the raw enamel (or paste) is oen very dierent from the color aer ring. ese complex and
unpredictable behaviors explain the need for meticulous testing but also the multitude of possibilities.
Moreover, pure or simply mixed coloring oxides do not make it possible to obtain all the desired shades of
color. Manufacturers have developed very complete palettes of reliable colorings which oen have similar shades
before and aer cooking. ese dyes are obtained from coloring oxides and other compounds, puried, reduced
to ne powder, measured, and mixed. ese mixtures are calcined and sintered at high and precise temperatures.
e mass obtained is crushed, washed with water, ground for a long time, and then dried. e composition of
these dyes is oen very complex and protected. Manufacturers oer palettes of dyes specic to dierent uses:
mass dyes for clay products (paste and slip), enamel dyes, dyes for high temperatures (or large re colors), and
dyes for low temperatures (or small re colors).
In this context, it seems almost out of reach to predict, classify, or compare the dierent reectivity of these
very complex materials even if chemometrics tools demonstrated some capabilities with binary or ternary
mixtures35. Nevertheless, we observed clearly dierent reectivity behavior correlated to some overlayer paintings
such as the pink color on the trees, dark green on the leaves, and other colors on the Phoenix’s throat and
chest. An alternative analysis is under investigation to understand this clear dierent behavior due to chemical
compounds or the presence of structural dierences (nanoparticles) leading to a substantial modication of the
dielectric response.
Conclusion
In this article, we used terahertz time-domain spectroscopy in reection geometry as a tool for the earthenware
art science. We were able to nd hidden fractures that were initiated near a large one. On the large plate enamel
colored sample, the bilayer structure of the ceramic and the substrate underneath the surface were presented.
To perform this analysis, terahertz imaging with a Z-tracking system allowed for nding the real shape and
dimensions of the plate. We could merge temporal information with good reectivity while keeping the distance
between the sample and the sensor constant over one centimeter in altitude. Such geometrical information was
used to reconstruct in 3D the plate and could be extended to any non-at object inspection. We demonstrated
that terahertz spectroscopy and imaging can provide very useful information for the evaluation of the damage
and conservation state of ceramic artwork or artist technique. Single point measurements by this technique
can be used for a qualitative assessment of the internal structure of a piece, while imaging can be useful for
careful measurement of the geometry and provides information about the artistic procedure. Finally, our
terahertz imaging technique shows interesting results about the reectivity of some colored pigments with
clear dierentiation. We performed an intensive and non-invasive analytical technique, X-ray uorescence
(p-XRF) and hyperspectral imaging spectroscopy (HSI), to characterize the glaze and the coloring agents in the
decorative ceramic plate. Very complex compositions with many dierent elements were detected and provided
an interesting understanding of the ceramic industrial palette. We can consider that terahertz spectroscopy and
imaging will become useful tools in the eld of art restoration and the earthenware community in the near future
owing to their non-contact and non-destructive nature.
Data availability
e datasets used and/or analysed during the current study available from the corresponding author on reason-
able request.
Annex
Let’s remind you that the depth-of-eld of the terahertz (THz) reection is limited, as a result of the comparatively
long (0.1mm to a few mm) wavelength of the THz radiation used. In order to obtain the best potential lateral
resolution, a short focal length must be used to focus the THz onto the target or sample. is results in a depth-of-
eld that is a few times the wavelength. Ideally, when imaging a planar object, we want to keep the target surface
within a few hundred micrometers of the focal plane. With a nonplanar surface, it is impossible to maintain the
surface at the focal plane over a large imaging area. For the Z-tracking, we have mounted a laser-range gauge at
a 30-degree angle near the terahertz focusing optic with the THz head. en the laser gauge provides a robust
measurement of the distance to the target surface. Updates to the instrument embedded control soware provide
the control-loop to permit the head to track the target surface. In order to keep the terahertz focus at the object
surface the instrument control system performs a raster scan with the laser range gauge some 10–20 scans before
the terahertz focal spot as shown in Fig.9. In order to track the target surface at the position of the THz focus,
the instrument control system will perform a raster image scan with the raster orientation such that the laser
gauge leads the THz focal point. us, the instrument can record the surface measured by the laser gauge ahead
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of the THz position. e height of the Z-axis is updated continuously using the surface prole measured a few
lines earlier in the measurement process. is keeps the terahertz beam focused on the surface of the object.
e soware allows the User to either keep the focus at the surface of the object or to oset the focus by up to
3mm to examine the subsurface structure. We have veried the z-axis surface tracking control loop stability by
stimulating the control system with a series of step-changes in the target position with a logarithmic increasing
range of step sizes. e step sizes range from 100µm up to 20mm. e responsiveness of the control loop may
be adjusted by a parameter set in the instrument conguration. For testing and demonstration purposes, we
have used a small attened aluminum scale to provide a non-planar test sample. e width is about 50mm with
dierent heights from 100µm to 1mm. We have used this to test the surface tracking algorithm.
In Fig.10, B-scan reveals the time position of the metal interface. A Gaussian beam acquires a phase shi
along the propagation direction; this phase shi diers from that of a plane wave propagating with the same
optical frequency. is dierence is known as the Gouy phase shi. at results in dierent images in frequency
domain as shown in Fig.10 and then artifacts. e result is a clear artifact since the metal interface is presenting
dierent reection coecients varying with the time of ight out of the Rayleigh length, so a loss of intensity at
dierent frequencies. With the tracker, only interfaces between two successive metallic levels are distinguishable
in the B-scan since the distance between the focal silicon lens attached to the PCA and the target is kept constant,
resulting in the same amplitude of the reected THz beam on the dierent metal scale at the same frequency.
Combining a perfect positioning of the sample and the time of ight obtained with the tracking gives a precise
3D rendering of the rst interface, allowing the 3D shape of any samples.
Hyperspectral imaging (HSI) in visible and near‑infrared (Vis–NIR) range
e visible near-infrared (Vis–NIR) hyperspectral camera, developed by SPECIM (Finland), was positioned
vertically on a translation rail controlled by the Spectral IDAQ soware. e rail, axed to two tripods, measured
1.30m and enabled the horizontal movement of the camera system. e two halogen lamps were positioned at a
45° angle from the sample, moving along the translation rail simultaneously with the camera. is synchronized
Offset distance
Lasergauge raster
THzraster
Figure9. e osets raster trajectories of the laser gauge target point relative to the THz focus are illustrated.
Implementation into the setup.
Figure10. Demonstration of Z tracker: (a) Peak-to-peak images, B-scan and Amplitude of the FFT at 1 and 2
THz without tracking, (b) with tracking option.
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movement guaranteed consistent illumination throughout the entire scanning process. e hyperspectral system
has an HS-XX-V10E CCD detector with a spectral resolution (FWHM) of 2.8nm, a spectral sampling of 0.7nm,
and a pixel size of 54.7μm2. e moving camera allows for the building of a data cube where the rst two
dimensions are spatial (1600pixels × Xpixels) (with X corresponding to the number of pixels in the width of the
image). e third dimension is a spectral dimension (840 bands). e wavelength range extends between 400
and 1000nm. e focal length is 23mm. For the study, the experimental conditions were a working distance
of 84cm, scan speed of 13mm/s, and an exposure time of 25ms. Spectral IDAQ soware provided spectra
acquisition, storage, and calibration. Before each session, both white and dark eld calibrations were conducted.
A Spectralon uoropolymer (99% reectance) was used as white reference to calibrate the reectance spectra. A
following acquisition with the shutter closed was performed and used as a dark reference to quantify the detector’s
electronic noise. e ceramic plate was illuminated with two halogen lamps oriented at 45° from the sample.
Data treatment
e data cube was treated with ENVI 5.2 +IDL soware (Harris Geospatial). False color images were generated
by substituting an infrared band for the red channel and selecting two other bands in the visible range for the
green and blue channels (R → 900 nm; G → 650nm; B → 550nm) in the ENVI soware. e IRFC (Infrared
False color) images helped distinguish chromogen agents of similar color but dierent compositions. ENVI’s
Spectral Hourglass Wizard (ENVI-SHW) was implemented to identify, extract, and map the reference spectra.
e processing ow implemented the Minimum Noise Fraction (MNF) technique to reduce noise and enhance
data quality, followed by the isolation of ‘spectrally pure’ pixels through the Pixel Purity Index (PPI) using 10,000
reectance spectra with a threshold of 2.5. e n-D Visualizer (a 3D scatter plot) was employed to manually
identify the clusters dening the dataset’s purest pixels (endmembers). Once the endmembers were extracted,
the SAM mapping method visualized their distribution in the image. is algorithm calculates, for each pixel
(in radians), the angle between the image’s reectance spectrum and the chosen endmember. e histogram
threshold was manually adjusted for each case to ensure a close match between the spectral characteristics of the
identied spatial pixels and the target endmember. e tolerance range of the study varied for the hyperspectral
cube, ranging from 0.17 to 0.4 radians. Only endmembers representative of the chromophores were selected and
investigated. e spectra of the endmembers have all been presented as Reectance.
Handheld XRF analysis
e elemental analysis was performed using the portable Olympus Vanta VCR-CCX-G2 analyzer. e detector is
large 13 mm2 silicon SDD with < 140eV FWHM resolution at Kα of Mn. e spot is 10mm or 3mm in diameter
with an integrated camera (variable collimator). Two beams (40kV and 10kV) are automatically sequenced, and
the total analysis time is 30s. Data treatment was performed using PyMCA soware.
Results from spectroscopic and elemental analyses
Hyperspectral imaging spectroscopy (HSI) and p-XRF were jointly employed to identify the coloring agents. e
elemental composition of the glaze was ascertained through portable X-ray uorescence spectroscopy (p-XRF).
Silicon (Si), lead (Pb), sulfur (S), potassium (K), and calcium (Ca) were identied in all areas, exhibiting the
same proportions in the colored regions and the white area on the back of the plate.
From hyperspectral analysis, we have identied and localized 15 main endmembers that characterize the
pigments used in Longwy plate coloration. Figure11 depicts the ensemble of these spectra and their represen-
tation through a false-color image obtained by SAM classication. e characterization of these endmembers
was then sorted by color; detailed descriptions of blues and greens are provided in section“Discussion” of the
Figure11. False color representation of the pigment distribution determined by SAM classication (le) and
reectance spectra of the extracted endmembers (right).
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article, while the remaining colors (red, pink, yellow, gold leaf, black pigments, and the green ink used in the
label) are presented and discussed here.
Red and pink colors
e bright red (endmember7) and the more violet color of the bird’s neck (endmember8) were attributed to
pigments based on chromium and iron. e two absorbance spectra exhibit an inection point at 650nm; the
violet also shows two absorbance peaks attributable to Fe3+ (420nm and 440nm). e denitive identication of
the chromium-doped red pigment is challenging. e absence of elements like Sn or Zn rules out the possibility
of Cr-doped malayaite (CaSnSiO5) or Cr-doped gahnite (Cr: ZnAl2O4)17. e lead content appeared with the
same intensity as the other colors investigated, suggesting the exclusion of lead chromite. Possible alternatives
include a synthesized spinel (Cr3+: MgAl2O4) or Al(2-x)CrxO3-based pigments. Spinels were rst synthesized at
the Manufacture de Sèvres by Ebelmen around the middle of the nineteenth century36. Further analyses, such
as XRD, are required.
Two additional hues of red have been identied in the scene as depicted in Fig.12, one utilized for the bird’s
chest (endmember9) and the other for painting the upper part of the wings (endmember 10). e hypothesis
of coloration through the precipitation of mixed CdSxSe(1−x) nanocrystals has been formulated based on p-XRF
results, as both cadmium and selenium were detected in these two areas. e reectance spectra further support
the assumption of semiconductor nanoparticles. According to the literature, the inection point of the curve
shows a shi towards higher wavelengths when changing the chemical composition from CdS (yellow colors)
toward CdSe (red colors), passing through dierent CdS-CdSe solid solutions (orange colors)37. e inection
points of the two investigated regions were at 570nm for the more orange color (endmember10) and at 598nm
for the more reddish one (endmember9), showing good consistency with previous research38.
e elemental analysis also provided the observation of other elements, including barium, iron and, in the
darker hue, traces of cobalt.
e elemental analysis of the pink hue observed in the trees and bushes revealed the presence of iron
and copper. e integration with the spectroscopic data strongly suggested the implementation of copper
nanoparticles (Cu0) whose surface plasmonic resonance (SPR) is approximately 555 nm39. Commonly, variations
in positions and shapes observed in the SPR band are related to the changes in size and dimension of the copper
nanoparticles.
Yellow color and gold leaf
Elemental analysis showed in Fig.13 the presence of chromium and iron in the yellow color. e absorbance
peak at 425nm and an inection point at 515nm observed in the reectance spectrum (endmember12) might
suggest the use of yellow chrome pigment (PbCrO4)40. However, further investigations are required to conrm
this hypothesis. e elemental composition of the gold leaf resulted in a gold alloy comprised of gold, traces of
silver, palladium, zirconium, and iron. e reectance spectrum presented an absorbance at around 520nm and
an inection point at 590nm.
Black color
Based on experimental data and existing literature, it was assumed that the black pigment was produced through
the calcination of various oxides, resulting in the formation of the spinel structure composed of di- and trivalent
cations41. e elemental composition analysis of Fig.14 revealed the presence of chromium, manganese, iron,
cobalt, copper, and zinc. e reectance spectra also detected dierent absorbance peaks corresponding to the
present cations. For instance, the broad absorbance at 800nm was attributed to Cu2+, while the absorption band
around 648nm suggested the contribution of Co2+ and Cr3+ absorption.
Figure12. Reectance spectra of red (le) and pink (right) hues.
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Figure13. Reectance spectra of yellow and gold leaf.
Figure14. Reectance spectrum of black hue.
Figure15. Reectance spectrum of the green ink used for the ceramic production label.
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Green ink used for the ceramic production label
Attached to the plate there is the original label from the plate’s manufacturing company, Longwy. Analyses of
the green ink used in this label revealed the presence of copper, chromium, barium, and iron. HSI absorbance
spectra exhibit a broad band at 660nm, characteristic of Cr3+ absorption (4A2g—> 4T2)6. In Fig.15, Cu2+ ions
were identied by a slight absorbance peak between 800 and 900nm.
Terahertz time‑domain spectroscopy
e samples were measured with a ber-coupled, commercial Terapulse Lx THz-TDS spectrometer manufactured
by TeraView LTD. e setup was arranged in a reection conguration. An 18mm focal length silicon lens
collimates the THz radiation from the emitter and focuses it onto the sample. Likewise, the THz radiation is
reected through the sample, and an 18mm focal length silicon lens focuses it into the receiver. For each sample,
ten measurements at random positions on the sample were recorded followed by a single reference measurement
where the sample was absent. All the measurements were performed under the same ambient experimental
conditions recording time traces with a length of 50ps and 1000 acquisitions (scan speed: 60traces/s). Before
calculating the reection function in the frequency domain as described in equation (1), the obtained time-
domain signals were articially extended to 60ps by zero-padding to ensure that the measured pulse was
positioned before the midpoint of the time window.
Received: 10 April 2024; Accepted: 6 August 2024
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Acknowledgements
is research beneted from the scientic framework of the University of Bordeaux’s IdEx “Investments for the
Future” program/ GPR “Human Past”.
Author contributions
P.M. designed the experiments and wrote the manuscript; FF. performed the experiments, PM, RC, FG,FF, PFT,
AM, A BA, and GM made spectroscopic data analysis, and wrote the section of the manuscript on the data
analysis. All authors contributed to writing the manuscript; PM. supervised all aspects of the project.
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https:// doi. org/
10. 1038/ s41598- 024- 69697-6.
Correspondence and requests for materials should be addressed to P.M.
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