Ep 58 — Fun Case Compilation (Part 2): Art Authentication, Archaeological Discoveries, Forensic Stories

Series: Encyclopedia of Infrared Spectroscopy: From Principles to Practice
Chapter: Chapter 6 · Practice and Extension — From the Lab to Infinite Possibilities
Audience: All series viewers, especially those interested in cultural heritage conservation, archaeology, and forensic science
Prerequisites: Ep 05–06 (functional group frequencies), Ep 31 (forensics and criminal investigation), Ep 32 (cultural heritage conservation), Ep 36 (micro-FTIR)
Reading Time: Approximately 30 minutes

Introduction: The Human Face of Infrared Spectroscopy

In the previous episode, we soared into space and saw how infrared spectroscopy "sniffs out" the Martian atmosphere. In this episode, we return to Earth — not to the laboratory, but to museums, archaeological sites, and courtrooms — places seemingly unrelated to "analytical chemistry."

Infrared spectroscopy plays a far more fascinating role in these scenarios than one might imagine:

• Is a priceless Renaissance fresco genuine or fake? Hyperspectral infrared imaging can tell you [1]
• A 3,000-year-old mummy — what spices were used in its embalming? μ-FTIR can find answers from tissue residues
• A hit-and-run case leaves only a small paint chip at the scene. Can it identify the vehicle? FTIR can

"Spectroscopic techniques are becoming indispensable tools in cultural heritage research and forensic investigations, revealing information invisible to the naked eye without damaging the sample." [1]

In this episode, we explore several "story-driven" cases to see how infrared spectroscopy bridges science and the humanities.

1. Art Authentication Stories

1.1 Macro Hyperspectral Infrared Imaging of Perugino's Fresco

In 2025, Rosi et al. published a study in ACS Sensors on macro infrared hyperspectral imaging of a Renaissance fresco by Perugino [1].

Background: Pietro Perugino (1446–1523) was a leading figure of the Umbrian School of the Renaissance and the teacher of Raphael. His frescoes are scattered in churches across Italy. After five hundred years, pigment layers have aged, restoration overlays have been applied, and colors have faded. The core need for conservators is to analyze the pigment composition without touching the fresco.

Method: The research team used macro FTIR hyperspectral imaging to perform a non-contact, full-area scan of the fresco. Similar to the micro-FTIR imaging discussed in Ep 36, but scaled from micrometers to meters — covering the entire fresco surface.

Findings:

• Identified multiple historical pigments, including lead white, vermilion, malachite green, and ultramarine
• Reconstructed spatial distribution maps of pigments based on characteristic infrared peaks
• Discovered underdrawing lines (sinopia) beneath the paint layer, revealing the artist's creative process

"Macro infrared hyperspectral imaging allows us to perform molecular-level compositional analysis of an entire fresco without contact or sampling, which is of great significance for the conservation and study of precious cultural relics." [1]

What did infrared spectroscopy do here? Each pigment has a unique infrared fingerprint:

• Lead white (2PbCO₃·Pb(OH)₂): ~1400 cm⁻¹ CO₃²⁻ vibration
• Vermilion (HgS): relatively "silent" in the infrared region, but its absence can confirm its presence
• Malachite (Cu₂(CO₃)(OH)₂): ~1500 cm⁻¹ and ~3400 cm⁻¹

🔗 Carbonate, hydroxyl, and other functional groups in pigments can be queried at ftir.fun:

• Hydroxyl: ftir.fun/ir/group/hydroxyl
• Carbonyl (carbonate-related): ftir.fun/ir/group/carbonyl

1.2 The 2013 cm⁻¹ Characteristic Peak of Bone Black and Oil Painting Dating

Bone black is an ancient black pigment made by charring animal bones, used from prehistoric cave paintings to 19th-century oil paintings. In 2018, Daveri et al. discovered that bone black has a unique characteristic peak in the infrared spectrum — 2013 cm⁻¹ [2].

Why is 2013 cm⁻¹ so special?

The main components of bone black are hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and carbon. Phosphate (PO₄³⁻) vibrations appear at 1000–1100 cm⁻¹, but the peak at 2013 cm⁻¹ is a combination or overtone band related to the specific mineral-carbon composite structure in bone black.

"The characteristic absorption peak of bone black at 2013 cm⁻¹ can serve as a reliable indicator for identifying whether this ancient pigment was used in an oil painting, helping to determine the creation date." [2]

Dating significance:

• After the 19th century, synthetic black pigments (carbon black, Mars black) gradually replaced bone black
• If the infrared spectrum of a purported "16th-century oil painting" lacks the 2013 cm⁻¹ peak, it suggests the use of modern pigments → likely a forgery
• If the 2013 cm⁻¹ peak is present, combined with other evidence, it supports the use of ancient pigments

Figure: Schematic of the characteristic infrared peak of bone black pigment. Source: ACS Sensors [1]

1.3 Forgery Detection: Infrared Differences Between Modern and Ancient Pigments

The core logic of infrared spectroscopy for forgery detection is: Different eras used different pigments, and each pigment has a unique infrared fingerprint.

"If a painting supposedly from the 17th century shows infrared features of titanium white, it is almost certainly a 20th-century forgery — because titanium white was not commercially produced until the 1920s." [2]

🔗 Functional groups involved in pigments can be queried: carbonyl (carbonates), hydroxyl, aromatic ring (phthalocyanine).

2. Infrared Spectroscopy in Archaeological Discoveries

2.1 Analysis of Ancient Pottery Glaze Composition

Pottery is the most common man-made artifact in archaeology. The composition of the glaze on pottery surfaces can reveal firing techniques, trade routes, and technological transmission.

Typical analysis workflow:

1. Sampling: Scrape a microgram amount of glaze from the pottery shard surface
2. Measurement: μ-FTIR (micro-FTIR) or ATR-FTIR
3. Interpretation: Identify silicates, lead, alkali metal salts, and other components in the glaze

Infrared characteristics of common glaze components:

• Lead glaze: PbO-SiO₂ system, ~1000 cm⁻¹ Si-O stretching
• Alkali glaze: Na₂O/K₂O-SiO₂ system, slightly shifted Si-O peak position
• Ash glaze (common in East Asian porcelain): Contains apatite (~1040 cm⁻¹ PO₄³⁻) + feldspar

"By analyzing ancient pottery glazes with infrared spectroscopy, archaeologists can distinguish products from different kilns and trace ancient trade routes. For example, the specific mineral combinations in Tang dynasty sancai glazes can serve as provenance indicators." [3]

2.2 Identification of Residual Materials in Mummy Tissues

Ancient Egyptian mummification used large quantities of spices, resins, and preservatives. Infrared spectroscopy analysis of residues in mummy tissues can reconstruct ancient embalming recipes.

Typical cases:

• Pine resin / myrrh: Contains terpenoids, C=C stretching ~1640 cm⁻¹, C-H stretching ~3000 cm⁻¹
• Beeswax: Long-chain alkanes and esters, C-H stretching 2850–2950 cm⁻¹, C=O stretching ~1735 cm⁻¹
• Bitumen: Complex polycyclic aromatic hydrocarbons, aromatic C=C ~1600 cm⁻¹, aliphatic C-H 2850–2950 cm⁻¹

🔗 The characteristic ester peak in beeswax can be viewed at ftir.fun/ir/group/ester. Alkyl C-H peaks at ftir.fun/ir/group/alkyl-c-h.

"Infrared analysis of a mummy from 1000 BCE revealed that its embalming materials contained a mixture of resin and beeswax, consistent with the ancient Egyptian embalming practices recorded by Herodotus."[3]

2.3 Identification of Ancient Textile Fibers

Textiles can survive for thousands of years in dry environments. Infrared spectroscopy is a non-destructive method for identifying ancient fiber types:

🔗 For amide features of protein fibers, see ftir.fun/ir/group/amide.

The key to distinguishing wool and silk lies in the Amide I position: wool ~1630 cm⁻¹ (predominantly β-sheet), silk ~1620 cm⁻¹ (β-sheet, sharper)[3].

3. Forensic Stories

3.1 Matching Paint Fragments from Crime Scenes using μ-FTIR

This is a classic forensic scenario: hit-and-run. Only a few millimeter-sized automotive paint chips are left at the scene. How to find the vehicle?

IR spectroscopy workflow [4]:

1. Sampling: Collect paint chips from the scene (maybe only 1–2 mm)
2. μ-FTIR analysis: Analyze the chips layer by layer with micro-FTIR (automotive paint typically has multiple layers: primer + basecoat + clearcoat)
3. Database comparison: Match against an automotive paint IR database
4. Narrow down: Determine make, model, year, color

Why is IR effective?

The resins and pigments in automotive paint have distinct IR features:

• Polyurethane clearcoat: N-H stretch ~3350 cm⁻¹, C=O stretch ~1690 cm⁻¹, Amide II ~1540 cm⁻¹
• Acrylic resin: C=O stretch ~1730 cm⁻¹, C-O-C stretch ~1150 cm⁻¹
• Alkyd resin: C=O stretch ~1735 cm⁻¹, long chain C-H 2850–2950 cm⁻¹

"μ-FTIR can obtain high-quality spectra from microgram-level samples and is one of the standard methods for paint chip comparison in forensic science. Automotive paints from different brands and years have distinguishable IR fingerprints."[4]

🔗 Functional groups in paint can be queried:

• Amide (polyurethane): ftir.fun/ir/group/amide
• Ester (acrylic/alkyd): ftir.fun/ir/group/ester
• Carbonyl: ftir.fun/ir/group/carbonyl

3.2 Fast Drug Screening with IR Spectroscopy

Infrared spectroscopy is an efficient tool for drug screening for three reasons:

1. Non-destructive: Does not consume the sample, preserving it as court evidence
2. Fast: ATR-FTIR yields results in seconds
3. Specificity: Different drugs have unique IR fingerprints

IR features of common drugs:

"ATR-FTIR has become a standard tool for on-site drug screening. Combined with portable instruments, preliminary identification can be completed within minutes, greatly improving law enforcement efficiency."[4]

🔗 Functional groups in drugs: ester, carbonyl, amine, hydroxyl, aromatic.

Forensic considerations: IR spectroscopy is often used as a screening method. Court convictions typically require confirmatory methods such as GC-MS. However, the "non-destructive" advantage of IR is unmatched by MS.

3.3 Document Authentication: Ink Age Analysis

In document authentication, a key question is: When was the ink on this document written?

IR spectroscopy can approach this from two angles:

Angle 1: Ink Composition Analysis

Inks from different eras use different components:

• Ancient ink (iron gall ink): Tannic acid-iron complex, O-H ~3400, C=O ~1700
• Early fountain pen ink (19th–early 20th century): Sulfates + dyes
• Modern ballpoint pen ink: Alkyd resin + dyes, C=O ~1735, C-O-C ~1100
• Gel pen/rollerball ink: Polyethylene glycols, C-O-C ~1100, O-H ~3400

"If a document alleged to be from the 19th century shows modern alkyd resin features in its ink IR spectrum, it can be judged as a later forgery."[4]

Angle 2: IR Luminescence Imaging

Some inks luminesce under IR light (infrared fluorescence), and aged inks change in fluorescence properties. IR luminescence imaging can:

• Detect erased or obscured writing
• Differentiate overlapping handwriting in different inks
• Assess the aging degree of ink

4. O-PTIR in the Analysis of Cultural Heritage Glass Corrosion (Cross-reference Ep 39)

The submicron distribution of corrosion products on 16th-century glass is a typical cultural heritage case for O-PTIR (Marchetti et al. Science Advances 2022)[5]. Principles, resolution, and full experimental details are in Ep 39; this episode only retains the storyline:

• Corrosion forms hydrated silicate gel and sulfate/carbonate products with heterogeneous spatial distribution;
• Conventional μ-FTIR is limited by the diffraction limit, making it difficult to resolve submicron structures;
• O-PTIR provides both organic and inorganic IR information with ~0.5 μm spatial resolution, aiding conservation strategies.

🔗 Related corrosion products: hydroxyl, sulfate, carbonate/carbonyl related

5. Summary of Cases: The Interdisciplinary Value of IR Spectroscopy

5.1 Common Pattern

Reviewing all cases in this episode, we find a common pattern:

Problem (Humanities/Legal) → Sampling (minimally/non-destructive) → IR Analysis → Molecular-level Information → Answer the Question

Whether in art authentication, archaeological analysis, or forensic investigation, the core value provided by IR spectroscopy is: turning "invisible molecular information" into "evidence that can answer questions".

5.2 Advantages of IR Spectroscopy in the Humanities

5.3 Cooperation with Other Techniques

IR spectroscopy often does not work alone in these fields but cooperates with multiple techniques:

VI. Application of ftir.fun in These Cases

The functional group analysis in these humanities and social science cases can also be queried and verified on ftir.fun:

"ftir.fun, as an online infrared knowledge base, not only serves chemical laboratories but also provides quick verification tools for conservators, archaeologists, and forensic scientists."[6]

Summary of This Episode

Discussion Questions

1. If you work in a museum and receive a painting allegedly from the 17th century, how would you design an identification protocol based on infrared spectroscopy? Which characteristic peaks of pigments need to be detected? What does it imply if titanium white (TiO₂) is found?

2. The characteristic peak of bone black at 2013 cm⁻¹ is used for dating paintings. But is this method absolutely reliable? Under what circumstances might "false positives" or "false negatives" occur? (Hint: restoration, pigment compounding)

3. When forensic scientists use μ-FTIR to compare paint fragments, why do they analyze multilayer structures (primer + color coat + clear coat) rather than just the outermost layer? How does this improve the reliability of matching?

4. The core advantage of O-PTIR over traditional FTIR is spatial resolution. In glass corrosion analysis, why is the ~10 μm resolution of traditional FTIR insufficient? What new information can submicron resolution reveal?

References

[1] Rosi, F. et al. "Macro FTIR Hyperspectral Imaging of Perugino's Renaissance Frescoes." ACS Sensors, 2025, 10(10):7334–7342.
https://doi.org/10.1021/acsse…

[2] Daveri, A. et al. "Infrared Spectroscopy for the Characterization of Bone Black Pigment." Journal of Analytical Methods in Chemistry, 2018, 6595643.
https://doi.org/10.1155/2018/…

[3] Edwards, H. G. M. & Vandenabeele, P. "Archaeological and Artistic Applications of Raman and IR Spectroscopy." Spectrochimica Acta Part A, 2015.
https://doi.org/10.1016/j.saa…

[4] Suzuki, E. M. & Marshall, W. P. "IR Spectra of US Automobile Original Topcoats (1974–1989)." Journal of Forensic Sciences, 1997.
https://doi.org/10.1520/JFS14…

[5] Marchetti, A. et al. "Optical Photothermal Infrared Spectroscopy for the Analysis of Glass Corrosion in 16th-Century Artifacts." Science Advances, 2022, 8(9).
https://doi.org/10.1126/sciad…

[6] ftir.fun Project. Infrared Spectral Functional Group Database.
https://ftir.fun

Preview of Next Episode: Ep 59 — User Innovation and Community Projects: Spectral Geeks on GitHub
Infrared spectroscopy is not exclusive to large companies and research institutions. On GitHub, a group of "spectral geeks" is transforming the field with open-source code—from the microplastic analysis platform OpenSpecy, to the AI functional group identifier SSIN, to the domestic tool ftir.fun. In the next episode, we step into the open-source spectroscopy community.