Microscopic view of porcelain shards with glowing blue pigments and spectral analysis lines.

Cobalt Blue in Ancient Porcelain: New Insights into Raman Spectroscopy Analysis

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Jordan Keane in Science & Nature April 2025 • 4 min read.

"Unlocking the secrets of historical pigments: A reassessment of cobalt blue in blue and white porcelain reveals surprising insights into art and archaeology."


Color profoundly influences human existence, shaping both our personal lives and cultural expressions. Among the spectrum, blue holds a special place, inspiring feelings of calm and serenity, often associated with the sky, water, and peace. This unique appeal has made blue a prized color in various art forms throughout history.

One striking example is blue and white porcelain, known as Qinghua in Chinese, which flourished during the Yuan Dynasty (1279-1368 AD). This exquisite art form gained immense popularity for its harmonious blend of intricate blue patterns against a pristine white background. The scarcity of natural blue colorants, such as lapis lazuli, further elevated the status and allure of blue in artistic creations.

Before the Industrial Revolution, only a limited number of synthetic blue pigments existed. These included Egyptian/Han blue (copper silicates), Maya blue (guest indigo in clay), and cobalt blue. Cobalt-based pigments, in both amorphous and crystalline forms, have been widely used for various purposes since 3500 BP, from Egyptian artifacts to modern creations, owing to their remarkable tinting strength. While ground cobalt glass (Smalt) had limitations in medieval times due to its susceptibility to bleaching, cobalt aluminate pigments have become popular and known for it's corrosion resistance

The Cobalt Blue Reassessment

Microscopic view of porcelain shards with glowing blue pigments and spectral analysis lines.

Identifying cobalt-based colorants in antiquities is critical for archaeological research and artwork authentication. Spinel end-members, which share a similar structure, can be challenging to differentiate, making precise analysis complex. Raman spectroscopy, a fast and non-invasive technique, has emerged as a promising method for distinguishing spinels like cobalt aluminate using unique peaks, or 'fingerprints.'

Recent Raman studies have dominated this field, with representative spectra attributed to cobalt blue in porcelain. However, discrepancies exist between these spectra and those of synthesized cobalt aluminate spinels. A significant difference is the absence of a high-intensity peak around 201 cm⁻¹ in the antiquity group, a band typically associated with the Co-O vibration in spinels. Some researchers attributed this to differences in the pigment's environment, laser excitation, crystallinity, and orientation.
To clarify the discrepancies, the study highlights a new approach:
  • The previous study of cobalt blue pigment displays discrepancies in Raman spectra.
  • An approach of combining MRS and SEM is proposed to clarify this issue.
  • The microstructure analysis shows pigment particles are enwrapped by anorthite.
  • The accurate spectrum of cobalt aluminate pigment is obtained by the use of MRS.
Published microstructural investigations suggest that cobalt-rich particles in blue and white porcelain are tightly enwrapped by anorthite crystals, making direct cobalt signal detection difficult. Characteristics of 'cobalt blue' Raman bands in art and archaeometry studies closely resemble mineral anorthite. The study couples spectroscopic analysis with structural observation to characterize pigments in complex environments, enhancing the understanding of Raman spectroscopy in art and archaeology.

Conclusion

This research highlights discrepancies in published spectroscopic results of cobalt aluminate, clarifying that previous studies on cobalt blue pigments in ancient ceramics captured spectra from the surrounding anorthite mineral (CaAl2Si2O8), not cobalt blue (CoAl2O4). The misidentification occurred because of a strong band around 507 cm⁻¹ present in both anorthite and cobalt blue. The strong T2g mode at 202 cm⁻¹ from cobalt blue is missing in most prior spectra. The study underscores the importance of combining micro-Raman and microstructure analysis in art and archaeology to address complex target systems.

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