Structure-Optical Properties Relationships in Cobalt-Based Purple Pigments Used by Robert Delaunay

Area of Science:

• Art Conservation Science
• Inorganic Chemistry
• Materials Science

Background:

• Purple pigments, particularly cobalt-based ones, gained popularity in late 19th- and early 20th-century art following industrial advancements.
• Despite their widespread use in modern and contemporary paintings, the chemical and structural properties of cobalt phosphate and arsenate purple pigments remain underexplored.
• Robert Delaunay, a prominent French avant-garde artist, utilized these purple pigments in his early works.

Purpose of the Study:

• To chemically characterize the inorganic purple pigments used by Robert Delaunay.
• To establish the relationship between the crystal structure and color (hue) of cobalt phosphate and arsenate compounds.
• To investigate the in situ composition and optical properties of pigments in Delaunay's masterpieces.

Main Methods:

• Synthesis of reference cobalt phosphate and arsenate materials.
• Characterization using synchrotron radiation-based high-angular-resolution X-ray powder diffraction (XRD).
• Optical analysis via UV-visible-Near-Infrared (UV-vis-NIR) spectroscopy.
• In situ XRD (imaging mode) and fiber optic UV-vis-NIR reflectance spectroscopy on artworks.

Main Results:

• Established structure-color relationships for inorganic compounds in the cobalt-phosphorus/arsenic (Co-P/As) family for the first time.
• Identified specific structural and chemical compositions responsible for the observed purple hues.
• Confirmed the presence and characteristics of these pigments in Robert Delaunay's early paintings.

Conclusions:

• The structural chemistry of inorganic purple pigments significantly influences their color properties.
• This research provides crucial chemical insights into the materials used by Robert Delaunay and potentially other artists of the era.
• Understanding these pigment compositions aids in the conservation and analysis of early 20th-century art.