PR 81 – rhodamine 6G

Widely used to dye paper and for making inks.

Manufacturer/ product code:  Sigma-Aldrich
Chemical Description:  C₂₈H₃₁N₂O₃Cl
Color: red
Color Index: PR 81
More info: 

“Handheld surface-enhanced Raman scattering identification of dye chemical composition in felt-tip pen drawings” J Raman Spectrosc. 2018;1–10

PR 81 – Rhodamine 6G is a synthetic organic colorant belonging to the xanthene dye family, known for its extremely bright, fluorescent red-orange hue. Chemically classified as a rhodamine dye, PR 81 is primarily a dye rather than a true pigment; when converted into an insoluble lake form, it may be used as a pigment under the Colour Index designation PR 81. Its extraordinary chromatic intensity and fluorescence distinguish it sharply from conventional non-fluorescent red pigments.

The molecular structure of Rhodamine 6G features a xanthene core responsible for its strong absorption in the green region of the spectrum and intense emission in the orange-red range. This fluorescence produces a visual brightness far exceeding that of ordinary pigments, making PR 81 highly effective in applications requiring maximum visibility. However, this same structure renders the colorant chemically and photochemically unstable. PR 81 exhibits poor lightfastness, with rapid fading under UV and strong visible light exposure.

Historically and industrially, Rhodamine 6G has been widely used in printing inks, security printing, signage, textile dyeing, tracing dyes, and fluorescence-based scientific applications. In artistic and decorative contexts, its use has been largely limited to posters, commercial graphics, and short-lived applications where brilliance outweighs permanence.

From a conservation and analytical perspective, PR 81 is significant as a marker of modern, dye-based coloration. It cannot be detected by XRF and typically requires UV-induced fluorescence imaging, UV–Vis spectroscopy, Raman spectroscopy, or chromatographic methods for reliable identification.

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Raman Spectroscopy 532 nm – ElviRa Spectrometer for Art

 

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Raman spectroscopy 785 nm

Pigments (powder, no binder)

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Pigments with acrylic binder (Pigments Checker)

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Raman spectroscopy 830 nm

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Raman spectroscopy 1064 nm

Pigments (powder, no binder)

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XRF spectroscopy

XRF cannot identify this pigment.

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Reflectance Spectroscopy (200-1000 nm) – GorgiasUV Spectrometer

Reflectance Spectroscopy (200-1000 nm) (GorgiasUV) Pigments-Checker Database (93078 downloads )

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Reflectance Spectroscopy (350-950 nm) – Gorgias Spectrometer

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NIR Reflectance Spectroscopy (930 – 1690 nm)

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FT-IR Spectroscopy (Non-invasive Diffuse Reflectance Probe)

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