Practical identification of cobalt-based blue pigments detecting the induced photoluminescence by a He-Ne laser using a Raman spectrometer

Since the early 19th century, the pigment historically known as cobalt blue has been one of the most widely used in artistic painting. Its many and excellent properties led to the rapid development of different synthesis processes for the pigments known today under the generic name of cobalt-based pigments. The differentiation and characterization of these pigments is often difficult, because many of them are made of the same raw materials, mainly cobalt oxides (or salts) and alumina (aluminum oxide), even when Raman spectroscopy is used. In this research, six cobalt-based trademark pigments and two chemically pure compounds, cobalt aluminate and calcined alumina, have been analyzed using a Raman spectrometer with a red He-Ne laser (632 nm) as the excitation source. It should be noted that only the Raman band around 517 cm⁻¹ associated with the cobalt aluminate has been detected by the authors in cobalt based pigments with this configuration. In order to obtain additional information, a fundamental aim of this work has been to detect the photoluminescence (PL) emitted by the leftover of calcined alumina (α-alumina) in these compounds, particularly in the form of a strong doublet located at 1367 and 1396 cm⁻¹. From an experimental point of view, it is remarkable that both Raman and PL information can be acquired within the same spectral range of the spectrometer. Additionally, the relationship between the intensities of the Raman band around 517 cm⁻¹ and those of the doublet can be used as an indicator to estimate the quality of each pigment.