By Roxanne Radpour, associate editor of archaeological pigments
A recent review of shellfish purple by Karapanagiotis (2019), also known as Tyrian purple, has produced a comprehensive look at the composition, archaeological/historical use and scientific characterization of the rarest dye pigment utilized in human history [1]. Shellfish purple, in use since the 18th century BCE, was popularized by Phoenicians around the 9th-7th century BCE, but fell out of production at the end of the Byzantine empire, due to the conquest by Ottomans of Constantinople and the decree by Pope Paul II to opt for kermes in the dyeing of ecclesiastical robes.
Derived from the mollusk family Muricidae, found in the Mediterranean basin, three types of shellfish have been identified as the source of coloring compounds: Hexaplex trunculus L., Bolinus brandaris L., and Stramonita haemastoma. The most prominent compounds, 6,6’- dibromoindigotin (DBI), indigotin (IND), 6-bromoindigotin (MBI), 6,6 ́-dibromoindigotin (DBI) and 6,6 ́-dibromoindirubin (DBIR), and in smaller amounts, indirubin (INR), 6 ́-bromoindirubin (6′MBIR) and 6-bromoindirubin (6MBIR), are produced from the hypobranchial glands of the mollusk shells, by way of photochemical and oxidation reactions undergone by constituent chromogens (organic precursors). While DBI has been considered most prevalent, studies have shown that the other compounds have been found to be in highest concentration in certain shell extracts and are specific to the mollusk species, thus establishing themselves as important identifying markers for shellfish purple along with DBI [1].
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| Top: most prominent colouring compounds found in the hypobrachial glands of the Muricidae mollusks; Bottom: colouring compounds/DMSO solutions (image adapted from Ioannis 2019) |
The earliest use of shellfish purple was identified in textiles from Syria (18-16th century BCE). As a pigment for wall paintings, the first uses were documented in Santorini and Rhodes (18-17th century BCE). However, GC-MS analysis of Minoan pottery vessels from Crete (17-16th century BCE) provided the first scientific evidence of the shellfish purple dying industry. It is suggested that pigment synthesis may have been carried out simultaneously in vat dyeing workshops. Scientific characterization of shellfish purple has been reported in the literature using HPLC-DAD, XRF, LC-MS, GC-MS, Raman spectroscopy; and FTIR [1]. Recent technical studies of synthetic shellfish purple have aimed to open the door to non-invasive luminescence imaging approaches [2], as this approach has proven successful for imaging of the ancient luminescent pigments Egyptian blue and madder lake. In Verri et al. (2019), the photophysical properties synthetic DBI in dimethyl sulfoxide (DMSO) solutions and solid state powder were characterized by spectrophotometry and fluorescence lifetime studies, guided by time-dependent density functional theory (DFT) calculations. Preliminary luminescence imaging of the solid state utilized laser excitation at 405 nm, facilitating a fluorescence capture between 800-900 nm. It was also noted that SEM-EDX characterization of the synthetic DBI did not reveal the presence of Br, often considered a marker for Tyrian purple in elemental identification techniques. Another analytical study by Vasileiadou et al. (2019), addressing conservation concerns of objects and paintings featuring the purple pigment, explored the effect of UV light on the purple pigment and dyed textiles. HPLC-DAD analyses showed dramatic decrease of coloring compound concentrations due to artificially-induced UV aging, and visual observation supplemented by colorimetry confirmed discoloration [3].
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| Left: color image of Tyrian purple powder packed with metal sample holder; Right: indicted near infrared photoluminescence of Tyrian purpose from 405nm laser excitation (image adapted from Giovanni et al. 2019) |
These papers give an indication of potential next steps for characterization and conservation of Tyrian purple, which may take into account other mollusk-based coloring compounds, influences of impurities, binders, and application to the substrate on diagnostic spectral signatures. Additionally, mixtures of Tyrian purple with other red organic colorants, as identified in archaeological evidence, feature overlapping absorption and emission profiles which will provide interesting analytical case studies to effectively isolate the shellfish pigment non-invasively.
References cited:
[1] Ioannis K., 2019. A Review on the Archaeological Chemistry of Shellfish Purple." Sustainability 11 (13), 3595.
[2] Giovanni V., Martin de Fonjaudran, C., Acocella, A., Accorsi, G., Comelli, D., D’Andrea, C., Nevin, A., Zerbetto, F., Saunders, D., 2019. An ‘imperial radiation’: Experimental and theoretical investigations of the photo-induced luminescence properties of 6,6′-dibromoindigo (Tyrian purple). Dyes and Pigments 160, 879-889.
[3] Athina Vasileiadou, Ioannis Karapanagiotis, Anastasia Zotou, 2019. UV-induced degradation of wool and silk dyed with shellfish purple. Dyes and Pigments 168, 317-326.
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