The mystery of the discolored flints. New molecules turn prehistoric lithic artifacts blue

Abstract

Prehistoric artifacts turning blue in the store rooms of the Natural History Museum in Verona, Italy recently raised serious issues for heritage materials conservation. Our analytical investigation showed that the unusual discoloration process of the flint tools is caused by the surface presence of at least three previously unknown pigmenting molecules of the triphenylmetane dyes class: 6-(bis(2,2,4-trimethyl-1,2-dihydroquinolin-6-yl)methylene)-2,2,4-trimethyl-2,6-dihydroquinolinium and its hydrogenated derivatives 2,2,4-trimethyl-6-((2,2,4-trimethyl-1,2,3,4-tetrahydroquinolin-6-yl)(2,2,4-trimethyl-1,2-dihydroquinolin-6-yl)methylene)-2,6-dihydroquinolinium and 6-(bis(2,2,4-trimethyl-1,2,3,4-tetrahydroquinolin-6-yl)methylene)-2,2,4-trimethyl-2,6-dihydroquinolinium. The peculiar formation of the molecules is possibly catalyzed within the silica pore surface starting from a well-known rubber stabilizer 2,2,4-trimethyl-1,2-dihydroquinoline released by the plastic pads flooring the storing cabinets. The investigated reaction and its surprising blue product represent a case study of the application of modern materials science to conservation and a serious warning towards the unpredictable challenges faced in the preservation of our cultural heritage.

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Acknowledgment

The Soprintendenza per i Beni Archeologici del Veneto in the person of Dr. Vincenzo Tinè authorized the investigation and the publication of the results. A. Barbon, S. Barison, A. Boaretto, S. Boesso, M.C. Dalconi, S. Fiameni, C. Giorio, D. Marton, M. Meneghetti, C. Pagura, G. Salvaro, L. Soldà, R. Spiess, A. Venzo, and F. Zorzi for helping in the preliminary analyses.

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Correspondence to Andrea Tapparo.

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Tapparo, A., Artioli, G., Angelini, I. et al. The mystery of the discolored flints. New molecules turn prehistoric lithic artifacts blue. Anal Bioanal Chem 399, 2389–2393 (2011). https://doi.org/10.1007/s00216-010-4625-4

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Keywords

  • Prehistoric artifacts
  • Blue flints
  • Triphenylmethane dyes
  • Mass spectrometry