Metal oxalates in paints: a Raman investigation on the relative reactivities of different pigments to oxalic acid solutions

A. Zoppi, C. Lofrumento*, N. F. C. Mendes, E. M. Castellucci

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

39 Citations (Scopus)

Abstract

One degradation phenomenon that occurs in artworks is the formation of metal oxalates on their surfaces. In order to gain insight into the inclination of pigments to produce oxalates, nine pigments including Na, Ca, Fe, Pb and Cu cations were selected to react with oxalic acid solutions at different concentrations (1 M, 0.1 M, 0.01 M and 0.005 M). Micro-Raman spectroscopy was used to detect the different reaction products. Pigments containing calcium (calcite, gypsum and Volterra gypsum) showed a high tendency to form weddellite as well as whewellite, especially at high acidic concentrations; among copper-based pigments (malachite, azurite, verdigris), the formation of moolooite was observed for high concentrations of acid and down to the lowest concentration (0.005 M) in the case of verdigris. Lead oxalate was detected on lead white. No iron oxalates were observed for hematite; the formation of calcium oxalate crystals was observed instead. Ultramarine blue reacted to produce elemental sulfur. According to the results obtained, calcite and verdigris showed the highest reactivity in oxalic acid environments, resulting in a high tendency to form calcium and copper oxalates, even at very low acidic concentrations; this behavior seems to arise from the high solubilities of these pigments in acidic environments.

Original languageEnglish
Pages (from-to)841-849
Number of pages9
JournalAnalytical and Bioanalytical Chemistry
Volume397
Issue number2
DOIs
Publication statusPublished - May 2010
Externally publishedYes

Keywords

  • Micro-Raman
  • Moolooite
  • Oxalates
  • Oxalic acid
  • Weddellite
  • Whewellite

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