Characterization of an unusual black patina on the Neang Khmau temple (archaeological Khmer area, Cambodia): a multidisciplinary approach

Abstract

In the archaeological Khmer complex (Cambodia), an unusual black patina covering both the exterior and interior walls of the Neang Khmau temple, has been observed during a recent conservative survey. In fact, it is present over all the four facades of the temple, covers a relevant part of the vertical surfaces protected from rainfall and is also homogeneously distributed on the interior of the monument. The upper and the lower areas show, on the contrary, a relevant lichen growth. The temple was built by using laterite bricks, a natural material common in tropical areas as a weathering product of pre‐existing rocks. A preliminary investigation on the patina samples together with ecological considerations excluded a biological origin of the black alteration, thus prompting a more detailed study. In this work, we have investigated the physico‐chemical origin of this unusual patina by identifying its mineralogical and chemical composition. Raman and FTIR spectroscopies, SEM‐EDS, micro‐XRF and XRPD have been applied to characterize the alteration products. Analyses have been performed both on single fragments and on polished cross sections collected from all four temple facades and from one area in the temple interior. Optical observations of the cross sections have evidenced irregularities in the thickness of the patina. Raman and FTIR spectra have revealed that the black layers are mainly composed of manganese minerals (hollandite, romanechite and manganite). This peculiar alteration pattern is interpreted as because of the tropical climatic conditions of Cambodia. Indeed the variation of wet and dry seasons favours the migration of iron and manganese ions and their successive immobilization in regions not directly subject to leaching. Copyright © 2016 John Wiley & Sons, Ltd.

Raman spectra of natural manganese oxides

“…Moreover, these compounds have been used in various applications, for example, in art works as pigments for pottery, mural paintings, and stained glass. Therefore, their proper characterization might provide an important tool in cultural heritage studies, or when addressing the degradation of art works due to the weathering …”

Recent advances in linear and non‐linear Raman spectroscopy. Part XI

“…Casanova Municchia and co‐workers characterized an unusual black patina on the Neang Khmau temple (archaeological Khmer area, Cambodia) using a multidisciplinary approach. They investigated the physico‐chemical origin of this patina by identifying its mineralogical and chemical composition with Raman and FTIR spectroscopies, SEM‐EDS, micro‐XRF, and XRPD . Centeno et al used Raman and FTIR to study the composition and spectroscopic properties of historic Cr logwood inks.…”

“…They investigated the physico-chemical origin of this patina by identifying its mineralogical and chemical composition with Raman and FTIR spectroscopies, SEM-EDS, micro-XRF, and XRPD. [126] Centeno et al used Raman and FTIR to study the composition and spectroscopic properties of historic Cr logwood inks. The Raman spectra acquired in the lab samples were used to characterize the ink used by Vincent van Gogh in Street in Saintes-Maries-de-la-Mer.…”

Applications of Raman spectroscopy in art and archaeology

“…The black patina is present in the four facades covering a relevant part of the vertical surfaces protected from rainfall and is also homogeneously distributed on the interior of the monument. Casanova Municchia et al studied the mineralogical and chemical composition of the patina using a multianalytical approach to characterize the alteration products. Analyses performed both on single fragments and on polished cross sections collected from all four temple facades and from one area in the temple interior showed the systematic presence of manganese mineral phases: hollandite (Ba(Mn 4+ 6 Mn 3+ 2 )O 16 ), romanechite ((Ba,H 2 O) 2 (Mn +4 ,Mn +3 ) 5 O 10 ) and manganite (MnO(OH)).…”