FTIR spectroscopy of woods: A new approach to study the weathering of the carving face of a sculpture

doi:10.1016/j.saa.2014.10.011

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Volume 136, Part C, 5 February 2015, Pages 1255-1259

https://doi.org/10.1016/j.saa.2014.10.011 Get rights and content

Highlights

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We are looking for a new approach to determine if a wooden sculpture is ancient.
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We have studied the water content inside wood through FTIR characteristics.
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In particular, the ratio of two signals around 3400 cm⁻¹ and 1610 cm⁻¹ is explored.
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The ratio of the two signals from the surface to the inner part of the wood decreases for ancient wood.
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The first data indicate that the ancient wood has a behavior in opposite with young wooden sculptures.

Abstract

Fourier transform infrared spectroscopy (FTIR), a useful method to study woods, has been applied in order to measure an eventual gradient of water absorbed (more precisely, hydroxyl groups, OH) between the surface and the inner part of an ancient and a modern wooden sculptures. The methodology adopted has consisted in comparing FTIR bands within two spectral ranges, one corresponding to the OH content in the wood (around 3400 cm⁻¹) and the other to the lignin molecules (aromatic C double bond C, around 1610 cm⁻¹). The ratio between the intensities of the peaks leads to a self-normalization of the measurements which avoids the effects of some experimental parameters (such as sample preparation).

The results obtained show that there is a significant and progressive penetration of OH groups from the surface to the inside part of the ancient sculpture (more OH on the surface than inside the wood, due to the aging of the surface and the creation of sites which are able to fix OH groups). In contrary, for the modern one, we measure an increase of OH contents from the surface to the inner part of the sculpture.

Graphical abstract

Introduction

In order to study ancient wooden objects of art we usually use various methods which give information about the structure of the surface and its weathering, as SEM [1], [2], [3], [4]. In parallel, we complete the approach by dating the cut of the tree used to carve the object with the well-known C14 method [5], [6], [7]. With time, fakes made to lure experts of art take into account the knowledge brought by the scientific analysis. So, some false objects are recently carved into old woods, others have their surface artificially aged with chemical or various treatments.

We have then to find a new approach to analyze this kind of objects, and especially the antiquity of the sculpture phase. We have chosen to test the possibility to use Fourier transform infrared spectroscopy (FTIR), usually exploited to characterize wood structure [8], [9], [10], [11], [12], [13]. FTIR gives data of the molecular structure of wood, a scale which is not enough known by fakers to be able to find an artificial procedure to simulate natural weathering. Wood modifications with various causes were studied with FTIR [11], [14], [15], [16], which is a no time-consuming method used for a rapid determination of molecular groups and which needs small amount of sample. In the present paper we have focused our methodology on the possibility to study the effect of water, as a weathering agent, and its integration inside the molecules of the wood (cellulose, hemicelluloses and lignin).

Section snippets

Samples

Samples were collected from two different objects. The first one is an «ancient» sculpture (Borneo, Shorea species, around 15th century AD for the cutting period of the wood) for which we presume that the surface was anciently carved. The second one is a modern object (species unknown, age less than 50 years old). They are described in Fig. 1 and Table 1.

For each sampling area, a powder of wood was collected within three different depths, from the surface to the internal part of the material

Results and discussion

Fig. 3 shows the infrared spectra recorded for the 3 wood samples: E1 (Fig. 3a), E2 (Fig. 3b) and E3 (Fig. 3c). The assignment of all the bands can easily be made using the data summarized in Table 2. Some little shifts can be observed as a function of the wood species studied. Indeed, samples E1 and E2 (which come from the same wood species) show infrared spectra with the same profile whereas some differences occur with sample E3 (which comes from another wood species). If the positions of the

Conclusion

In this paper we have applied a new methodology in order to study the antiquity of the carving phase for wooden objects. The same analysis performed on an ancient and a modern wood, the comparison between samples taken from different depths and different localizations of an object, the use of FTIR spectrometry and the calculation of ratios of intensities for two ranges of absorptions have led to various interesting observations which should be developed with more objects. Indeed, we have put

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FTIR spectroscopy of woods: A new approach to study the weathering of the carving face of a sculpture

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Samples

Results and discussion

Conclusion

Polym. Degrad. Stab.

Int. Biodeter. Biodegrad.

Carbohydr. Polym.

Spectrochim. Acta Part A

J. Microscopy

Science

Archaeometry

Radiocarbon

Ann. Sci. For.