Decay of a Roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic resonance and portable nuclear magnetic resonance

Abstract

Wood is a hygroscopic biodegradable porous material widely used by men in the past to create artworks. Its total preservation over time is quite rare and one of the best preservation modalities is waterlogging. Observing the anatomy of waterlogged archaeological wood could also be complicated because of its bacterial degradation. However, the characterization of wood morphology and conservation state is a fundamental step before starting any restoration intervention as it allows to extract information about past climatic conditions and human activities. In this work, a micro-invasive approach based on the combined use of high-resolution magnetic resonance imaging (MRI) and diffusion-nuclear magnetic resonance (NMR) was tested both on a modern and an ancient pine wood sample. Furthermore, a completely non-invasive analysis was performed by using portable NMR. This multi-analytical NMR approach allowed to highlight the effect of decay on the wood microstructure, through alterations in the pores size, tortuosity, and images contrast of the ancient pine compared to the modern one. This work pointed out the different but complementary multi-parametric information that can be obtained by using NMR and tested the potential of high-field MRI and low-field portable NMR in the detection of wood diagnostic features.

A Multi-Parametric Investigation on Waterlogged Wood Using a Magnetic Resonance Imaging Clinical Scanner

“…The T 2 *-maps enhance the magnetic field inhomogeneities [17,51,62] generated by the magnetic susceptibility difference at the interface between different wooden tissues, wood and water, and wood and air, showing a decidedly different contrast as compared with what we observe in the X-ray images. In addition to this complementary information, as compared with that provided by X-rays, T 2 *-w images are also sensitive to magnetic field inhomogeneities generated by paramagnetic substances and/or deposits of heavy metals [13,17,43]. This information is particularly useful in analyzing submerged archaeological wood, as we show later.…”

“…Therefore, thanks to the inhomogeneous distribution of water [50], for each species of wood, it is easy to distinguish macroscopic anatomical characteristics such as growth rings, veins, parenchyma rays, texture, the difference between sapwood and heartwood, possible knots, and plant defects, without having to dissect the samples. In particular, some darker areas and black spots appearing in the T 2 *-w image of some species (Mitragyna ciliata, Aningeria altissima, Entandrophragma cylindricum, Toona ciliata, Pinus ponderosa) may be due to magnetic field inhomogeneities generated by air pockets confined in the pores, or by paramagnetic substances present in the wood [13,35,43]. The T 2 map (Figure 4d) allows us to immediately identify areas where the water is more mobile (longer T 2 ), and therefore, where wood is less dense and more homogeneous in structure.…”

“…The T 2 * map allows us to identify areas characterized by a higher inhomogeneity of magnetic field (lower T 2 * values) due to the magnetic susceptibility difference between adjacent wood tissue [51] (such as water in vessels and vessels cell membrane). These are highly heterogeneous areas and/or areas with the presence of paramagnetic agents or heavy metals that contribute to decreasing the value of the T 2 * parameter [43]. The proton density (PD) map shows the spatial distribution of the S(0) parameter which is related to the NMR signal intensity without the relaxation effects (see Equation ( 4)).…”

“…Furthermore, the development of non-destructive techniques for investigating wood structure and the state of preservation of wooden products without compromising their integrity is highly desirable. In fact, it is not always possible to extract a sample from an analyzed object because most of the ancient artifacts are painted or strongly degraded [7][8][9][10][11][12][13][14] and therefore, it difficult to choose a good sampling area.…”

“…Interesting results have been obtained in the field of dendrochronology [27,35], forest sciences [32,33,36], materials sciences, and archaeological woods [13,31,37]. Despite the potential usefulness in the physiological and microstructural study of wood, the presence of papers in the literature concerning the application of MRI for the recognition of wood characteristics, wood properties, and their conservation status is still limited [27,31,33,[38][39][40][41][42][43]; therefore, MRI remains to be a non-consolidated technique for the above-mentioned purposes. Moreover, in recent years, the development of MRI clinical scanners has been remarkable, allowing us to obtain high-quality images for the investigation of human tissues.…”