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


ACTA IMEKO 
ISSN: 2221-870X 
March 2022, Volume 11, Number 1, 1 - 10 

 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 1 

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

Valeria Stagno1,2, Silvia Capuani2,3 

1 Earth Sciences Department, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy  
2 National Research Council - Institute for Complex Systems (CNR-ISC) c/o Physics Department Sapienza University of Rome, Piazzale Aldo  
  Moro 5, 00185 Rome, Italy  
3 Centro Fermi - Museo Storico Della Fisica e Centro Studi e Ricerche Enrico Fermi, Piazza del Viminale 1, Rome 00184, Italy 

 

 

Section: RESEARCH PAPER  

Keywords: Archaeological waterlogged wood; micro-MRI; diffusion-NMR; portable NMR 

Citation: Valeria Stagno, Silvia Capuani, Decay of a Roman age pine wood studied by micro magnetic resonance imaging, diffusion nuclear magnetic 
resonance and portable nuclear magnetic resonance, Acta IMEKO, vol. 11, no. 1, article 12, March 2022, identifier: IMEKO-ACTA-11 (2022)-01-12 

Section Editor: Fabio Santaniello, University of Trento, Italy 

Received March 3, 2021; In final form March 15, 2022; Published March 2022 

Copyright: This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, 
distribution, and reproduction in any medium, provided the original author and source are credited. 

Corresponding author: Valeria Stagno, e-mail: valeria.stagno@uniroma1.it  

 

1. INTRODUCTION 

Wood is a porous material with complex morphology. In the 
past, it has been widely used by men to produce artworks. For 
this reason, wood is widespread in the cultural heritage world and 
its microstructure has always been studied for the species 
identification, for dendrochronological analyses and for 
extracting important information about ancient human activities 
[1]. Two principal types of wood can be recognized: softwood 
and hardwood. Softwood has a very homogeneous structure 
mainly composed of tracheids and fibre-tracheids [2]. The resin 
canals are one of its main characteristics that allow it to be 
distinguished from the hardwood. In addition, its annual rings 
are usually well separated through the annual ring limit and they 

are made of an earlywood area, with pores characterized by larger 
lumens and thinner walls, and a latewood area with thicker walls 
and smaller lumens [2]. However, these two areas are not always 
well differentiated. Some softwoods present an abrupt passage 
from earlywood to latewood, while some others a gradual one 
[2]. Among the many anatomical elements described above, the 
growth ring is surely one of the most important because its 
characterization provides the age of the tree and the climatic 
conditions in which it has grown [3], [4], as well as being crucial 
for the species identification. Because of its biodegradability [4] 
wood is hardly preserved with no microstructural alterations. 
Waterlogged wood is usually well preserved for the microscopic 
observation of its annual rings, especially when the wooden 
object was buried under the sea sediments [6]. In fact, thanks to 
the anaerobic conditions fungal attacks are more or less excluded 

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.. 

mailto:valeria.stagno@uniroma1.it


 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 2 

[6] and if the object was buried under the seabed also the marine 
borer activity is limited [7]. In this environment, the main 
degradation can be attributed to erosion bacteria [8]-[10]. 
Conversely, at a macroscopic level the wood rings are not always 
visible with the naked eye because of changes in colour, 
consistency, and superficial morphology of the wood structure 
[11]. As a consequence, microscopic imaging techniques are 
always required. Moreover, the evaluation of the effect of age is 
also very important, for example, to determine the conservation 
state of an artwork. Knowing the state of wooden remains, such 
as its degree of decay and its pore size and morphology, is useful 
for planning the restoration and choosing the restoration 
materials [11], [12]. In the literature, several works [13]-[22] about 
the use of nuclear magnetic resonance (NMR) on wooden 
artworks have proven its utility in the microstructural 
characterization and the evaluation of their conservation state. 
The above cited works employed invasive or destructive NMR 
techniques, while some others [22]-[25] pointed out the potential 
of non-invasive portable NMR. Among these, our previous work 
Stagno et al. [23] showed how 1D and 2D low-field NMR 
experiments can be used as complementary techniques to study 
water compartmentalization in archaeological waterlogged wood 
with the support of optical microscopy and magnetic resonance 
images. Furthermore, we suggested the ability of low-field NMR 
in detecting cell wall decay and paramagnetic impurities. In 
respect to our previous work [23], in this study we investigated a 
decayed pine wood of the Roman age by using MR images with 
higher resolution and by characterizing the wood structure also 
with the pore size distribution extracted from the relaxation 
measurements. The results obtained from the archaeological 
sample were compared with the results obtained from its modern 
counterpart. Specifically, the aim of this work was to test the 
potential of NMR multi-analytical approach to evaluate 
archaeological wood decay. We compared micro-invasive 
magnetic resonance imaging (MRI) and diffusion-NMR [26]-[29] 
with non-invasive portable NMR [30], [31]. The potential of MR 
images as an alternative approach to the conventional techniques 
for the characterization of the annual rings, as well as of all the 
diagnostic features of waterlogged wood, was tested. Moreover, 
high-field diffusion and low-field portable NMR were used to 
highlight the effect of decay on the wood microstructure, such 
as variations in the pore size of the ancient compared to the 
modern pine.  

2. BACKGROUND THEORY 

2.1.  Diffusion-NMR and pore size 

Molecules constituting a fluid whose absolute temperature is 
greater than zero Kelvin are in constant movement because of 
their kinetic energy [27]. This process is called self-diffusion. 
When a fluid totally fills porous medium, molecules moving with 
random motion are subject to continuities deviations of their 
trajectories due to collision with the pores walls. Diffusion-NMR 
techniques investigate diffusion dynamics by following in time 

the fluid molecules. The root mean square distance travelled, 𝓁D 
(or diffusion length), increases with time as long as no boundaries 

are encountered, according to the Einstein relation: 𝓁D 

=√2 𝑛 𝐷 𝑡 where 𝑛 = 1, 2, 3 is the space dimension and 𝐷 is 
the bulk diffusion coefficient that can be measured by a Pulsed 

Field Gradient sequence [26]. At a fixed diffusion time 𝑡 = ∆ 
(where ∆ is the delay time between the two gradient pulses) and 
fixed pulse magnetic-field gradient duration 𝛿 such that 𝛿 ≪ ∆, 

it is possible to vary the magnetic-field gradient strength 𝑔, so 
that the NMR-signal amplitude 𝑆(𝑔) is given by [28], [32]:  

𝑆(𝑔) = 𝑆(0) e
−𝛾2𝑔2𝛿2𝐷(∆−

𝛿
3

)
 ,  (1) 

where 𝛾 is the gyromagnetic ratio of protons, 𝑏 = is the so called 
𝑏-value, 𝐷 is the diffusion coefficient obtained at a specific 
diffusion time ∆ and 𝑆(0) is the signal at 𝑔 =0. Since during ∆ 
in a Pulsed Gradient Stimulated Echo (PGSTE) experiment 

longitudinal relaxation decay 𝑇1 would occur, the maximum Δ 
accessible is limited by the relation ∆ < 𝑇1  [33]. Geometrical 
restrictions of the medium, such as the pores walls, lead to a 

𝐷(∆) decreasing with time. In a heterogeneous porous system as 
wood, it is possible to derive useful information about pores size, 
pores interconnection and membrane permeability [29] by 

studying the 𝐷(∆) behaviour. 
In the case of long ∆ limit (i.e., ∆ ≫  𝐿2 𝐷0⁄ , where 𝐿 is the 

mean pore diameter) and impermeable wall, the water diffusion 
coefficient varies according to: 

𝐷(∆) =
𝐿2

2 ∆
 . (2) 

Equation (2) indicates that 𝐿 can be obtained from the slope 
of 𝐷 vs ∆−1. However, deviation from equation (2) can occur for 
semi-permeable walls [34]-[35]. For materials with 

interconnected pores, at very long times 𝐷(∆) approaches an 
asymptotic value 𝐷∞ that is independent of the diffusion time ∆ 
and directly related to the tortuosity 𝜏 of the porous material:  

𝜏 =
𝐷0
𝐷∞

 . (3) 

Tortuosity is an intrinsic property of a porous medium usually 
defined as the ratio of actual flow path length to the straight 

distance between the ends of the flow path. So, 𝜏 of the porous 
system reflects the connectivity degree of the porous network 
[36]-[38]. 

2.2.  Transversal relaxation and pore size 

The transversal or spin-spin relaxation time 𝑇2 is due to the 
loss of coherence of the spins among themselves. When an 
ensemble of spins is considered, their magnetic fields interact 
(spin-spin interaction), slightly modifying their precession rate. 
These interactions are temporary and random. Thus, spin-spin 
relaxation causes a cumulative loss in phase resulting in 

transverse magnetization decay [26]. In a porous medium, the 𝑇2 
relaxation of the fluid (e.g., water) is given by the sum of different 
contributions [39]-[43]: 

1

𝑇2
=

1

𝑇2b
+

1

𝑇2s
+

𝐷(𝛾 𝐺 𝑇E)
2

12
 , (4) 

where 1 𝑇2⁄  is the total spin-spin relaxation rate, 1 𝑇2b⁄  is the 
relaxation rate of bulk water and 1 𝑇2s⁄  is the relaxation rate of 
water on the pore surface. The term 𝐷(𝛾 𝐺 𝑇E)

2 12⁄   is related 
to the dephasing caused by the presence of a magnetic field 

gradient 𝐺, named “diffusion relaxation” term. 𝐷 is the diffusion 
coefficient of water in the porous system, 𝛾 is the hydrogen 
gyromagnetic ratio and 𝑇E is the echo time. 

When the 𝑇2  is measured by a Carr-Purcell Meiboom-Gill 
(CPMG) sequence, the contribution of diffusion relaxation is 

averaged out by means of an echo train if a very short 𝑇E is 
selected [44], [45]. In the presence of a porous system, the 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 3 

dominant relaxation rate is 1 𝑇2s⁄  while the term 1 𝑇2b⁄  can be 
neglected because of the low efficiency of bulk water relaxation 
[46]. Equation (4) becomes:  

1

𝑇2
≈

1

𝑇2𝑠
= 𝜌

𝑆

𝑉
 , (5) 

where 𝜌 is the surface relaxivity [46], 𝑆 𝑉⁄  is the surface-to-
volume ratio of the pore. Therefore, by measuring the spin-spin 

relaxation in a porous medium the pore diameter 𝑑 can be 
calculated by [46]-[48]: 

1

𝑇2
= 𝜌

2 𝑛

𝑑
 , (6) 

where 𝑛 is a shape factor (for our samples a spherical shape was 
considered, 𝑛 = 3). 

3. MATERIALS AND METHODS 

3.1. Samples 

Two cylinders-like wood samples of a water-soaked modern 
wood and an archaeological waterlogged wood were studied. 
Their size was less than 15 mm in length and 8 mm in diameter, 
suitable for a 10 mm NMR tube. The archaeological sample 
detached from an ancient pole of the Roman harbour of Naples, 
dated to the V century AD [49], [50]. It was well preserved in 
waterlogged conditions and for this reason was always kept in 
water during the analysis. The modern wood, instead, was 
previously maintained at the environmental conditions of 20 °C 
and 50 % of relative humidity. In order to perform NMR 
acquisitions, the modern sample was imbibed with distilled water 
until the saturation was reached. Both the species of modern and 
archaeological wood was stone pine (Pinus pinea L.) [51], [52]. 

3.2.  High-field NMR acquisitions 

All the high-field NMR analyses were performed by using a 
400 MHz Bruker-Avance spectrometer with a 9.4 T static 
magnetic field and a micro-imaging unit equipped with high-
performance and high-strength magnetic field gradients for MRI 
and diffusion measurements. The gradients maximum strength 
was 1240 mT/m and the rise time 100 µs. 

To measure the diffusion coefficient 𝐷 and the longitudinal 
relaxation time 𝑇1, useful to choose the longest observation time 
∆ accessible during the diffusion experiments according to the 
∆ < 𝑇1 relation, the soaked samples were inserted without 
additional water in the NMR tube, which was sealed on the top 
with Parafilm in order to prevent the sample dehydration. The 
longitudinal relaxation time was measured with a Saturation-

Recovery 𝑆R sequence with 128 points from 10 µs to 10 s, 
repetition time 𝑇R of 10 s, number of scans 𝑁S equal to 4. The 
acquisition time was 1 hour and 40 minutes for each sample. For 
the measurement of the water diffusion coefficient, a PGSTE 
sequence [28], [29] was used. The diffusion was evaluated along 
the x axis (i.e., perpendicular to the main direction of the wood 
grain) corresponding to the radial direction. The PGSTE signal 

was obtained using 𝑇R = 5 s, echo time 𝑇E = 1.9 ms, pulse 
gradient duration 𝛿 = 3 ms, 32 steps of the gradient strength 𝑔, 
from 26 to 1210 mT/m, for each diffusion time ∆ and 𝑁S = 16. 
The ∆ values used were 0.04 - 0.08 - 0.12 - 0.16 - 0.2 - 0.3 - 0.4 s.  
The 𝑏-value spanned from a minimum of 1.6 × 107 s/m2 to a 
maximum of 9.5 × 1011 s/m2. The acquisition time was about 6 
hours for each sample. 

For the acquisition of MR images, the samples were inserted 
with distilled water in the NMR tube and sealed with Parafilm in 

order to prevent water evaporation. In this way, 𝑇2
∗-weighted 

images were performed with a Gradient Echo Fast Imaging 
(GEFI) sequence [26] in the transversal, tangential and radial 

direction. The images were weighted on the 𝑇2
∗ parameter, which 

depends on both 𝑇2 and magnetic field inhomogeneities. The 
optimized parameters used in the GEFI sequence are reported 
in Table 1, where STK is the slice thickness, FOV the field of 
view, MTX the image matrix and R the in-plane resolution. 

3.3. Low-field portable NMR acquisitions 

For the low-field NMR acquisitions the samples were just 
placed on the surface of the sensitive area of the portable 
spectrometer. A Bruker minispec mq-ProFiler with a single-sided 
magnet that generates a static magnetic field of 0.35 T with a 1H 
resonance frequency of 15 MHz and dead time of 2 µs was used. 
The single-sided NMR was equipped with a RF coil for 
performing experiments within 2 mm starting from the sample 
surface to the inside of the sample [53].  

The portable spectrometer has a constant magnetic gradient 
field with strength of about 3 T/m. 

The transversal relaxation time 𝑇2 was measured with a 
CPMG sequence with 𝑇E = 42 µs, 6500 echo times, 𝑇R = 1 s, 
𝑁S = 128 and acquisition time of about 5 minutes. 

3.4. Data processing 

The longitudinal relaxation time was obtained by plotting the 

signal intensities 𝑆 as a function of 𝑡 = 𝑆R  delays for fitting the 
following equation: 

𝑆(𝑡) = 𝑀 [1 − e
−

𝑡
𝑇1 ] (7) 

to data, where 𝑇1 is the longitudinal relaxation time and 𝑀 is the 
associated equilibrium magnetization. 

The diffusion coefficient values were obtained by fitting the 
following equation: 

𝑆(𝑏) = 𝑀1 e
−𝐷1𝑏 + 𝑀2 e

−𝐷2𝑏 (8) 

to data acquired at different 𝑏-values, where 𝑆(𝑏) is the NMR 
signal as function of the 𝑏-value, 𝐷1 and 𝐷2 are the two 
components of the diffusion coefficient associated with the 

magnetizations 𝑀1 and 𝑀2, respectively. The fit goodness was 

evaluated by the 𝑅2̅̅̅̅  (i.e., the 𝑅2 corrected for the number of the 
regressors). Fits of equation (7) and equation (8) were performed 
by using the OriginPro 8.5 software. 

Plots of the diffusion coefficient 𝐷 vs. the diffusion time ∆ 
were performed with MatlabR2019b. From the 𝐷 vs. ∆ trend, the 
first and last points, corresponding to the free water diffusion 
and the diffusion through semi-permeable membranes (i.e. the 

wood cell walls), were removed and the pores radius 𝐿 was 

Table 1. Acquisition parameters of T2*-weighted images. 

Parameters Modern pine Archaeological pine 

TE/TR (ms) 3/1200 5/1500 

Number of Slice 3 3 

NS 128 128 

STK (µm) 200 300 

FOV (cm2) 0.9 × 0.9 / 1.4 × 1.4 0.9 × 0.9 

MTX (pixels) 512 × 512 512 × 512 

R (µm2) 18 × 18 / 27 × 27 18 × 18 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 4 

estimated by the linear fit of equation (2) [54]. From the pores 

radius 𝐿, the pores diameter 𝑑 was then calculated. 
In addition, the value for ∆ = ∞ of the normalized diffusion 

coefficient 𝐷(∆) 𝐷0⁄ , where 𝐷0 is the free water diffusion 
coefficient equal to 2.3 × 10-9 m2/s, was calculated and used to 
evaluate tortuosity according to equation (3) [54]. Finally, the 
Inverse Laplace Transform [55] in Matlab (Matlab R2019b) was 

used to obtain the low-field 𝑇2 distribution and the pores 
diameter distribution according to equation (6). The surface 

relaxivity 𝜌 was previously estimated for the modern and ancient 
pine from the slope of the line given by 𝑇2 time vs. pore size 𝛼, 
where 𝛼 was calculated from diffusion measurements by the 

relation 𝛼 = 2 √𝐷∆  [46], [47]. 

4. RESULTS 

𝑇2
∗-weighted images of the transversal section of modern and 

archaeological stone pine are displayed in Figure 1a) and Figure 
1b), respectively. Here, all the anatomical elements observable 
with conventional optical microscopy [51], [52] are detectable. 
First, the annual rings limit (white arrows) is well visible in both 
Figure 1a) and Figure 1b) with two separate areas with different 
contrast. In both modern and archaeological pine, the darker area 

corresponds to structures with low 𝑇2
∗ values, while the brighter 

ones to structures with high 𝑇2
∗ values [56]. These structures 

correspond to tracheids, which are considered as the 
predominant constituents of all softwoods [57]. The dark area is 
the latewood (light blue circles) while the bright area is the 
earlywood (green circles). 

Resin canals (red circles), likely with the presence of resin, can 
also be observed. Moreover, rays (pink arrows) can be seen in 
both the samples but in Figure 1b) the archaeological pine shows 
black spots and artefacts (yellow circles) located along the rays 
and on the edge of the sample. These artefacts are typical of MR 

images of waterlogged woods [13] because of the deposition of 
impurities, i.e., bacterial erosion wood products and seabed 
sediments, in the wood microstructure during the burial period. 

These paramagnetic impurities provide a black contrast in 𝑇2
∗-

images [58] revealing the distribution of the degradation zones 
since they are stored in the decayed structures of wood [13].  

Figure 2a) and Figure 2b) display the radial section of modern 
and archaeological pine, respectively. In both the images, the 
anatomical element observable is the annual rings limit (white 
arrows) and in Figure 2b) the above-mentioned distribution of 
paramagnetic inclusions (yellow circles). 

The tangential section of the modern and archaeological 
samples is shown in Figure 3a) and Figure 3b), respectively. The 
red circles highlight the tangential resin canals and the pink 
arrows the medullary rays. Again, in Figure 3b) the archaeological 
pine shows decayed zones (yellow circles) corresponding to black 
spots and artefacts produced by the presence of paramagnetic 
agents. 

In Table 2 the relaxation time 𝑇1 measured at high magnetic 
field and obtained by equation (7) is displayed. Both modern and 

archaeological pine show a 𝑇1 around 500 ms. This limited the 
observation time ∆ of the diffusion measurements, whose 
maximum value was set to 400 ms. In Figure 4a) and Figure 4b) 

the first (𝐷𝑥1) and second (𝐷𝑥2) diffusion component as function 
of the diffusion time ∆ obtained by equation (8) are displayed.  

In Table 3 the pores diameter and the tortuosity calculated 
from the high-field measurements by equation (2) and equation 

(3), respectively, are reported. In Figure 5a) the 𝑇2 time 
distribution obtained by portable NMR is showed both for the 
modern (dashed line) and the ancient (solid line) pine. The pores 
size distribution calculated by equation (6) from the above 

mentioned low-field 𝑇2-distribution is presented in Figure 5b) 
for the modern pine (dashed line) and the archaeological pine 
(solid line). 

 

Figure 1. T2*-weighted MR images of the transversal section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 T). In both 
the samples there are resin canals (red circles), earlywood (green circles), latewood (blue circles), annual rings limit (white arrows) and rays (pink arrows). 
The ancient pine shows artefacts induced by paramagnetic ions in decayed areas (yellow circles). 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 5 

5. DISCUSSIONS 

Compared to the conventional methods used to observe 
wood anatomy, i.e., optical microscopy or scanning electron 
microscopy, our MR images allowed to recognize some 
important diagnostic characters of wood especially in the 
transversal section (Figure 1).  

Conversely, fewer characters were observed in the tangential 
(Figure 3) and radial (Figure 2) sections due to the current 
limitation on the resolution of MRI, whose maximum value is 
around 10 µm. Indeed, when the species of a softwood has to be 
recognized, the radial section is of fundamental importance 
because of its diagnostic features, such as pits of cross-fields and 
rays, which allow to discriminate among quite similar softwood 
structures (e.g., pine and spruce). However, the resolution of 

 

Figure 2. T2*-weighted MR images of the radial section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 T). In both the 
samples the annual rings limit (white arrows) is observable. The ancient pine shows artefacts induced by paramagnetic ions in decayed areas (yellow circles). 

 

Figure 3. T2*-weighted MR images of the tangential section of modern pine (a) and archaeological pine (b) obtained at high magnetic field (9.4 T). In both 
tangential resin canals (red circles) and rays (pink arrows) are observable. The ancient pine shows artefacts induced by paramagnetic ions in decayed areas 
(yellow circles). 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 6 

MRI is not a physical limit but it depends on the characteristics 
of the instrumentation used.  

In this work, MR images showed in Figure 1, Figure 2 and 
Figure 3 aim at investigating the decay effect and their specific 
contrast can be used to reconstruct the decay distribution and to 
detect the presence of paramagnetic impurities. All the sample 
volume can be imaged with MRI, whereas optical and scanning 
electron microscopy only provide images of a small portion of 
the wood sample. Moreover, the mechanical preparation of the 
sample in optical and scanning electron microscopy leads to its 
destruction if compared with the virtual sectioning operated by 

MRI. The longitudinal relaxation time 𝑇1 could provide 
information about the water molecules surrounding 
environment, the sample structure and composition [26]. 

However, in a porous medium 𝑇1 can be influenced by 

paramagnetic ions [59], [60] that usually cause its reduction, as 
found in Stagno et al. [23]. In our ancient wood many 
paramagnetic inclusions were detected as artifacts in the MR 

images [13] and 𝑇1 values, displayed in Table 2, seem to be 
exchange averaged [59], therefore they do not include such 
detailed information about different structural compartments as 

𝑇2 value. For the aforementioned reason, the longitudinal 
relaxation time of our modern and ancient pine cannot be used 
to describe structural changes among them. Nevertheless, the 

measurement of 𝑇1 is useful for setting the diffusion observation 
time. From the comparison of the transversal, radial and 
tangential section of the modern (Figure 1a, Figure 2a and Figure 
3a) and the ancient (Figure 1b, Figure 2b and Figure 3b) pine, it 
is possible to observe morphological differences. First of all, the 
black spots (see section 4), can be associated with a degradation 
process operated by microorganisms with the inclusion of 
paramagnetic impurities (i.e., bacterial erosion wood products 
and burying sediments) into the wood structure. These black 
zones are not present in the modern wood and they are mostly 
located along the rays and on the edge of the sample (Figure 1b, 
Figure 2b and Figure 3b yellow circles). This indicates a strong 
degradation in these zones. The image contrast allows to observe 
that both the woods show well delimited growth rings. While the 
modern wood does not show particular changes in the annual 
ring thickness, in Figure 1b) the ancient pine has some thinner 
annual rings with no latewood. We can hypothesize that this is 
due to a climatic change during the tree growth (before the V 
century AD). In fact, thinner rings are usually attributed to not 
rainy periods [4], [11]. However, the ring thickness can be also 
influenced by other factors, for example the tree age. Therefore, 
more than one sample should be analysed to confirm our 
hypothesis. 

The plots of the diffusion coefficient vs. the diffusion time in 
Figure 4a) and 4b) show that both wood samples have two main 
diffusion compartments but the diffusion in the modern pine is 
slower than in the ancient pine. The difference is around one 

order of magnitude for both the compartments 𝐷𝑥1 and 𝐷𝑥2. 
This situation can be explained as the consequence of the 
degradation process that occurred in the ancient pine. In fact, the 
decay of the cellular structure and wood polymers may have 
produced the cell walls thinning and the lumens enlargement in 
the archaeological pine. From the point of view of a porous 
system, ancient pine is characterized by larger pores compared to 
modern pine. The existence of two different diffusion 
compartments means that both the woods have at least two main 

pores sizes as shown in Table 3. The two sizes 𝑑1 and 𝑑2 
calculated from the high-field NMR diffusion measurements can 
be identified as the earlywood and latewood tracheids diameter 
[60], considering that tracheids are the main constituents of 
softwood. By comparing the calculated diameters of the modern 
and the ancient pine, despite in the earlywood the diameter seems 
to be similar between the two samples, for the latewood there is 
an increment of the tracheids size in the archaeological pine. A 
possible explanation is that the decay is higher in areas with high 
concentration of wood polymers, such as the latewood cell walls. 
These polymers are predominantly cellulose and hemicellulose 
that, as pointed out by high-resolution NMR spectra of both 
modern and ancient wood [19], are more degraded in respect to 
the usually well-preserved lignin. This result means that the 
greater the thickness of the cell wall, the stronger its 
deterioration. This is in agreement with the distribution of 
paramagnetic impurities revealed by the MR images, which 
follows the distribution of decay that is mainly located in rays 

Table 2. Longitudinal relaxation time T1. 

Sample T1 ± SE (ms) 

Modern pine 541 ± 13 

Archaeological pine 511 ± 19 

 
Figure 4. Plots (a) and (b) show the Dx1 and Dx2 decay as function of Δ for 
modern pine (circle markers) and archaeological pine (triangle markers). 
Dashed lines are for illustration purpose only. 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 7 

having thick cell walls. Further information can be deduced from 

the tortuosity 𝜏 (Table 3). The modern pine shows a higher 
tortuosity (7.0 ± 1.1) compared to the archaeological pine 
(3.5 ± 0.6). However, for the ancient pine the normalized D data 
did not reach a limit value (see Figure 4). Therefore, we likely 
underestimated the value of tortuosity. Nevertheless, the 
tortuosity value obtained for modern pine seems to be in 
agreement with the literature value around 10 calculated for 
thermally modified Pinus sylvestris , also considering the effect of 
thermal modification and that we used a different species (Pinus 
pinea) [61]. Moreover, tortuosity is also in good agreement with 
the diffusion coefficient and diameters results. In fact, the higher 
the tortuosity, the more complicated the water routes. This 
means that the modern wood has a complex structure within 
which water cannot move easily. Conversely, the ancient pine has 
lost this complexity because of the structure’s degradation, which 
has produced new voids and widened the existing pores lumens 
making the water motion easier. Specifically, the two different 
tortuosity values corroborate the pore size results obtained by 
NMR diffusion. Indeed, the low tortuosity measured for the 
ancient pine is compatible with its larger and more 
interconnected pores.  

Differences in the pore size of the modern and the ancient 
pine were also found by using the low-field portable NMR, in 
good agreement with the high-field results. Specifically, in Figure 

5a) the 𝑇2 time distribution obtained by using the portable NMR 
is displayed and indicates that the archaeological pine (solid line) 

has longer 𝑇2 than the modern pine (dashed line). The only 
exception is the shortest component around 1 ms that is longer 
for the modern wood than for the archaeological one. Since in 
previous works [62], [63] a component around 1 ms was 
attributed to bound water in the cell walls, we can suggest that 
bound water of the archaeological pine is highly influenced by 
paramagnetic impurities, whose accumulation is greater in more 

degraded areas, i.e. in the cell walls. This is confirmed by other 

studies [23], [64], where the 𝑇2 component around 1 ms 
ascribable to bound water in the cell walls was not detected due 

to their strong degradation state. Since the 𝑇2 is proportional to 
the degree of decay, we can suggest that the increment of 𝑇2 for 
the ancient pine is a consequence of decay. This also means that 
water in the archaeological pine is located in larger structures and 
the water content has raised as shown by the probability 

associated with 𝑇2 (Figure 5a). The pores diameter calculated 
from high-field diffusion-NMR (Table 3) can be compared with 

the pores diameter obtained by low-field 𝑇2 distribution (Figure 
5b). However, compared to high-field diffusion, the low-field 
relaxation measurements allowed to provide a global pore 
distribution of the sample while the intrinsic resolution of the 

diffusion-NMR is limited by the 𝑇1. In our case, the maximum 
distance (𝓁D) accessible is 43 µm [65] at ∆ = 400 ms and 𝐷 =
 2.3 × 10-9 m2/s (the free water diffusion coefficient). 
Nevertheless, we think that in case of the archaeological pine the 

𝐷 vs ∆−1 behaviour is still affected by the presence of free-like 
water due to the existence of very large pores, not detectable with 
diffusion NMR techniques [54]. This explains why the 
archaeological pine shows pore distribution around 70 µm 
(Figure 5b) that was not detected by NMR diffusion. The pore 
size distribution displayed in Figure 5b) confirms the 
enlargement of pores in the archaeological pine as predicted by 

diffusion and 𝑇2 measurements. However, the peak around 
70 µm is quite broad indicating a continuous distribution among 
peak with diameter from about 55 µm to 90 µm. The most 
intense peak for the ancient pine is around 17 µm. It should be 
noticed that both the diameters obtained from diffusion (Table 
3) and associated with earlywood and latewood are included in 
the peak around 17 µm. This indicates a continuous distribution 
among earlywood and latewood tracheids, likely with water 
exchange. For modern wood, the two predominant peaks as well 
as their probability in Figure 5b) are in good agreement with the 
diameters calculated from diffusion analyses, with the MR images 
and with the literature values [61], [66] in which the mean lumen 
diameter was found around 15 µm – 20 µm. The existence of 
two separate peaks shows distinct spins populations for 
earlywood and latewood. 

Table 3. Pores diameter, associated magnetizations and tortuosity obtained 
by NMR diffusion. 

Sample d1 (µm) d2 (µm) τ 

Modern pine 16.0 ± 1.0 5.4 ± 0.6 7.0 ± 1.1 

Archaeological pine 18.4 ± 3.5 13.6 ± 1.2 3.5 ± 0.6 

 

Figure 5. T2 relaxation time distribution (a) and pore diameter distribution (b) for the modern pine (dashed line) and the ancient pine (solid line) obtained by 
using portable low-field NMR. 



 

ACTA IMEKO | www.imeko.org March 2022 | Volume 11 | Number 1 | 8 

Smaller pore sizes can be associated to parenchyma cells and 
voids of the cell walls. Also in this case, the values obtained by 
equation (6) and displayed in Figure 5b) confirm that the ancient 
pine has larger pores (from 0.1 µm to 3 µm) than the modern 
pine (from 0.1 to 1 µm). 

Particularly, the increment of size of the cell walls voids 
reveals the degradation of polymers constituting the cell wall 
itself. 

6. CONCLUSION 

This work suggests that by using high-field micro-MRI and 
high-field diffusion-NMR it is possible to obtain information 
about the archaeological wood decay. Information related to 
changes in the mean pore size, caused by wood decay, can be also 
obtained by low-field NMR relaxometry. The use of different 
NMR techniques and instrumentations provided the same result 
about the increment of the mean pore size for the archaeological 
pine wood. By comparing the modern pine sample and the 
ancient pine sample, the effect of the degradation process on the 
wood microstructure can be observed through the contrast of 
the MR images and quantified by the diffusion coefficient of 
water, the tortuosity and the pore size. The high-field NMR 
showed that the decay in pine wood mostly occurred in areas 
with high concentration of polymers, such as rays and latewood 
cell walls, with the enlargement of the pores lumen and the loss 
of wood structural complexity. MRI can also reveal 
morphological aspects of wood that are not observable with the 
naked eye, such as the annual rings, which can inform about past 
climate changes. In the pore size determination obtained by using 

portable low-field NMR, the method based on the 𝑇2-
distribution seems to be superior to the high-field diffusion 

method, whose resolution is limited by 𝑇1. In conclusion, high-
field techniques require a sampling; however, compared to other 
conventional analyses, they are non-destructive towards the 
sample that can be relocated in its original position in the 
artwork. The low-field portable NMR, instead, is mobile and 
suitable for in-situ analysis on samples of any size thus being 
non-invasive and non-destructive. Compared to high-field NMR 
it is also low cost, with shorter acquisition and processing time. 
We suggest that single-sided portable NMR is a powerful 
technique for revealing porosity changes of the entire wood 
structure produced by decay.  

ACKNOWLEDGEMENT 

The authors would like to thank the “Istituto Centrale per il 
Restauro” (ICR) of Rome (Italy) for providing the archaeological 
wood sample.  

We acknowledge funding of Regione Lazio under the 
ADAMO project No. B86C18001220002 of the Centre of 
Excellence at the Technological District for Cultural Heritage of 
Lazio (DTC). 

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