# Inside the kink-bands of archaeological flax artefacts via sub-micrometer resolution micro-CT: A comprehensive microstructural analysis to better understand degradation mechanisms of fibres

Loren Morgillo, Alessia Melelli, Mario Scheel, Raymond Wightman, Timm Weitkamp, Camille Goudenhooft, Anita Quiles, Darshil U Shah, Marwa Abida, Johnny Beaugrand, et al.

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## ► To cite this version:

Loren Morgillo, Alessia Melelli, Mario Scheel, Raymond Wightman, Timm Weitkamp, et al. Inside the kink-bands of archaeological flax artefacts via sub-micrometer resolution micro-CT: A comprehensive microstructural analysis to better understand degradation mechanisms of fibres. Composites Part B: Engineering, 2025, 298, pp.112347. 10.1016/j.compositesb.2025.112347. hal-05049276

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# Inside the kink-bands of archaeological flax artefacts via sub-micrometer resolution micro-CT: A comprehensive microstructural analysis to better understand degradation mechanisms of fibres

Loren Morgillo<sup>a</sup>, Alessia Melelli<sup>b, c</sup>, Mario Scheel<sup>b, d</sup>, Raymond Wightman<sup>c</sup>, Timm Weitkamp<sup>b</sup>, Camille Goudenhooft<sup>a, d</sup>, Anita Quiles<sup>d, e</sup>, Darshil U. Shah<sup>e</sup>, Marwa Abida<sup>f</sup>, Johnny Beaugrand<sup>g</sup>, Alain Bourmaud<sup>a, *</sup>

<figure><img src="image_4.png" /></figure>

## ARTICLE INFO

### Keywords:

Flax fibres

Structural defects

Archaeology

Egyptian textiles

Synchrotron radiation

## ABSTRACT

This work explores how the morphology of kink-band zones in flax fibres impacts the mechanical properties of the elementary fibres. Kink-bands are structural defects and are particularly sensitive to physical and biological stresses, on isolated fibres or in bio-based composite materials. To this end, a panel of archaeological samples from different time periods and preserved under different environmental conditions were selected and studied using synchrotron micro-tomography. It is demonstrated that although kink-bands are generally more numerous in ancient fibres, their degree of severity is sometimes less. This underlines the importance of fibre extraction methods, which are principally responsible for kink-band formation. The results also show that kink-band are weaknesses points, allowing rapid development of internal porosity (up to 25 %) when the fibres are used or stored in extreme environments, and that this porosity can also extend to healthy areas of the fibres. However, in some cases, even after millennia of conservation, it appears that the fibres can present morphologies comparable to modern samples, probably due to their good initial quality. Thanks to the findings of the present work, simplified schemes of degradation in kink-band zones, useable on single fibres but also in composite materials, are proposed. These results confirm the importance of fibre extraction processes on fibre quality and durability, and subsequent use for sustainable and high-performance composite materials and textiles.

## 1. Introduction

Flax fibres have structural defects, known as kink-bands, which represent highly sensitive areas [1,2]. These defects have a major impact on the behaviour and mechanical properties of both fibres and their composites [3–5]. When mechanically loaded, kink-bands are potential fracture zones; they cause fibres to break in the polymer flow during extrusion or injection processing [4], as well as fibres in a composite in the solid state. They are also the preferred sites for hydrolytic enzyme

development in fibre [6]. At the composite scale, it was shown that kink-bands may have a greater impact in initiating and propagating damage than porosities [5]. The number and intensity of these fibre defects are therefore key factors in the industrial development of bio-based composites, and limiting their presence is a major challenge.

The contribution of these defects to fibre damage is mainly explained by the very specific microstructure of the zones concerned. Flax fibres consist mainly of crystalline cellulose, oriented almost linearly with the fibre axis within their majority S2-G layer [7]. In the kink-band zones, a

This article is part of a special issue entitled: ECCM21 published in Composites Part B.

* Corresponding author.

E-mail address: alain.bourmaud@univ-ubs.fr (A. Bourmaud).

<footnote><sup>a</sup> Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, France</footnote>

<footnote><sup>b</sup> Synchrotron SOLEIL, F-91190 Saint-Aubin, France</footnote>

<footnote><sup>c</sup> Microscopy Core Facility, Sainsbury Laboratory, University of Cambridge, Bateman Street, Cambridge CB2 1LR, UK</footnote>

<footnote><sup>d</sup> Institut Français D'Archeologie Orientale Du Caire, Le Caire, Egypt</footnote>

<footnote><sup>e</sup> Centre for Natural Material Innovation, Department of Architecture, University of Cambridge, Cambridge, UK</footnote>

<footnote><sup>f</sup> Univ. de Lorraine, UMR-CNRS 7239, LEM3, F-57078 Metz, France</footnote>

<footnote><sup>g</sup> UR1268 Biopolymères Interactions Assemblages, INRAE, F-44316 Nantes, France</footnote>
range of ultrastructural studies using Raman spectroscopy [8], electron microscopy (SEM), atomic force microscopy (AFM) or multi-photon spectroscopy (SHG) have revealed marked disorientations of the cellulose, with angles of up to 30–45°, instead of around 5° in the healthy zones [1]. This has a direct influence on the mechanical behaviour of the fibre, increasing stiffness at the start of the curve but generally reducing elongation at break, which is already low [9]. In addition, the presence of numerous micro-scale porosities in these kink-band zones has been visualized, with a concentric distribution around the fibre lumen, most probably linked to the decohesion of the cellulose parietal layers during the bending, compression or buckling mechanisms at the origin of kink-band formation [10,11]. Porosities also have a major impact on the mechanical performance of fibres, by reducing the fibre cross-sectional area (CSA) and by inducing high stress concentrations. Indeed, a ratio of 7.5 in terms of local stresses was highlighted by Quereilhac et al. between healthy (king-band free) and damaged (king-band) zones [12]. Subject to a history of controversy in literature, it is now established that the origin of these kink-band defects is principally attributed to the processes related to the mechanical extraction of fibres and rather than due to stresses during plant growth. SEM and micro-CT tomography work helped to clarify the origin of the development of kink-bands in extracted flax fibres and stems [10,13].

There is renewed interest in the study of these defects due to the increase in industrial applications of plant fibres, particularly for semi-structural biocomposite applications [14]. The question of how these defects change over extended periods of time (physically, biochemically and so on), and also change with environmental conditions under which the fibres are aged or preserved, is a major topic that was specifically addressed by few, if any studies to date. In this sense, archaeological textile samples represent an extraordinary material source and database for studying the effects of time and conservation methods on the ultrastructural properties of plant fibres.

Flax, in particular, had enormous importance and wide uses in Ancient Egypt [15]. The dry climate of the geographic region and the favourable conditions of preservation (e.g. buried in sand or within tombs, away from direct sunlight) mean that many of these archaeological textile samples are well-preserved [16]. A great deal of scientific work focused on the biochemical [17] and sometimes mechanical characteristics of these materials [15,18,19]. In contrast, tomography was seldom used at the fibre scale [20,21]; its application was mainly reserved for assessing the condition of works of art or investigating the hidden enclosed interiors of objects or sarcophagi [22,23].

The main purpose of this work is to gain a better understanding of the impact of time and environmental conditions over periods of long-term conservation on the microstructural evolution of flax fibres, and in particular kink-band areas. To this end, 4 archaeological samples of ancient flax, dating from 3000 BCE to 800 C E and having various uses and ageing modes (i.e. funerary linen wrapping mummies, fishing nets, or linen found buried in the ground) were selected. Their morphology was studied using SEM and micro-tomography. The results suggest defect morphologies specific to long-term ageing conditions. The porosities associated with the lumen were also quantified, both in the healthy areas of the fibres and by focusing on the kink-band regions. All these results were discussed in light of contemporary fibres.

## 2. Materials and methods

### 2.1. Flax samples

Contemporary flax of Eden variety, cultivated and mechanically scutched in 2022 in a family farm (Zefta, Egypt), was used as reference. In addition, four small pieces of archaeological flax textile artefacts were collected at Le Louvre Museum (Paris, France). A brief contextual description of the samples and associated images is provided in Table 1.

<caption>**Table 1**</caption>

Description of the five flax samples (CE: common era; BCE: before common era). All samples are stored at Le Louvre Museum (Paris, France).

<table><thead><tr><th>Sample and abbreviation</th><th>Image</th><th>Origin</th><th>Dating</th><th>Brief description</th></tr></thead><tbody><tr><td>Modern flax MOD</td><td></td><td>Zefta, Egypt</td><td>2022</td><td>Eden variety. Cultivated, water retted and mechanically scutched.</td></tr><tr><td>AF 11260 TOMB</td><td></td><td>Deir-El-Medina (Tomb 1407)</td><td>200–300 C E</td><td>Funerary fabric from the Pebos mummy, in contact with embalmed body.</td></tr><tr><td>E 27376 SOIL 1</td><td></td><td>Gebel El-Zeit galena mining site (site 1)</td><td>1533 - 1432 BCE</td><td>Linen clothes from terracotta figurines. The two fabrics were found in soil, in the same archaeological dig.</td></tr><tr><td>E 27370 SOIL 2</td><td></td><td>Gebel El-Zeit galena mining site (site 1)</td><td>1617 - 1451 BCE</td><td></td></tr><tr><td>E 286 NET</td><td></td><td>Unknown (Egypt)</td><td>4324 - 4053 BCE</td><td>The exact origin of this fishing net is unknown.</td></tr></tbody></table>
All the ancient samples were radiocarbon dated [24]. Complementary information can be found in [25].

180 s.

## 2.2. SEM analysis

Some small fragments of fibres were manually extracted from ancient textile artefacts and contemporary scutched fibres. Samples were glued on conductive plots and observed using a Jeol JSM 6460 L V SEM (Akishima, Tokyo, Japan) to observe surface and characteristic zones of kink-bands. Before observations, samples were sputter coated with gold using an Edwards Scancoat Six device (Burgess Hill, UK) for

## 2.3. Confocal laser-scanning microscopy (CLSM)

For confocal microscopy, fibres were observed using a Leica SP8 confocal laser scanning microscope using 405 nm excitation and a HyD spectral detector set to 490-525 nm emission width with a scan speed of 400 Hz and a scan format of 512 × 512 pixels and line average of 4. In addition, observations were made with the Leica HC PL APO CS2 20×0.75NA objective on flax bundles embedded in agar resin (epoxy resin low viscosity (LV) resin agar, Scientific agar UK). The resin block

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Morphological analysis of the four archaeological samples and modern reference using micro-CT (left, centre) and SEM (right) observations.</figcaption></figure>
containing the bundles was transversely or longitudinally pre-cut,
depending on the direction of the fibres. Finally, the surface was pre-
pared using ultramicrotomy (Ultramicrotome Leica EM UC7, Leica
Microsystems, Nanterre, France).

## 2.4. X-ray microtomography

The tomographic investigations were made at the ANATOMIX
beamline of the Synchrotron SOLEIL facility. Details of the beamline and
available setups are described in Ref. [26]. Microtomography scans were
obtained in a parallel-beam setup. Before data acquisition, small bundles
of fibres were extracted from the scutched fibres and ancient artefacts
and then glued at the tip of medical needles. They were then glued on
the sample holder. A monochromatic X-ray beam with a photon energy
of 10 keV was selected and the sample was mounted at a distance of 5
mm from the detection plane. The detector consisted of a single-crystal
lutetium aluminum garnet (LuAG:Ce) scintillator, microscope optics
with 20 × magnification (Mitutoyo, Japan) and a scientific-grade CMOS
camera with 2048 × 2048 pixels (Hamamatsu Orca Flash 4.0 V2),
resulting in an effective pixel size of 0.325 µm. For each tomography
scan, 2000 projections were acquired over an angular range of 180° with
an exposure time of 250 ms per projection (about 9 min for a full scan),
the diameter and height of the tomographic volume was 0.66 mm.
Volumes were reconstructed using the standard data processing pipeline
at the beamline, with the PyHST2 program [27] developed at ESRF
synchrotron (Grenoble, France) as the backend. A Paganin filter [28]
was applied to correct ring artefacts. After experimental acquisition,
volumes were analysed using FIJI (ImageJ-win 64, National Institutes of

Health, USA) [29] and Avizo version 2021.1 software (Thermo Fisher
Scientific), with a voxel size of 0.325 µm using the PyHST2 program
developed by ANATOMIX beamline. After cropping and conversion to
8-bit integer format with FIJI, the 3D renderings were segmented using
Avizo to separate the fibres in three parts: fibres, lumen and pores ac-
cording to the method described in Ref. [3]; each fibre was analysed on a
length of 660 µm.

# 3. Results and discussion

## 3.1. Overall morphology of the selected samples

SEM and X-ray microtomography are wonderful tools for identifying
the unique features of the five samples, and the remarkable preservation
of the yarns enabled high-quality images to be captured. Fig. 1 presents
one longitudinal micro-CT slice, 3D renderings and detailed SEM views
of the five samples. In addition, Fig. 2 shows confocal observations on
the fibre's cross-sections. This panel of visualisations reveal important
differences in structure, size and preservation between the samples.

Starting with the TOMB sample (Fig. 1B), the fibres remained ori-
ented in the monofilament yarn and are extremely well individualized
without any middle lamellae (see intact one in Ref. [30]) or cortical
residue. Observation of the cross-section demonstrates that the fibres are
only slightly degraded with small lumen; kink-bands are also clearly
detectable on the surface of the fibres. These observations were
confirmed using confocal analysis, revealing pores attributed to
kink-band zones (Fig. 2B) despite a relatively good preservation of cell
walls.

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Confocal micrographs on fibres cross sections for MOD (A), TOMB (B), SOIL 1 (C), SOIL 2 (D) and NET (E) samples.</figcaption></figure>
Sample SOIL 1 is a double-twisted yarn with apparent cohesion, with the fibre bundles are poorly individualized and show some cortical residue. It is important to remember here that twisted yarns are reputed to be of better quality, stronger and more consistent. In particular, double-twisted yarns produce a softer fabric with greater durability, dimensional stability and abrasion resistance [31]. The 2D cross-section of the yarn reveals (Fig. 1B and Fig. 2C) very dense fibres with very little porosity, even the lumens are difficult to see.

Although it was found in the same archaeological dig and exposed to similar environmental conditions, the SOIL 2 sample appears to be much more degraded than SOIL 1. It is a monofilament that has lost its cohesion with a less pronounced twist angle than the two previous samples (TOMB and SOIL 1). In addition, the fibres in SOIL 2 are extremely degraded; cross-section analysis evidence a pronounced alteration of the cell walls, mainly internally from the lumen, similar to observations on composted flax fibres [32], but also from the outside of the cell wall as clearly evidenced in Fig. 2D through confocal analysis, the fibres edges appear to be highly degraded with a lower fluorescence intensity, suggesting a possible mineralization of flax fibres after long term ageing in mine soil [25].

The NET sample has a double-twist structure with highly cohesive fibres. A coating was probably applied to the net to improve its durability during its initial use in water [25]. There is also a great disparity between the fibres, with some appearing cellulose rich while others are hollow with degradation from the lumen, in the same way as SOIL 2 sample.

Finally, compared to archaeological artefacts, the modern sample (MOD) exhibits very homogenous fibres, with low lumen size but also kink-bands, presence of residues such as cortical parenchyma. The observed fibre bundle appears to be very cohesive. Interestingly, the confocal image clearly highlights the middle lamellae (Fig. 2A), with high fluorescence attributed to lignin. After the scutching stage, middle lamellae are rarely fully degraded [3]. It is not the case for the ancient samples, mainly due to water retting commonly used in ancient Egypt [33] but also due to effect of time, the pectic rich middle lamellae [34] being highly influenced by environmental conditions, especially humidity and temperature [35].

## 3.2. Investigation of single fibres morphology using X-ray phase contrast micro-tomography

Image analysis on X-ray micro-tomography data is used to segment the volume of the fibre, i.e. the raw material, from the pores and the lumen. Two fibres from each yarn were analysed. Fig. 3 provides quantified data on the total volume of fibrous material, porosity, and lumen of each studied fibre. At first, three fibres appear more degraded than the others: one from the NET sample and two from the SOIL 2 sample. The other fibres have similar specificities, complicating direct comparison.

These first analyses are consistent with the first observations (Fig. 1), demonstrating a huge degradation of SOIL 2 sample and a heterogenous NET sample, with one fibre having a porosity content close to 20 % and another one poorly affected by time. The other fibres exhibit relatively low porosity content, similar, and even better than modern MOD fibres, especially when the lumen size is considered. Therefore, to have a better view and precise distribution of defects, it is essential to extend the analysis to the distribution of porosity levels (including lumen) along the fibre length. A specific focus is proposed on kink-band regions, easy to identify on 3D rendering pictures but also to shear bands and 45° oriented pores with the longitudinal axis of the fibre [11].

First, Fig. 4 shows a comparison of pore distribution between one modern (MOD) and one tomb (TOMB) fibre.

Both samples exhibit a good state of preservation, porosity content in kink-band regions is higher than defect-free ones. Additionally, a comparison of the graphs reveals a peculiarity: the MOD sample has fewer kink-bands, with three zones with 10–15 % of porosity. In contrast, considering exactly the same length, the TOMB sample has more kink-bands, with nine zones exceeding 7.5 % porosity including three zones exceeding 12.5 % and sometimes reaching 21 %. So, there are more kink-bands, and they generally exhibit more important porosity content. This is clearly observable on kink-band cross-sections, especially at the extreme right of the TOMB sample.

Then, the two soil samples are compared with the same method in Fig. 5. The SOIL 1 fibres (Fig. 5A) exhibit numerous small kink-bands that are less porous, with no zones exceeding 7.5 % porosity, with a multitude of peaks around 5 %. From our perspective, it seems unlikely that ageing alone could account for the differences in the quantity of kink-bands, as these are primarily created by human mechanical action. One hypothesis we can propose is that these differences originate from the initial extraction of the fibres during the scutching process. This mechanical treatment likely played a significant role in forming the kink-bands observed in the various samples. This is particularly interesting, as a recent study on the impact of combing demonstrated that intensive combed flax fibres exhibited more pronounced kink-bands, with greater porosity in these specific areas. Conversely, fibres with minimal mechanical treatment showed fewer kink-bands but numerous small porosities along the fibre length [3]. It is plausible that ancient hand-worked fibres [36] experienced different level of mechanical stress compared to contemporary mechanically processed fibres, which might partly explain why contemporary fibres (MOD) have severe kink-bands compared to some older fibres (SOIL 1). SOIL 1 sample exhibits kink-bands with low porosity but also significant fraction of cortical residues (Fig. 1C), this could suggest that Egyptians extracted these fibres by hand and likely with no water-retting step. Also, this peculiar sample which has two yarns turned in z direction and then twisted together in S (S2z) suggests an ancient method. Furthermore, the 1700 years separating the SOIL 1 sample from the TOMB sample could also have an influence on the fibre. Over these centuries, scutching

<figure><img src="image_7.png" /><figcaption>Fig. 3. Quantification of cell wall fibre, lumen and porosity in archaeological artefacts and modern samples. For each batch, 2 fibres were analysed using micro-CT volumes.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Comparison of the pore distribution (lumen + pores) in modern (A) and tomb (B) samples. The distribution curve is supplemented by local cross sections from kink-band regions.</figcaption></figure>

techniques likely evolved with more advanced mechanical tools, applying more stress on the fibres. This evolution in methods may have contributed to the differences observed in the samples which can highly vary according to the tools or extraction methods used [13].

These analyses highlight the challenges in comparing samples, as a fair comparison requires considering numerous factors such as dating, conservation environment, initial use, and yarn structure, some of which such as exact use or conservation remain unknown. Limited number of samples examined through micro-CT scan is also a strong limit. Nevertheless, we can gain insights into why SOIL 2 is one of the most degraded of our samples (Fig. 5B), with some NET fibres. The yarn structure is particularly interesting for this sample. It is a relatively fine mono-filament, with fibres that are much less cohesive and less twisted compared to the other samples. This lack of cohesion and twist probably contributes to the more pronounced biodegradation observed in the fibres, as the slightest impurity, bacteria or even fungi can enter easily between or inside the fibres [19,25,37]. The SOIL 1 sample, made of two twisted yarns is much more cohesive and compact and exhibits better conservation. Regarding the dating range, the two yarns were found in the same region, had similar uses and are from the same period; the slightest difference in age cannot explain the structural variations found.

Finally, considering the storage environment, we cannot conclude that the dry desert conditions accelerated the degradation of the sample compared to the other samples. SOIL 1 and SOIL 2 were found at Gebel El-Zeit, where the climate is extremely arid, especially from approximately 500 C E with a stabilization of rain at a very low level; before 500

CE, peaks of aridity alternated with periods of wetter conditions [38].

For instance, in 1980, the average temperature in Luxor was 24.9 °C ± 8.7 °C throughout the year, with only 28 mm of rainfall. This is in stark contrast to a European city like Paris, where the average temperature in the same year was 11.2 °C ± 3.1 °C, and annual rainfall was 690 mm (https://www.infoclimat.fr/). These low-humidity conditions contributed to the exceptional preservation of the samples over thousands of years, in the same way as the TOMB sample, which was extremely well-preserved in stable conditions inside a tomb. The older NET sample (4324-4053 BCE) has probably been impacted by meteorological cycles with succession of arid and wetter periods.

It is also interesting to note that within the same yarn, some fibres may be considerably more degraded than others. The NET sample is a good example. Fig. 6 shows a comparison between the two fibres extracted from the NET sample. NET 1 fibre has little porosity and no kink-bands on the surface, while the NET 2 fibre is completely open along the entire length of the fibre and the lumen is no longer distinguishable. This information gives rise to numerous hypotheses and questions regarding the initial areas of fragility. A fibre without significant defects can endure over time in a dry, stable environment, even with a potential use in water for fishing activity. Conversely, a degraded fibre is more vulnerable to external aggressions and will deteriorate more rapidly. The most degraded NET fibres exhibit similarities with SOIL 2 sample; if the uses and conservation conditions are considered, degradation phenomena are not necessary the same with a biodegradation for the fishing net sample whereas the SOIL 2 sample was
<figure><img src="image_9.png" /><figcaption>Fig. 5. Comparison of the pore distribution (lumen + pores) in SOIL 1 (A) and SOIL 2 (B) samples. The distribution curve is supplemented by local cross sections from kink-band regions.</figcaption></figure>

probably initially biodegraded and the organic material partially replaced by ions from the soil that formed a pseudomorph [20,25].

To go further, the following section will focus on these porous areas and regions of significant degradation to better understand their impact, weakness and local specificities.

### 3.3. Microstructural analysis of kink-band regions

The previous observations led us to examine kink-bands zones. One fibre from each sample was chosen and examined in three regions exhibiting significant degradation; the least degraded fibre was chosen for the NET sample to be able to clearly differentiate the kink-bands from the defect-free zones. The volumes of porosity in kink-band regions were quantified and shown in Fig. 7.

Results confirm what we evidenced in the previous section, the SOIL 2 sample displays the most pronounced degradation. The kink-bands observed in the MOD contemporary fibres and TOMB fibres exhibit higher porosity compared to those in the SOIL 1 and NET fibres. The difference of a porosity content could be attributed to differences in action tools, modern mechanical scutching lines being possibly much more aggressive, compared to ancient manual tools. Interestingly, the porosity content observed in modern kink-bands is very similar to what is observed in TOMB sample. The preservation of kink-band zones in samples SOIL 1 and NET is also remarkable with porosity content

between 2.1 % and 5.9 % which is significantly lower than is the modern sample.

For most of the samples, the volume of porosity in each kink-band zone is relatively similar, evidencing a homogenous ageing of the samples, even in kink-band regions. It is not the case for the NET sample, and to a lesser extent for the SOIL 2 sample, showing that even within a fibre, this sample exhibits heterogeneity. In addition, the volume of porosity attributed to kink-bands of this latter is in the same range as literature data [10,11], confirming the pertinence of the results.

For all samples, except for the much-degraded SOIL 2, the porosity content in free of defects regions ranges between 1.5 and 2.2 %, which is consistent with values of lumen porosity reported in the literature for flax fibres [39,40]. This also confirms that except in lumen zones, there are not substantial additional pores induced by long term ageing of the flax fibres analysed; it is an important finding which demonstrates that kink-bands regions are the most sensitive and can degrade with time, even in conditions very favourable to conservation, such as in desertic zones. Conversely, in the same conditions, defect-free regions stay intact a long time.

Next, let's delve into the degradation within these regions. Fig. 8 shows the longitudinal and transverse distribution of pores in three kink-band zones for each sample. Despite severely degraded zones, the selected fibres did not fracture, even in instances where porosity exceeded 40 % in the SOIL 2 fibre. Porosity is clearly arranged in a
<figure><img src="image_10.png" /><figcaption>Fig. 6. Comparison of the pore distribution (lumen + pores) in 2 fibres of the NET sample. The distribution curve is supplemented by local cross sections from kink-band regions.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 7. Detailed analysis of the porosity content for each batch. For each sample, one fibre was analysed, and 3 kink-band zones investigated. The first column represents the average porosity in the defect-free regions quantified individually.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Distribution of pores in kink-bands regions for each sample. Three kink-bands are analysed for each fibre. Distribution is shown longitudinally and, in a cross-section (from the middle of each area) as well. Porosity attributed to lumen and cell walls are shown in blue and red, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)</figcaption></figure>

concentric way around the lumen of the fibre, for MOD and SOIL 1 samples [11]. The corresponding hypothesis is a decohesion of cellulose layers [41], deposited during the filling of fibres during the growing stage of the plant. Compared to modern MOD sample, the conservation state of SOIL 1 sample seems remarkable, considering the kink-band morphology but also porosity content. For samples TOMB and NET, porosity degree is similar to MOD sample, discrepancy in the arrangement of pores in kink-band zones are noticed; a circular arrangement could be slightly detected but the main difference is the size of pores, which is more important in TOMB and NET samples compared to MOD and SOIL 1 samples. In severe conditions, long term ageing can probably induce a coalescence of pores.

A fine analysis of the number and size of pores in kink-bands zones is shown in Fig. 9, on the same regions as in Fig. 8. As expected, the very degraded SOIL 2 sample exhibits a huge number of large size pores. If

volume and number of pores are viewed as markers of quality and severity of kink-bands, the best-preserved sample is the fibre extracted from the NET, as already underlined (Figs. 7 and 8). The TOMB sample analysis confirms the visual feeling from Fig. 8, attributed to a possible aggregation or coalescence of the circular pores in kink-bands, with 2 large porosities.

The last observed sample, SOIL 2, is highly degraded, as discussed in previous section. This is strongly confirmed by Fig. 7, demonstrating that kink-band zones are very sensitive parts, with a huge development of pores in case of severe ageing.

### 3.4. Main typologies of defects in kink-band zones over long time

One of the key points to emerge from these analyses is that ageing of flax fibres does not probably lead to the appearance of kink-bands;
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Number and size of pores in kink-band regions for each sample. The analysed areas are exactly those shown in Fig. 8. The circles' area is linearly correlated with the volume of each measured pore.</figcaption></figure>

defects are inherently connected to the way humans use and treat flax. Nevertheless, the kink-bands created when the fibres are extracted will serve as main initial sites for accelerated degradation and this is due to the porosity present in these areas. In kink-band regions, pores arranged in concentric circles around the lumen are described in literature [11]. The contemporary fibres analysed in the present study exhibit also this specific organization (Fig. 8). Along time, the structural observation previously described reveals different degradation mechanisms depending on the location of the porosity initially present in the fibre band of the ageing conditions. Fig. 10 illustrates three common degradation patterns observed in the samples and described in the previous sections:

First, a phenomenon of porosity coalescence is observed, mainly on the TOMB sample. In this case pores always exhibit a concentric organization, but their size is increased, suggesting an aggregation of initial porosity, possibly induced by a continuous decohesion of cellulose layers. This phenomenon could be enhanced by the increase of the non-cellulosic matrix brittleness, induced by changes in biochemical composition after ageing and especially poly-galacturonan and

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Main degradation mechanisms in kink-band zones addressed from the present study: Coalescence of pores (A), degradation from the lumen (B) and from the outside of the fibre (C).</figcaption></figure>

rhamnogalacturonan type-I polymers [35]. Once the non-cellulosic polymer components have been reduced, the cell walls become more fragile and the cellulose is probably more mobile without its embedding matrix; these factors encourage disorganisation, particularly in the kink-bands regions due to the existing concentric pores. These pores can then group together and give rise to the coalescence described above.

A second degradation mechanism involves the opening of the lumen. In certain areas, the lumen cracks and breaks perpendicular to the tangles of cellulose microfibrils. Over time, the secondary cell wall layer is eroded away from this crack and the lumen, unlike the primary cell wall (PCW). The cellulose, main component of the S2-G and S2-Gn layers, degrades faster than the non-cellulosic polysaccharides (NCP) and lignin components of the PCW. This phenomenon was observed in various studies, particularly on wood, where the lumen act as a channel for bacteria and fungi, causing the material to degrade [42]. Ageing studies were also conducted on PLA/flax composites. After 125 days in compost, the single fibres within the composite exhibit internal holes through the lumen similar to our observations [32], suggesting pronounced biodegradation. It's extremely interesting to find the same phenomenon in our samples, proving that this mode of fibre degradation via the lumen is a common characteristic of all flax fibres, regardless of their origin and variety. In our panel of sample, this degradation mode is clearly observable on samples SOIL 2 and NET.

Finally, fibres can be weakened by the outer layer, leading to a high rate of degradation. When a crack breaches the primary cell wall (PCW) and the S1 layer, it propagates significantly, deteriorating all internal layers and resulting in very high porosity levels. The emergence of these cracks can be explained in two ways. Firstly, external mechanical actions, such as various stresses, may cause fibres to crack without completely breaking in the kink-band areas, creating favourable sites for degradation. Secondly, it is possible that fibres have been attacked by a disease. For instance, some studies have investigated the impact of Verticillium wilt caused by the fungus *Verticillium dahliae*, finding localized degradation at the fibre's periphery that extends to the lumen [37]. Therefore, some of the fibres examined in this study may have been contaminated by fungi, for example NET sample and almost SOIL 2 sample, conserved in soil for a long period.

## 4. Conclusion

In the present research, synchrotron micro-tomography investigations with sub-micron resolution were performed on one modern flax fibre sample and four archaeological artefacts, selected according to their period but also their initial use and possible ageing conditions. The distribution of pores was studied and detailed in the whole fibres and with a specific focus on the sensitive kink-band zones. Several key points and conclusions can be argued.

* At the yarn level, it is crucial to study the structure of old samples, focusing on torsion angles and fibre density. Cross-referencing these findings with ageing studies on contemporary yarns will help to understand the impact of yarn structure on the ageing process.

* The present results demonstrate also that sometimes, ancient samples exhibit less pronounced kink-bands compared to modern flax fibres. This interesting result can be explained by the evolution in extraction tools and methods, inducing probably different level of stress and impact on fibre structure. Kink-band development can also be strongly linked to use of textile along time. They could also be markers of ancestral use and extraction mode, proving information and findings on ancient technological practices.

* Nevertheless, pores are generally more numerous in ancient fibres with a possible collapsing in kink-band zones, inducing very high pore volume, up to 25 %. In extreme cases, the pore network propagates to the whole fibre, beyond kink-band regions.

* Finally, comparing defect-free zones and kink-band zones, it appears that if flax fibres are subjected to good conditions of storage or
conservation, defect-free zones remain intact, even the lumen, confirming a potential excellent stability of flax fibre structures over long time periods.

The study confirms the impact of kink-bands on degradation of fibres, the more damaged fibres exhibit very high pore size and density in kink-band regions, responsible for a strong decrease of the fibre properties. In future work, a link will be established with mechanical performances of the fibres and environmental ageing in laboratory will be implemented to better understand the main impacting factors on kink-band severity. Nevertheless, some ancient fibres show only a weakly-degraded microstructure, even after several millennia, probably because they initially have a limited number of low-size kink-bands; this underlines the strong interest in developing softer extraction methods to preserve at best as possible flax fibres, in order to increase their durability and performances along time. The work demonstrates the key role of kink-band in damage evolution; these sensitive regions are also one of the main weaknesses of plant fibre-reinforced composite materials; to produce plant fibre preforms and reinforcements, an accurate selection must be done and the mechanical extraction, in particular, must be optimized to limit as much as possible the kink-bands quantity.

### CRediT authorship contribution statement

**Loren Morgillo:** Writing – original draft, Visualization, Software, Investigation, Data curation, Conceptualization. **Alessia Melelli:** Writing – review & editing, Investigation. **Mario Scheel:** Writing – review & editing, Software, Methodology, Investigation. **Raymond Wightman:** Methodology, Investigation. **Timm Weitkamp:** Writing – review & editing, Methodology. **Camille Goudenhooft:** Writing – review & editing, Methodology, Investigation. **Anita Quiles:** Writing – review & editing. **Darshil U. Shah:** Writing – review & editing. **Marwa Abida:** Writing – review & editing, Supervision. **Johnny Beaugrand:** Writing – review & editing, Supervision, Conceptualization. **Alain Bourmaud:** Writing – review & editing, Supervision, Methodology, Funding acquisition, Conceptualization.

### Declaration of Competing interest

The authors declare that they have no known competing financial interests or personal relationships with other people or organizations that could have appeared to influence the work reported in this paper.

### Acknowledgments

The authors gratefully thank the ANR (Agence Nationale de la Recherche) for funding the FLOEME and ANUBIS projects (ANR-21-CE10-0008 – FLOEME; ANR-21-CE43-0010 – ANUBIS) and Synchrotron SoleIL for the ANATOMIX beamline time (proposal n° 20211421). ANATOMIX is an Equipment of Excellence (EQUIPEX) funded by the Investments for the Future program of the ANR, project NanoimagesX, grant no. ANR-11-EQPX-0031.

### Data availability

Data will be made available on request.

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