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Stabilisation of waterlogged archaeological wood: the application of structured-light 3D scanning and micro computed tomography for analysing dimensional changes

Heritage Science volume 10, Article number: 60 (2022) Cite this article

Abstract

Cultural heritage objects made of wood can be preserved under waterlogged conditions for many years, where decay is slowed down and the wood structure is more or less completely filled with water. Depending on the degree of degradation, finds may collapse and shrink when they are allowed to dry in an uncontrolled manner after excavation, leading to total loss of the object and its information. Conservation measures are taken to prevent damage of objects and dimensional stability is an important criterion. In this study, structured-light 3D scanning and micro-computed tomography were used to analyse the dimensional stability of wood after conservation, as well as its long-term stability. 83 samples from a reference collection established between 2008 and 2011 allowed this comparative study of the most common conservation methods at that time. The effects of conservation methods using alcohol-ether resin, melamine-formaldehyde (Kauramin 800®), lactitol/trehalose, saccharose, and silicone oil on dimensional stability were investigated. In addition, different polyethylene glycol (PEG) treatments with subsequent freeze-drying were also investigated: one-stage with PEG 2000, two-stage with PEG 400 and PEG 4000 and three-stage with PEG 400, PEG 1500 and PEG 4000. The data received from analyses of both volume and surface gave detailed information about the success of each conservation method. Attempts were made to quantify the damage patterns, specifically shrinkage, collapse, and cracks. While PEG and freeze-drying, alcohol-ether-resin, as well as the Kauramin 800® method gave the best results, analysis also highlighted the failures of each method.

Introduction

Wood can be preserved in waterlogged anoxic environments for thousands of years. There, only a few microorganisms such as bacteria and fungi that use wood as a nutrient are viable in an environment with a low oxygen level [1,2,3,4,5,6,7]. During burial, microorganisms have had time to degrade the wood by actually consuming the material from it so that physical weakening of the structure has occurred. Water swells the wooden structure and fills the pore spaces—the capillaries and the microcapillaries. As the cell wall suffers from material loss more water will fill up the internal voids. Though, the moisture content of decayed wood is raised. The maximum moisture content is related to the state of preservation of wood and is a universally used indicator [8,9,10].

Depending on its condition, waterlogged archaeological wood will change its dimensions in two stages upon drying, due to collapse and shrinkage [11]. Above the fibre saturation point, cell cavities or lumina will collapse meaning irregular distortion unforeseeable in extent and distribution. The cause of collapse is capillary tension exerting compressive forces on the cell wall. As a result, the considerably weakened cell walls inevitably collapse [8, 12, 13]. Because of its heterogeneous state of preservation, there will be stresses developed in the object. In particular, the decayed shelf will dry first while the core is still wet [14].

Below the fibre saturation point, the cell walls will shrink [8, 15, 16]. The wood will then contract to a minimum of the original dimensions. Shrinkage in the tangential direction is generally more severe than radial and longitudinal shrinkage. But volumetric shrinkage is directly proportional to the water content of archaeological wood [10, 14].

There are several criteria to assess the effectiveness of conservation methods [14, 16,17,18]. One of the main criteria is to prevent the wood from shrinkage and to stabilise the volume of the object. The stabilisation involves the preservation of the shape of the object, which contains information such as the manufacturing technique and the function of the object [19]. The dimensions of the actual but swollen waterlogged state should be preserved and intervention should be kept to a minimum [20].

To avoid shrinkage and collapse of waterlogged wood upon drying is the main challenge for the conservation of archaeological wet finds [21, 22]. The damaging effect of air-drying and the question of how to solve this problem has already been noted in the nineteenth century as being not an easy task [23, 24]. Since then, a variety of methods and conservation agents have been tested in conservation [16, 25,26,27,28,29,30,31,32,33]. The heterogeneous material, the different types of wood and especially the widely varying states of preservation make it difficult to assess the success of a conservation method, and studies to compare different conservation methods have been made since the beginning of wood conservation [8, 18].

To check the dimensional stabilisation of a method, the condition after the conservation is compared with the condition before conservation. In conservation science, several measurement methods have been applied to assess methods by comparing the dimensions before and after conservation: The extent of dimensional changes was evaluated on thin sections under the microscope [13] or on wooden samples with exact dimensions [16, 18, 19, 34,35,36,37,38,39,40,41,42]. The surfaces were also compared by drawing the outline of objects [43]. To assess the anisotropic character of archaeological wooden objects during drying, samples were cut in accordance with the specific alignments [17, 44] or stainless pins were introduced into the wood [15, 16, 34, 45,46,47,48,49,50,51]. In recent times, also optical 3D measurement methods are used in laboratory studies [45,46,47, 49, 50, 52] and on shipwrecks [53].

The aim of this study is to assess the dimensional stabilisation of a number of established and most commonly used conservation methods on larger sample series consisting of different wood species. The research in this study focuses on the volume changes after conservation using structured-light 3D scanning. In addition, the sample material studied allowed to investigate the volume stability of the conservation methods after 10 years.

Normally, the evaluation of the dimensional stability of conservation methods is limited to the outer surface of the wood. Micro-computed tomography (µCT) offers a non-destructive technology for the visualisation of the structures inside. Until now there are only a few cases that have also considered changes inside wood after conservation using tomographic methods [52, 54,55,56,57,58,59,60,61,62]. These investigations have shown that shrinkage of the wood can also result in cavities inside. In addition to cavities, cracks in the structure can also be detected [62]. However, it is the overall structure that provides information on whether an object has been stabilised. For this reason, the samples in this study were also examined by µCT after their conservation, which allowed internal defects such as cell collapse and cracks to be taken into account when evaluating the conservation result.

Materials and methods

Samples

The material under investigation is a reference collection held at the Römisch-Germanisches Zentralmuseum (RGZM), Leibniz Research Institute for Archaeology. In a research project that was performed from 2008 to 2011 the most established conservation methods were investigated where approximately 800 samples were conserved at different Institutions with their standard methods (Table 1, Fig. 1). A detailed overview of the conservation methods and the samples is given on the project homepage [45]. The methods employed were alcohol-ether-resin [18], melamine-formaldehyde (Kauramin 800®) [63], lactitol/trehalose [64], saccharose [26], silicone oil [65] and polyethylene glycol (PEG) with subsequent freeze-drying. PEG treatment followed either a one-stage process with PEG 2000 [66], two-stages with PEG 400 and PEG 4000 [67] or three-stages with PEG 400, PEG 1500 and PEG 4000 [45].

Table 1 Overview of conservation methods
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Fig. 1
figure 1

Samples of the KUR collection at the RGZM

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The samples were taken from archaeological objects that were collected from different sites. The objects were made of different types of wood and have different degrees of degradation, which were divided into the samples. The degree of degradation was determined by the maximum water content (Umax). The mass of the waterlogged wood was determined without vacuum impregnation with water before measuring. Umax is defined as the water present in the sample compared to the absolute dry wood substance [2, 10]:

$$Umax= \frac{mass\, wet\, wood-mass \,oven \,dried \,wood}{mass\, oven \,dried\, wood}\, [\%]$$
(1)

Values of Umax have been classified by de Jong [51]. The numbers correspond to the water content ranges of the samples: (1) Umax > 400%, (2) Umax 185–400%, and (3) Umax < 185%. The maximum water content was determined destructively for the control samples to be air-dried, and non-destructively for the samples to be conserved [45, 67]. The density of the cell wall substance is usually assumed to be 1.5 \(\frac{g}{{cm}^{3}}\) [10, 68]. The water content is proportional to the void volume in the wood and inversely proportional to the basic or conventional density (BD, standard ISO 13061) of the wood in question (amount of wood substance per volume, g/cm3) [2, 69, 70]:

$$BD=\frac{100}{\left(66.7+{U}_{max}\right)} \left[\frac{g}{{cm}^{3}}\right]$$
(2)

To get an impression of how much wood has been decayed, the residual basic density (RBD) was calculated, in percent, as the ratio between the measured density of the archaeological material and the average basic density for non-degraded wood of the same species [10, 69], as derived from the literature [14, 71]:

$$RBD=\frac{BD}{BD (fresh \,wood)}\bullet 100\left[\%\right]$$
(3)

The non-destructive determination of the condition of the wood does not always lead to reliable results, due to both mineral inclusions and inclusions of air in intact fibres with non-degraded pits and cell-walls. Therefore, the average value from all samples was taken (Appendix A).

This systematic collection of samples provides a unique chance to compare the structural differences of the conserved wood [43, 46]. Each test series of the collection includes samples derived from the same object (same wood species and finding place, similar state of preservation) that were conserved with the different treatments. Due to the different sizes of the objects, the number of samples varied. Therefore, not all test series contain all conservation methods. For the volume analyses in this study, the ten largest test series of the collection were selected in order to cover a high variety of methods and wood genera. Sample series of different wood species were investigated: three from oak (Oa1, Oa2 and Oa3), two from fir (Fi1 and Fi2) and one from alder (Al1), ash (As1), beech (Be1), pine (Pi1) and spruce (Sp1). 83 samples were analysed (Appendix A). Figure 2 gives an overview of the sampling and measurement methods of this research.

Fig. 2
figure 2

Graphical overview of the experimental procedure: sampling, conservation, measurements with structured-light 3D scanning and µCT

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Structured-light 3D scanning

From 2008 onwards, the wet samples were captured before conservation and in dry condition after conservation using a structured-light 3D scanner. The capturing device was an ATOS III Rev. 01 scanner from GOM (a ZEISS company) with a field of view of 500 mm × 500 mm × 500 mm and a point distance of 0.25 mm. After data capture, all scans were processed in the ATOS Professional 2016 Software using same parameters by employing Python scripts to control the workflow and obtain a reduced 3D mesh with a closed surface. To analyse volume changes after 10 years the conserved samples were captured again in 2020. Since the same sensor was no longer available, a successor model was used. The ATOS III Rev.02 Triple Scan from GOM (a ZEISS company) with a field of view of 320 mm × 240 mm × 240 mm and a point distance of 0.10 mm was used. The processing of scans results in 3D meshes of the samples.

Micro-computed tomography

Analysis of the condition of the wood structure inside the samples about 10 years after their original conservation was carried out at Lucerne University of Applied Sciences and Arts with a µCT-system. The tomographic analysis was performed on the in-house laboratory XCT system (Diondo d2, Germany). An optimised setup and acquisition protocol for the µCT measurements was developed for conserved wood. The measurements were conducted by setting the X-ray source (XWT-225 TCHE+ from X-ray works, Garbsen, Germany) in high power mode and choosing an operation voltage of 120 kV and a filament current of 167 μA with a 1 mm aluminum filter. The wood samples were mounted in a sample holder and placed in the sample chamber. The sample was rotated 360° in continuous mode during the acquisition. The radiographical projections were recorded with a 4343 DX-I X-ray detector (Varex, Salt Lake City, USA), with a pixel size of 139 μm. The distance between the X-ray source and the sample was between 160 and 250 mm and the distance between the X-ray source and the detector was 860 mm, giving a magnification between 3.4 and 5.4 and a nominal voxel size between 27 and 44 μm. A total of 3000 projection images were acquired during the sample rotation of 360°. The resulting projections were converted into a 3D image stack of approx. 3000 × 3000 × 3000 voxels using the CERA reconstruction software based on the filtered back projection Feldkamp algorithm [72] from Siemens. The achieved resolution of the µCT measurements depends on the size of the samples [73]. Appendix A gives an overview of the samples, their size and the achieved resolution of the µCT measurements. Image cross-sections and 3D renderings of the wood were visualised in VGStudioMax3.4© software.

Surface and volume determination and calculation of the dimensional changes

To get information about the surface and the volume of the samples measured with µCT the data was analyzed with VGStudioMax3.4. To obtain values of the wood volume concerning inner cracks and collapse, surface determination of the very heterogeneous material was done manually for each sample with the different segmentation tools of the software. Afterwards the surface and volume values of the segmented µCT data and that of the structured-light 3D scanning were calculated with the same software.

The evaluation of the conservation methods was done by determining the dimensional stability on the basis of the volume data that was derived from the surface of the whole sample. The values of the individual wood anatomical directions (tangential, radial and longitudinal) can be taken from the database [45].

To evaluate the volume changes in the measurement, data from structured-light 3D scanning was used to calculate the shrinkage (S) from the volume of the samples before (Vwet) and after (Vdry) conservation [74,75,76,77]:

$$S=\frac{{V}_{wet}-{V}_{dry}}{{V}_{wet}} \times 100\, [\%]$$
(4)

The anti-shrink efficiency (ASE) was determined from the shrinkage of a non-conserved control sample (So) and the shrinkage of the conserved sample (Scon) [15, 16, 78]:

$$ASE=\frac{{S}_{0}-{S}_{con}}{{S}_{0}} \times 100\, [\%]$$
(5)

100% ASE means a very good conservation has been achieved, whereas an ASE of 0% indicates a result equal to that accomplished by air-drying. An ASE of 75% seems to be acceptable [15, 16]. By using ASE, a statistical evaluation and comparison of conservation results is possible.

To probe the volume stability of conserved archaeological wood over time and the dimensional stability of the wood volume with respect to the inner structure, the values of the whole samples from structured-light 3D scanning and µCT were used. Analogous to the shrinkage after conservation analysis (Eq. 4), the volume change of the samples after 10 years was calculated from the volume after conservation and the volume 10 years after conservation. Similarly, the volume change concerning the inner structures of the wood was calculated from the volume of the structured-light 3D scanning 10 years after conservation and the volume of the µCT-data after 10 years. Surface changes (AO) were calculated additionally from the values of structured-light 3D scanning and µCT after 10 years:

$${A}_{O} change=\frac{{{A}_{O\, \mu CT}}-{{A}_{O\, scan}}}{{{A}_{O\, scan}}} \times 100 [\%]$$
(6)

To get an impression of the size and shape of the cavities inside the samples the volumes and surface areas were determined by the difference between structured-light 3D scanning and µCT data. From these values the sphericity ѱ was calculated:

$$\uppsi = \frac{{\sqrt[3]{{36\pi V^{2} }}}}{{A_{O} }}$$
(7)

The sphericity ѱ relates the shape of a body based on its volume (V) and its surface (AO) to the smallest possible surface of a sphere of the same volume [79]. The value of the sphericity ѱ for a sphere is 1. The lower a value of sphericity ѱ is for a shape, the larger the surface area is compared to a sphere.

Results and discussion

Volume changes

Appendix B shows the values for the volumes of the structured-light 3D scans before and after conservation. The measurements show extensive shrinkage of the unconserved wood samples. In Fig. 3 the loss of volume after air-drying of the untreated wood shows a relationship with the condition of the wood which is calculated as the residual basic density in per cent (Eq. 3). There is a clear trend showing that the better the wood is preserved, the less volume change occurs during air-drying. Figure 3 also demonstrates less shrinkage of all conserved samples in comparison to the air-dried samples. There is a difference between the conservation methods concerning the samples with a higher degradation and a lower residual basic density. It is evident that the stabilisation of these samples treated with saccharose, lactitol/trehalose or silicone oil is not as sufficient as compared to the other treatments.

Fig. 3
figure 3

Loss of volume after air-drying untreated samples and conserved ones in relation to the condition of the wood (residual basic density in %)

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This tendency is also confirmed by the calculation of the ASE (Eq. 5). Figure 4 shows the ASE of all sample series in relation to the preservation status. For a better overview, the average value (x̅) of the residual basic density is given here for each sample series. In Fig. 4 it becomes even clearer that strongly degraded sample series are less stabilised by the conservation agents saccharose, lactitol/trehalose and silicone oil than by the other conservation agents. In addition, it also becomes obvious that the overall volume stabilisation decreases with a better state of preservation. The threshold here is a residual basic density of about 60%. This limit indicates that the preservation state 2 after de Jong [51, 80, 81] is particularly difficult to conserve due to two different states of preservation in one object [17]. The µCT data confirms that in these test series (Pi1, Fi1, Oa1 and Oa2) samples have highly degraded and low degraded areas. In contrast, the more degraded sample series show a consistent picture in the µCT data. The cross-sections of the samples As1-K800 and Pi1-AlEt show the difference between one homogeneous state of preservation (Fig. 5) and two states of preservation in one object (Fig. 6).

Fig. 4
figure 4

Volume stability (ASE) dependent on the average residual basic density x̅. The conservation method is given on the data point

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Fig. 5
figure 5

µCT cross section of the ash sample conserved with Kauramin 800® (As1-K800) with one homogeneous state of preservation

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Fig. 6
figure 6

µCT cross section of the pine sample conserved with alcohol-ether-resin (Pi1-AlEt) with high degraded area left and low degraded area right

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Apart from the state of preservation, there are also considerable variances in the conservation methods in the individual test series with different wood species. All conservation methods improve stabilisation to some degree. Taking all the results into account and looking at the average of the volume changes for the different conservation methods, a general trend can be seen (Fig. 7). The best results are mostly achieved using PEG, the alcohol-ether method or Kauramin, although there are exceptions. The individual conservation methods with their deviations and special features will be discussed later in detail.

Fig. 7
figure 7

Average shrinkage of all selected samples after conservation, after 10 years and including the inner structures derived from µCT

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Appendix C shows the values of the volume changes after 10 years. Here, it is particularly noticeable that almost all unconserved wood samples are not stable. With a few exceptions, the changes in the conserved samples are very slight and no clear trends can be identified for the individual conservation methods. All conservation methods thus seem to permanently stabilise the samples, as can be seen from the average values for volume change after 10 years (Fig. 7). In Appendix C, the internal structures are considered and the extent to which this changes the volume. Since no µCT data of the previous state is available, the values of the µCT data are compared with the values of the surface scans of the current state 10 years after conservation. Since the original condition of the interior of the samples is unknown, this is primarily a comparison of methods. It should be noted here that the µCT not only considers internal cavities, but also, in contrast to the surface scan, considers and more accurately depicts crevices and depressions (Fig. 8). Nevertheless, differences in the conservation methods become apparent when looking at the average value of the volume change inside all samples. The conservation methods with Kauramin 800®, saccharose, lactitol/trehalose and silicone oil seem to lead to a greater change in volume inside the samples than the other conservation methods (Fig. 7).

Fig. 8
figure 8

µCT cross sections of two examples (Oa3-K800 left and Oa3-Sac right) for the comparison of the surface detection with structured-light 3D scanner and µCT

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Volume changes in relation to the surface

Another indicator for the stabilisation of a sample that can be obtained from surface scans and µCT are changes in surface values. Obviously, if only the outer surface is considered, it will also decrease with volume shrinkage. By comparing the surface data with the µCT data, a value for the surface inside the sample is obtained, which can provide information about cell collapse and cracks in the sample (Appendix D, Eq. 6).

If the surface area increases in the µCT data, we are dealing with cavities inside the sample. Again, it must be considered that due to the lack of the previous condition, it cannot be clearly proven whether the cavity was already inside the wood before the conservation. In some cases, there is also some uncertainty due to deeper cracks in the surface that were not detected by the structured-light 3D scanner (Fig. 8). When looking at the µCT data, however, cell collapse is definitely evident (Fig. 9). In addition, clear cracking can be seen in the samples conserved with PEG that were freeze-dried (Fig. 14) [46, 52].

Fig. 9
figure 9

µCT cross section of the alder sample treated with alcohol-ether-resin (Al1-AlEt) with cell collapse in the wood structure

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The values for the enlargement of the surface and the shrinkage in percent, resulting from the comparison of surface scan and µCT, are listed in Appendix D. From the values, it is possible to see, to some extent, if collapse and cracks appear inside the wood samples. If neither the volume nor the surface have changed much, this means that the sample is also stable inside. In contrast, it can be assumed that a large surface increase combined with a large volume loss can only be explained by a significant collapse inside the sample. An example of this is the alder sample (Al1-LaTr, Fig. 10). A surface increase of 305% and a shrinkage of 16%. If only the volume has changed considerably and the surface does not change so significantly, it can be assumed that isolated, large cracks or fissures have occurred within the wood. This is the case with the oak sample (Oa2-K800), which is also shown in the cross-section of the sample (Fig. 13). In relation to the volume loss with 15%, the surface with 144% has changed far less compared to the alder sample (Al1-LaTr, Fig. 10). If, on the contrary, the surface has increased considerably with a smaller change in volume, this is a sign of many fine cracks within the wood, as has already been mentioned for the wood conserved with PEG and freeze-dried. An example of this is the ash sample (As1-PEG1), which has a relatively stable volume with a shrinkage of 1% but a large surface enlargement with a value of 218%. The fine cracks, leading to these values, can be seen in Fig. 14. Such obvious changes in the wood structure and the measured volume and surface values are discussed below individually for each conservation method studied.

Fig. 10
figure 10

µCT cross section of the alder sample treated with lactitol/trehalose (Al1-LaTr) with cell collapse in the wood structure

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Another way to approximate the shape of these cavities inside the wood is to calculate the sphericity ѱ, which relates the shape of a body based on its volume and its surface to the smallest possible surface of a sphere of the same volume (Eq. 7) [79]. The lower this value for a shape, the larger the surface in relation to a sphere. The values determined in this way show a wide variation and are probably dependent on the conservation agent, the type of wood and the state of preservation. However, tendencies for the different conservation methods can be derived from the averaged values (Fig. 11). In contrast to the other methods, the values obtained from samples treated with PEG scatter far less. This could be an indication that a more uniform and reliable conservation result can be achieved here. Furthermore, the average values obtained from samples treated with PEG are noticeably low. The sphericity ѱ confirms the picture that fine cracks tend to form in these cases.

Fig. 11
figure 11

Sphericity ѱ (average values with standard deviation) of the cavities inside the samples determined for the different conservation agents from structured-light 3D scanning and µCT data

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Dimensional stabilisation provided by the conservation methods

Alcohol-ether-resin

The samples conserved with the alcohol-ether-resin show very good volume stabilisation overall. With two exceptions, the values determined for the ASE of the scans before and directly after conservation are above 90% (Appendix B). The oak sample (Oa2-AlEt) also has a very good ASE value of 88%, while the pine sample (Pi1-AlEt) having the lowest value, with an ASE of 76%. This could be due to the preservation state 2 according to de Jong [51, 80, 81] with two different levels of degradation (Fig. 6). The scans after 10 years show consistently good values for volume stability. Considering the internal structures resulting from the µCT data, the method also shows good volume stabilisation (Appendix C, Fig. 7). Only the alder sample (Al1-AlEt) shows a poor value with more than 5% additional volume loss. If the surface change of the samples preserved with alcohol-ether-resin is added, the predominantly positive overall picture is reinforced (Appendix D). The low surface increase when considering the internal structures on the basis of the µCT, together with the simultaneous low volume loss, indicates that there was little collapse or crack formation in the interior of the specimens, which is also confirmed by the cross sections of the samples in µCT data. An exception is again the alder sample (Al1-AlEt), which shows a comparatively high value of 137% surface increase, which in connection with the volume loss indicates an increased cell collapse inside the sample. This is supported by the observation of the µCT data (Fig. 9). It should be noted here that in this series, poorer values are observed overall in comparison with all conservation methods, which may be explained by the fact that cavities were already present in the wood before conservation. This positive assessment is also confirmed in previous studies [18] especially for broadleaved woods [82].

Kauramin 800®

The samples conserved with Kauramin 800® also show mainly very good results. With two exceptions, the ASE directly after conservation shows values above 90% (Appendix B). Five of the ten samples even show an ASE of 99% to 101%. The samples with lower values are the pine sample (Pi1-K800) with an ASE of 87% and the oak sample (Oa1-K800) with an ASE of 88%. The pine sample (Pi1-K800) comes from the same sample series as the previously mentioned pine sample (Pi1-AlEt), which, with two different areas of degradation (de Jong: 2), also has the worst value of the alcohol-ether-resin. Volume stability is guaranteed for Kauramin 800® even after 10 years. A somewhat different picture of the method emerges when looking at the µCT data. While the positive results for most samples are confirmed by a small additional volume loss, a different picture emerges for the oak samples (Oa1-K800 and Oa2-K800) with two different states of preservation (de Jong: 2, Appendix C). The additional volume loss of 10% and 15%, respectively, is also significantly reflected in the overall results of the method (Fig. 7). It is striking that this volume loss is caused, in both cases, by large gaps that run along the two different maintenance states in the wood (Figs. 12 and 13). The low values of surface area increase in relation to volume loss confirm here that large fissures are involved (Appendix D). Overall, the samples preserved with Kauramin 800® show greater surface area increase than the samples preserved with alcohol-ether-resin, suggesting that cell collapse and cracking have increased in these. This may be due to the fact that it is too stiff and is not easy to deform. This has been criticised in the literature regarding previous methods using melamine resins [18]. It has also been described that Kauramin 800® stabilises highly degraded wood very well, while well-preserved wood is stabilised more poorly. This could be due to poorer penetration of the amino resin prepolymer, resulting in collapse and shrinkage of the well-preserved wood [17]. This becomes obvious in samples with two different states of preservation.

Fig. 12
figure 12

µCT cross section of the oak sample treated with Kauramin 800® (Oa1-K800) with a large crack in the wood structure

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Fig. 13
figure 13

µCT cross section of the oak sample treated with Kauramin 800® (Oa2-K800) with large cracks in the wood structure

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Fig. 14
figure 14

µCT cross section of the ash sample treated with PEG 2000 and freeze dried (As1-PEG1) with cracks in the wood structure

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Lactitol/trehalose

The samples conserved with lactitol/trehalose were inferior to the previously mentioned methods. The ASE of the samples directly after conservation is between 76% and 92%, whereby only the beech sample (Be1-LaTr) with an ASE of 92% has a value above 90% (Appendix B). A change in volume after ten years is also not observed in the woods conserved with lactitol/trehalose. Looking at the internal structures based on the µCT of the samples preserved with lactitol/trehalose, this also confirms poor volume stabilisation (Appendix C, Fig. 7). In particular, the alder sample (Al1-LaTr) and the oak sample (Oa3-LaTr) have poor values with 16% and 14% additional volume loss, respectively. If one adds to the volume loss the strong surface change in these samples (Appendix D), this indicates that considerable collapse has occurred inside these samples. Figure 10 shows a cross-sectional image of the µCT, which illustrate the collapse of the wood structure.

Polyethylene glycol (PEG 2000) one-step and freeze-drying

The samples conserved with PEG 2000 and then freeze-dried show predominantly very good volume stabilisation (Appendix B). Except for two samples, the ASE values are above 90%. In six of the ten samples, the ASE is between 99% and 105%. The samples with the lowest values are the oak sample (Oa1-PEG1) with a value of 83% and the oak sample (Oa2-PEG1) with a value of 89%. The scans after 10 years also show consistently high values of volume stability for the samples conserved with PEG 2000. Similarly, the µCT data confirm good volume stabilisation inside the samples (Fig. 7). With just over 2% additional shrinkage inside the samples, the alder sample (Al1-PEG1) and one of the two fir samples (Fi2-PEG1) have the highest values (Appendix C). If the surface change is also considered, it is noticeable that these two samples also exhibit a large surface enlargement with values above 200%. With the exception of the first fir sample (Fi1-PEG1), such a surface increase is observed in all samples preserved with PEG 2000 and freeze-drying (Appendix D), which can be explained by the formation of cracks. Figure 14 shows these cracks, for example, in the ash sample (As1-PEG1). With the exception of the fir sample (Fi1-PEG1), these cracks occur in all samples conserved with PEG 2000. The first step in the freeze-drying process is the freezing of the impregnated wood. Freezing the aqueous solution in the wood leads to the expansion of its volume [83] leading to cracks. Specimens preserved with PEG exhibit low strength [18]. This fragility is further increased by the cracks that have formed as a result of the conservation.

Polyethylene glycol (PEG 400 and 4000) two-step and freeze-drying

The samples conserved in stages with PEG 400 and PEG 4000 and afterwards freeze-dried also show a predominantly favourable volume stabilisation (Appendix B). With the exception of the values of three samples, the calculated ASE is above 90%. The samples with the poorest values are the oak sample (Oa2-PEG2) with a value of 89%, the pine sample (Pi1-PEG2) with a value of 87%, and the second oak sample (Oa1-PEG2), which dropped sharply to 74%. The scans after 10 years show satisfactory volume stability, in line with the other conservation methods. Considering the results of the µCT, a similar picture appears as for the conservation with PEG 2000. The volume stabilisation inside the samples is good and with a value above 3%, the oak sample (Oa3-PEG2) has the highest additional shrinkage (Appendix C). An increase of the surface area with a good volume stabilisation inside the samples can also be observed, which in turn suggests the formation of cracks inside the samples (Appendix D). The cracks occur less strongly than with the other PEG methods. However, they are clearly visible in seven sample series. The fir samples (Fi1-PEG2, Fi2-PEG2) and the spruce sample (Sp1-PEG2) have no obvious cracks. In one case, in addition to cracks in the outer area, it can also be observed that the wood structure in the centre has collapsed (Fig. 15).

Fig. 15
figure 15

µCT cross section of the ash sample treated with PEG 400 and 4000 and freeze dried (As1-PEG2) with cracks in, and collapse of, the wood structure

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Polyethylene glycol (PEG 400, 1500 and 4000) three-step and freeze-drying

The samples conserved in three stages with PEG 400, PEG 1500 and PEG 4000 and then freeze-dried are among the best results of the methods with PEG (Appendix B). With the exception of two samples, the ASE values are above 90%. The samples with poorer values are the oak sample (Oa1-PEG3) with a value of 85% and the pine sample (Pi1-PEG3) with a value of 80%. The scans after 10 years show satisfactory volume stability (Appendix C). As with the other conservation methods using PEG, the µCT data show good volume stabilisation with a simultaneous increase in surface area inside the samples, again indicating the formation of fine cracks (Appendix D). The µCT cross-sections (Fig. 16) confirm that these cracks occur in all samples conserved with the three-step PEG method except the fir sample (Fi1-PEG3). In particular, the alder sample (Al1-PEG3) stands out with a shrinkage of almost 7% and a surface area increase of over 600%. This sample is an exception among the samples conserved with PEG because, in addition to the cracks, there is also considerable collapse in the conserved wood structure (Fig. 17).

Fig. 16
figure 16

µCT cross section of the ash sample treated with PEG (three-step) and freeze dried (As1-PEG3) with cracks in the wood structure

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Fig. 17
figure 17

µCT cross section of the alder sample treated with PEG (three-step) and freeze dried (Al1-PEG3) with cracks in, and collapse of, the wood structure

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Saccharose

The samples conserved with saccharose show a poorer conservation overall and the results vary greatly. Thus, the ASE of the ten samples directly after conservation ranges between 64% and 94%, with only the fir sample (Fi1-Sac) with 94% and the pine sample (Pi1-Sac) with 90% showing a value above 90% (Appendix B). The ASE of the pine sample (Pi1-Sac) is particularly remarkable in this context, as the other conservation methods consistently perform rather poorly in this series of samples. A change in volume after ten years is also not observed in the wood samples conserved with saccharose (Fig. 7). The µCT data also show very different results, with very poor values of volume stabilisation inside the samples in some cases (Al1-Sac, As1-Sac, Oa3-Sac, Appendix C). Together with the high values of surface change (Appendix D), this indicates a significant collapse of the wood structure, as confirmed by the images of the ash sample (As1-Sac, Fig. 18). However, low stabilisation is not solely related to low concentration of consolidant. Highly concentrated solutions can also be problematic. Their susceptibility to degradation by osmotolerant microorganisms can lead to the development of slime and gases, which can prevent consolidants from penetrating the wood. Even an excess of biocides, due to the low penetration of the same, could not prevent these fermentation processes. The decomposition of saccharose takes place by chemical or microbiological processes, whereby the sugar is converted into the monosaccharides of fructose and glucose. This prevents volume stabilisation and, as the degradation products are more hygroscopic, leads to a damp surface and poorer drying [84,85,86].

Fig. 18
figure 18

µCT cross section of the ash sample treated with saccharose (As1-Sac) with collapse of the wood structure

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Silicone oil

The samples conserved with silicone oil tend to show poorer volume stabilisation overall and the ASE of the samples is between 71% and 89% (Appendix B). No change in volume can be observed in the wood conserved with silicone oil after ten years (Fig. 7). As with the saccharose-conserved samples, the µCT data show very different results (Appendix C). Thus, there are also striking samples (Al1-Sil, As1-Sil, Sp1-Sil) with poor volume stabilisation and an interior surface enlargement, which again indicates the collapse of the wood structure (Appendix D). This is also confirmed, for example, by the images of sample As1-Sil (Fig. 19).

Fig. 19
figure 19

µCT cross section of the ash sample treated with silicone oil (As1-Sil) with collapse of the wood structure

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Conclusion

On the basis of the surface data from structured-light 3D scanning of ten test series with a total of 83 wood samples, it was possible to comparatively investigate the preservation results of different conservation methods with regard to their volume stabilisation. Wood is a very heterogeneous material and the successful conservation and volume stabilisation depends on different factors. This study showed that first of all the state of preservation is of great importance. The dimensional changes upon air-drying are dependent on the water content. This was also confirmed by other studies [2, 5, 13, 80].

There are also enormous differences between the conservation methods studied. It should be noted that all methods investigated in the study stabilised the wood to some extent, in contrast to air-drying. The investigation of the volume stability after 10 years further showed that all conserved samples remained stable in contrast to the air-dried samples. However, it should also be noted that only the methods using PEG, alcohol-ether-resin and Kauramin 800® guarantee reliable volume stabilisation. The other methods were subject to considerable fluctuations and in some cases showed a large volume loss due to cell collapse.

For the first time, the internal structures of a large sample series were considered by the application of µCT. In contrast to the structured-light 3D scans performed here, the µCT measurements show the considerable damage such objects can take during conservation due to cracks and cell collapse. Such disadvantages could also be observed in samples that were treated with the methods using PEG, alcohol-ether-resin and Kauramin 800®. For example, in a condition with two differently degraded areas in the wood, large gaps can form during conservation with Kauramin 800®. This is possibly due to the fact that the conservation agent has the ability to stabilise heavily degraded wood better than well preserved wood, which may be explained by poorer penetration into well preserved wood [17, 87]. In the samples conserved with PEG and subsequently freeze-dried, a large number of fine cracks in the wood were very often visible in the µCT data. This can be explained by the fact that the aqueous PEG solution expands during freezing, resulting in damage to the wood structure. The two-step method using aqueous solutions of PEG 400 and PEG 4000 showed slightly better results with fewer cracks. This may be attributed to the fact that low molecular weight PEG lowers the critical freeze-drying temperature and a proportion of the solution does not freeze. Though less volume expansion may occur [46]. In addition, it was observed in one sample that there is considerable collapse in the centre. That may be explained by the fact that the aqueous solution of PEG thawed during the freeze-drying process. Afterwards, the wood structure collapsed when drying from the liquid phase. It is likely the conservation agent was transported to the outside areas by capillary forces, thus, appearing as though there is no conservation agent present in this area in the µCT data. This has already been described in the literature as being a crucial parameter in the conservation treatment with PEG [17, 18, 54, 58].

These observations show the importance of tomographic methods and the need to take into account the internal structures when assessing the conservation result. However, these observations also show that further research and validation of the results is required. In order to be able to assess the changes in the wood caused by the conservation with certainty, it is necessary in further test series to also record the preliminary condition of the samples using tomographic methods. However, in the case of the waterlogged condition of wood, it should be noted that the presence of water in the wood reduces the quality of the µCT data and complicates the measurement procedure. Therefore, for waterlogged wood, magnetic resonance imaging, for example, would be a suitable method to better capture the preliminary wet condition [55, 88, 89]. It should be mentioned that in this study only volume stabilisation of conservation methods was considered for evaluation. Other factors such as reversibility, effort, toxicity of the ingredients and long-term stability are also of great importance in the choice of the procedure.

Availability of data and materials

The datasets generated and/or analysed during the current study are available in the repository of the RGZM, [www.rgzm.de/kur]. Further datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

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Acknowledgements

The authors thank the Federal Cultural Foundation and the Cultural Foundation of German States (KUR Programme) and all participants for their work in conserving the wooden samples.

The 3D scanning was done in cooperation with the University of Applied Science Mainz, i3mainz—Institute for Spatial Information and Surveying Technology.

Funding

Open Access funding enabled and organized by Projekt DEAL. The establishment of the reference collection for the conservation of waterlogged wood was funded by the Federal Cultural Foundation and the Cultural Foundation of German States. The cross-border research project is funded by the German Research Foundation—416877131 and the Swiss National Science Foundation—200021E_183684.

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Affiliations

  1. Römisch-Germanisches Zentralmuseum, Leibniz Research Institute for Archaeology, Ernst Ludwig Platz 2, 55116, Mainz, Germany

    Jörg Stelzner, Ingrid Stelzner, Markus Wittköpper, Waldemar Muskalla, Anja Cramer, Guido Heinz & Markus Egg

  2. Lucerne University of Applied Sciences and Arts-School of Engineering and Architecture, Technikumstrasse 21, Horw, Switzerland

    Jorge Martinez-Garcia, Damian Gwerder & Philipp Schuetz

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  1. Jörg Stelzner
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Contributions

JS designed, analysed and interpreted the data regarding the surface/shrinkage analyses of archaeological wood and was the major contributor in writing the manuscript. IS designed, analysed and interpreted the data and wrote the manuscript. DG performed the CT measurements. AC, GH were responsible for the structured-light 3D scanning. MW, WM, ME were responsible for the preparation of the KUR sample collection. DG, JM, MW, WM, AC, GH, ME, PS supported the analysis of the data as well as contributed to the manuscript writing. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Jörg Stelzner.

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Competing interests

The authors declare that they have no competing interests.

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Appendices

Appendices

Appendix A: Genus, conservation method, condition, dimensions and resolution µCT of the samples

Sample KUR-No Genus Conservation method Umax (%) Condition (de Jong) Basic density (g/cm3) Residual basic density (%) Dimensions (cm)
L, Ø/L, W, H 		Resolution µCT (µm)
Al1-Air V24-23 Alder Air dried 743 1 0.124 28 20, 3–5 27
Al1-AlEt V24-45 Alder Alcohol-Ether 706 1 0.129 29 12, 3–5 27
Al1-K800 V24-60 Alder Kauramin 800® 958 1 0.098 22 16, 3–5 30
Al1-LaTr V24-19 Alder Lactitol/trehalose 891 1 0.104 24 14, 3–5 30
Al1-PEG1 V24-13 Alder PEG (1) 853 1 0.109 25 17, 3–5 35
Al1-PEG2 V24-63 Alder PEG (2) 671 1 0.136 31 18, 3–5 27
Al1-PEG3 V24-21 Alder PEG (3) 1048 1 0.090 20 19, 3–5 30
Al1-Sac V24-34 Alder Saccharose 820 1 0.113 26 13, 3–5 27
Al1-Sil V24-56 Alder Silicone oil 807 1 0.114 26 15, 3–5 30
As1-Air V28-27 Ash Air dried 738 1 0.124 22 11, 12 43
As1-AlEt V28-23 Ash Alcohol-Ether 994 1 0.094 17 11, 12 44
As1-K800 V28-30 Ash Kauramin 800® 961 1 0.097 17 11, 12 35
As1-PEG1 V28-08 Ash PEG (1) 1291 1 0.074 13 11, 12 35
As1-PEG2 V28-10 Ash PEG (2) 1195 1 0.079 14 11, 12 35
As1-PEG3 V28-36 Ash PEG (3) 929 1 0.100 18 11, 12 44
As1-Sac V28-07 Ash Saccharose 1376 1 0.069 12 11, 12 35
As1-Sil V28-24 Ash Silicone oil 983 1 0.095 17 11, 12 35
Be1-Air V07-23 Beech Air dried 664 1 0.137 24 20, 8, 8 35
Be1-AlEt V07-18 Beech Alcohol-Ether 735 1 0.125 22 12, 8, 8 35
Be1-K800 V07-14 Beech Kauramin 800® 720 1 0.127 23 19, 8, 8 35
Be1-LaTr V07-Exp3 Beech Lactitol/trehalose 566 1 0.158 28 17, 8, 8 35
Be1-PEG1 V07-09 Beech PEG (1) 772 1 0.119 21 13, 8, 8 35
Be1-PEG2 V07-27 Beech PEG (2) 654 1 0.139 25 14, 8, 8 35
Be1-PEG3 V07-08 Beech PEG (3) 747 1 0.123 22 15, 8, 8 35
Be1-Sac V07-01 Beech Saccharose 748 1 0.123 22 16, 8, 8 35
Be1-Sil V07-37 Beech Silicone oil 808 1 0.114 20 18, 8, 8 35
Fi1-Air V22-28 Fir Air dried 381 2 0.223 59 30, 8, 2,5 32
Fi1-AlEt V22-09 Fir Alcohol-Ether 398 2 0.215 57 22, 8, 2,5 32
Fi1-K800 V22-33 Fir Kauramin 800® 487 1 0.181 48 26, 8, 2,5 32
Fi1-LaTr V22-26 Fir Lactitol/trehalose 352 2 0.239 63 24, 8, 2,5 32
Fi1-PEG1 V22-22 Fir PEG (1) 432 1 0.201 53 27, 8, 2,5 32
Fi1-PEG2 V22-31 Fir PEG (2) 3711 2 0.2281 601 28, 8, 2,5 32
Fi1-PEG3 V22-35 Fir PEG (3) 241 2 0.325 86 29, 8, 2,5 32
Fi1-Sac V22-11 Fir Saccharose 285 2 0.284 75 23, 8, 2,5 32
Fi1-Sil V22-05 Fir Silicone oil 333 2 0.250 66 25, 8, 2,5 32
Fi2-Air V30-Exp1 Fir Air dried 465 1 0.188 49 11, 6–7 40
Fi2-AlEt V30-33 Fir Alcohol-Ether 695 1 0.131 35 11, 6–7 40
Fi2-K800 V30-09 Fir Kauramin 800® 601 1 0.150 39 11, 6–7 40
Fi2-PEG1 V30-04 Fir PEG (1) 542 1 0.164 43 11, 6–7 40
Fi2-PEG2 V30-18 Fir PEG (2) 632 1 0.143 38 11, 6–7 40
Fi2-PEG3 V30-15 Fir PEG (3) 554 1 0.161 42 11, 6–7 40
Fi2-Sac V30-17 Fir Saccharose 599 1 0.150 40 11, 6–7 40
Fi2-Sil V30-24 Fir Silicone oil 548 1 0.163 43 11, 6–7 40
Oa1-Air V10-02 Oak Air dried 201 2 0.374 67 17, 7 32
Oa1-K800 V10-06 Oak Kauramin 800® 198 2 0.378 67 16, 7 32
Oa1-LaTr V10-33 Oak Lactitol/trehalose 160 3 0.441 79 15, 7 32
Oa1-PEG1 V10-29 Oak PEG (1) 179 3 0.407 73 11, 7 32
Oa1-PEG2 V10-31 Oak PEG (2) 176 3 0.412 74 12, 7 32
Oa1-PEG3 V10-39 Oak PEG (3) 181 3 0.404 72 13, 7 32
Oa1-Sac V10-03 Oak Saccharose 163 3 0.435 78 14, 7 32
Oa2-Air V16-15 Oak Air dried 362 2 0.233 42 12, 15, 4,5 33
Oa2-AlEt V16-09 Oak Alcohol-Ether 175 3 0.414 74 12, 15, 4,5 32
Oa2-K800 V16-25 Oak Kauramin 800® 228 2 0.339 61 12, 15, 4,5 44
Oa2-PEG1 V16-18 Oak PEG (1) 189 2 0.391 70 12, 15, 4,5 35
Oa2-PEG2 V16-32 Oak PEG (2) 237 2 0.329 59 12, 15, 4,5 44
Oa2-PEG3 V16-36 Oak PEG (3) 188 2 0.393 70 12, 15, 4,5 35
Oa2-Sac V16-17 Oak Saccharose 184 3 0.399 71 12, 15, 4,5 35
Oa2-Sil V16-13 Oak Silicone oil 187 2 0.394 70 12, 15, 4,5 32
Oa3-Air V27-17 Oak Air dried 577 1 0.155 28 12–14, 5–6 19
Oa3-AlEt V27-40 Oak Alcohol-ether 527 1 0.168 30 12–14, 5–6 32
Oa3-K800 V27-07 Oak Kauramin 800® 646 1 0.140 25 12–14, 5–6 26
Oa3-LaTr V27-30 Oak Lactitol/trehalose 566 1 0.158 28 12–14, 5–6 26
Oa3-PEG1 V27-12 Oak PEG (1) 691 1 0.132 24 12–14, 5–6 32
Oa3-PEG2 V27-02 Oak PEG (2) 578 1 0.155 28 12–14, 5–6 32
Oa3-PEG3 V27-01 Oak PEG (3) 557 1 0.160 29 12–14, 5–6 39
Oa3-Sac V27-21 Oak Saccharose 434 1 0.200 36 12–14, 5–6 26
Oa3-Sil V27-10 Oak Silicone oil 520 1 0.170 30 12–14, 5–6 26
Pi1-Air V03-01 Pine Air dried 390 2 0.219 52 20, 7, 9–12 39
Pi1-AlEt V03-17 Pine Alcohol-Ether 3321 2 0.2511 601 12, 7, 9–12 40
Pi1-K800 V03-41 Pine Kauramin 800® 3321 2 0.2511 601 19, 7, 9–12 40
Pi1-LaTr V03-28 Pine Lactitol/trehalose 3321 2 0.2511 601 17, 7, 9–12 40
Pi1-PEG1 V03-35 Pine PEG (1) 3321 2 0.2511 601 13, 7, 9–12 40
Pi1-PEG2 V03-32 Pine PEG (2) 3321 2 0.2511 601 14, 7, 9–12 40
Pi1-PEG3 V03-20 Pine PEG (3) 3321 2 0.2511 601 15, 7, 9–12 40
Pi1-Sac V03-45 Pine Saccharose 3321 2 0.2511 601 16, 7, 9–12 40
Pi1-Sil V03-42 Pine Silicone oil 3321 2 0.2511 601 18, 7, 9–12 40
Sp1-Air V23-20 Spruce Air dried 503 1 0.176 46 21, 10, 6 40
Sp1-K800 V23-08 Spruce Kauramin 800® 482 1 0.182 48 20, 10, 6 40
Sp1-LaTr V23-02 Spruce Lactitol/trehalose 454 1 0.192 51 19, 10, 6 40
Sp1-PEG1 V23-25 Spruce PEG (1) 427 1 0.203 53 15, 10, 6 40
Sp1-PEG2 V23-30 Spruce PEG (2) 280 2 0.288 76 16, 10, 6 40
Sp1-PEG3 V23-28 Spruce PEG (3) 282 2 0.287 75 17, 10, 6 40
Sp1-Sac V23-29 Spruce Saccharose 374 2 0.227 60 18, 10, 6 40
  1. 1 = Mean value of the sample series

Appendix B: Volumes of the samples before and after conservation with calculated shrinkage and ASE

Sample KUR-No Volume Scan1 (mm3) before conservation Volume Scan2 (mm3) after conservation Shrinkage (%) ASE (%)
Al1-Air V24-23 146,308 31,179 78.69 0
Al1-AlEt V24-45 102,902 99,115 3.68 95
Al1-K800 V24-60 160,907 162,261 − 0.84 101
Al1-LaTr V24-19 128,825 104,028 19.25 76
Al1-PEG1 V24-13 224,601 223,555 0.47 99
Al1-PEG2 V24-63 105,005 104,819 0.18 95
Al1-PEG3 V24-21 159,932 156,425 2.19 97
Al1-Sac V24-34 113,119 81,711 27.77 65
Al1-Sil V24-56 154,675 140,245 9.33 88
As1-Air V28-27 1,333,063 235,173 82.36 0
As1-AlEt V28-23 1,243,400 1,205,707 3.03 96
As1-K800 V28-30 571,461 573,401 − 0.34 100
As1-PEG1 V28-08 731,204 755,998 − 3.39 104
As1-PEG2 V28-10 416,108 421,655 − 1.33 102
As1-PEG3 V28-36 1,121,247 1,145,266 − 2.14 103
As1-Sac V28-07 757,823 604,063 20.29 75
As1-Sil V28-24 1,220,413 1,099,338 9.92 88
Be1-Air V07-23 772,702 151,476 80.40 0
Be1-AlEt V07-18 597,620 565,700 5.34 93
Be1-K800 V07-14 689,166 688,499 0.10 100
Be1-LaTr V07-Exp3 671,849 630,914 6.09 92
Be1-PEG1 V07-09 674,679 671,609 0.46 99
Be1-PEG2 V07-27 699,016 697,988 0.15 100
Be1-PEG3 V07-08 585,750 569,672 2.74 97
Be1-Sac V07-01 609,858 434,048 28.83 64
Be1-Sil V07-37 507,941 389,231 23.37 71
Fi1-Air V22-28 453,549 222,550 50.93 0
Fi1-AlEt V22-09 460,970 440,617 4.42 91
Fi1-K800 V22-33 384,611 371,581 3.39 93
Fi1-LaTr V22-26 360,264 328,850 8.72 83
Fi1-PEG1 V22-22 494,810 478,138 3.37 93
Fi1-PEG2 V22-31 445,544 434,733 2.43 95
Fi1-PEG3 V22-35 500,516 483,497 3.40 93
Fi1-Sac V22-11 480,933 465,727 3.16 94
Fi1-Sil V22-05 434,348 392,397 9.66 81
Fi2-Air V30-Exp1 88,000 49,539 43.71 0
Fi2-AlEt V30-33 427,312 414,753 2.94 93
Fi2-K800 V30-09 495,437 492,590 0.57 99
Fi2-PEG1 V30-04 307,024 306,696 0.11 100
Fi2-PEG2 V30-18 269,965 270,071 − 0.04 100
Fi2-PEG3 V30-15 421,682 411,711 2.36 95
Fi2-Sac V30-17 357,884 338,088 5.53 87
Fi2-Sil V30-24 404,852 386,070 4.64 89
Oa1-Air V10-02 725,355 399,675 44.90 0
Oa1-K800 V10-06 895,292 868,531 2.99 93
Oa1-LaTr V10-33 768,421 705,640 8.17 82
Oa1-PEG1 V10-29 1,008,166 930,839 7.67 83
Oa1-PEG2 V10-31 997,903 883,538 11.46 74
Oa1-PEG3 V10-39 969,533 883,916 8.83 80
Oa1-Sac V10-03 901,297 798,944 11.36 75
Oa2-Air V16-15 512,214 249,991 51.19 0
Oa2-AlEt V16-09 806,657 758,211 6.01 88
Oa2-K800 V16-25 1,214,955 1,160,158 4.51 91
Oa2-PEG1 V16-18 1,002,133 944,715 5.73 89
Oa2-PEG2 V16-32 1,258,876 1,188,430 5.60 89
Oa2-PEG3 V16-36 1,208,876 1,157,459 4.25 92
Oa2-Sac V16-17 1,056,104 961,673 8.94 83
Oa2-Sil V16-13 844,580 725,961 14.04 73
Oa3-Air V27-17 293,261 47,476 83.81 0
Oa3-AlEt V27-40 267,820 250,129 6.61 92
Oa3-K800 V27-07 340,889 338,726 0.63 99
Oa3-LaTr V27-30 266,238 220,194 17.29 79
Oa3-PEG1 V27-12 363,884 362,077 0.50 99
Oa3-PEG2 V27-02 283,059 268,144 5.27 94
Oa3-PEG3 V27-01 289,788 266,534 8.02 90
Oa3-Sac V27-21 316,392 249,489 21.15 75
Oa3-Sil V27-10 379,014 325,680 14.07 83
Pi1-Air V03-01 901,457 693,464 23.07 0
Pi1-AlEt V03-17 771,098 727,558 5.65 76
Pi1-K800 V03-41 1,067,031 1,034,388 3.06 87
Pi1-LaTr V03-28 776,356 755,903 2.63 89
Pi1-PEG1 V03-35 867,035 849,403 2.03 91
Pi1-PEG2 V03-32 983,539 953,583 3.05 87
Pi1-PEG3 V03-20 747,056 721,864 3.37 85
Pi1-Sac V03-45 752,562 734,681 2.38 90
Pi1-Sil V03-42 783,823 739,365 5.67 75
Sp1-Air V23-20 1,062,100 365,165 65.62 0
Sp1-K800 V23-08 906,595 850,773 6.16 91
Sp1-LaTr V23-02 839,765 768,510 8.49 87
Sp1-PEG1 V23-25 1,031,354 1,063,448 − 3.11 105
Sp1-PEG2 V23-30 955,998 944,487 1.20 98
Sp1-PEG3 V23-28 1,053,538 1,047,572 0.57 99
Sp1-Sac V23-29 885,828 771,872 12.86 80

Appendix C: Volume changes of the samples 10 years after conservation considering inner structures from µCT

Sample KUR-No Volume Scan2 (mm3) after conservation Volume Scan3 (mm3) 10 years after conservation Volume µCT (mm3) 10 years after conservation Shrinkage (%)
Scan 2 and 3 		Shrinkage (%)
Scan 3 and µCT
Al1-Air V24-23 31,179 24,760 17,706 20.59 28.49
Al1-AlEt V24-45 99,115 98,384 93,231 0.74 5.24
Al1-K800 V24-60 162,261 161,185 156,812 0.66 2.71
Al1-LaTr V24-19 104,028 102,996 86,192 0.99 16.31
Al1-PEG1 V24-13 223,555 223,018 217,914 0.24 2.29
Al1-PEG2 V24-63 104,819 102,585 100,481 2.13 2.05
Al1-PEG3 V24-21 156,425 154,216 143,465 1.41 6.97
Al1-Sac V24-34 81,711 78,017 68,037 4.52 12.79
Al1-Sil V24-56 140,245 139,209 132,425 0.74 4.87
As1-Air V28-27 235,173 216,694 193,375 7.86 10.76
As1-AlEt V28-23 1,205,707 1,197,414 1,184,689 0.69 1.06
As1-K800 V28-30 573,401 569,276 564,613 0.72 0.82
As1-PEG1 V28-08 755,998 754,140 744,974 0.25 1.22
As1-PEG2 V28-10 421,655 419,105 410,026 0.60 2.17
As1-PEG3 V28-36 1,145,266 1,147,825 1,137,275 − 0.22 0.92
As1-Sac V28-07 604,063 597,524 512,904 1.08 14.16
As1-Sil V28-24 1,099,338 1,089,794 939,898 0.87 13.75
Be1-Air V07-23 151,476 147,738 127,144 2.47 13.94
Be1-AlEt V07-18 565,700 559,303 558,498 1.13 0.14
Be1-K800 V07-14 688,499 686,595 677,402 0.28 1.34
Be1-LaTr V07-Exp3 630,914 630,508 593,651 0.06 5.85
Be1-PEG1 V07-09 671,609 668,946 662,300 0.40 0.99
Be1-PEG2 V07-27 697,988 693,637 681,991 0.62 1.68
Be1-PEG3 V07-08 569,672 566,796 559,662 0.50 1.26
Be1-Sac V07-01 434,048 433,702 427,680 0.08 1.39
Be1-Sil V07-37 389,231 390,118 379,094 -0.23 2.83
Fi1-Air V22-28 222,550 219,359 215,214 1.43 1.89
Fi1-AlEt V22-09 440,617 437,768 434,591 0.65 0.73
Fi1-K800 V22-33 371,581 369,032 364,899 0.69 1.12
Fi1-LaTr V22-26 328,850 326,867 326,097 0.60 0.24
Fi1-PEG1 V22-22 478,138 478,878 478,834 − 0.15 0.01
Fi1-PEG2 V22-31 434,733 436,025 433,782 − 0.30 0.51
Fi1-PEG3 V22-35 483,497 483,900 483,138 − 0.08 0.16
Fi1-Sac V22-11 465,727 464,262 461,471 0.31 0.60
Fi1-Sil V22-05 392,397 393,697 389,657 − 0.33 1.03
Fi2-Air V30-Exp1 49,539 48,737 45,156 1.62 7.35
Fi2-AlEt V30-33 414,753 413,424 409,599 0.32 0.93
Fi2-K800 V30-09 492,590 491,311 486,574 0.26 0.96
Fi2-PEG1 V30-04 306,696 305,457 298,768 0.40 2.19
Fi2-PEG2 V30-18 270,071 269,760 265,858 0.12 1.45
Fi2-PEG3 V30-15 411,711 412,560 403,988 − 0.21 2.08
Fi2-Sac V30-17 338,088 336,763 330,397 0.39 1.89
Fi2-Sil V30-24 386,070 385,594 384,166 0.12 0.37
Oa1-Air V10-02 399,675 375,634 360,929 6.02 3.91
Oa1-K800 V10-06 868,531 852,548 760,510 1.84 10.80
Oa1-LaTr V10-33 705,640 684,208 679,196 3.04 0.73
Oa1-PEG1 V10-29 930,839 925,324 922,177 0.59 0.34
Oa1-PEG2 V10-31 883,538 873,966 871,588 1.08 0.27
Oa1-PEG3 V10-39 883,916 884,323 863,624 − 0.05 2.34
Oa1-Sac V10-03 798,944 800,842 798,595 − 0.24 0.28
Oa2-Air V16-15 249,991 242,620 232,541 2.95 4.15
Oa2-AlEt V16-09 758,211 746,747 746,471 1.51 0.04
Oa2-K800 V16-25 1,160,158 1,159,215 980,995 0.08 15.37
Oa2-PEG1 V16-18 944,715 944,951 927,353 − 0.02 1.86
Oa2-PEG2 V16-32 1,188,430 1,175,774 1,171,048 1.06 0.40
Oa2-PEG3 V16-36 1,157,459 1,148,261 1,131,193 0.79 1.49
Oa2-Sac V16-17 961,673 965,032 960,526 − 0.35 0.47
Oa2-Sil V16-13 725,961 723,683 722,258 0.31 0.20
Oa3-Air V27-17 47,476 44,990 41,674 5.24 7.37
Oa3-AlEt V27-40 250,129 245,888 243,032 1.70 1.16
Oa3-K800 V27-07 338,726 338,272 325,887 0.13 3.66
Oa3-LaTr V27-30 220,194 220,472 189,715 − 0.13 13.95
Oa3-PEG1 V27-12 362,077 360,424 356,083 0.46 1.20
Oa3-PEG2 V27-02 268,144 266,536 258,136 0.60 3.15
Oa3-PEG3 V27-01 266,534 269,946 261,177 − 1.28 3.25
Oa3-Sac V27-21 249,489 246,670 226,558 1.13 8.15
Oa3-Sil V27-10 325,680 318,458 311,940 2.22 2.05
Pi1-Air V03-01 693,464 690,464 669,462 0.43 3.04
Pi1-AlEt V03-17 727,558 720,240 715,479 1.01 0.66
Pi1-K800 V03-41 1,034,388 1,029,360 1,024,444 0.49 0.48
Pi1-LaTr V03-28 755,903 747,161 744,845 1.16 0.31
Pi1-PEG1 V03-35 849,403 848,117 844,877 0.15 0.38
Pi1-PEG2 V03-32 953,583 941,285 937,230 1.29 0.43
Pi1-PEG3 V03-20 721,864 723,021 720,784 − 0.16 0.31
Pi1-Sac V03-45 734,681 736,946 733,326 − 0.31 0.49
Pi1-Sil V03-42 739,365 745,165 738,476 − 0.78 0.90
Sp1-Air V23-20 365,165 355,505 350,248 2.65 1.48
Sp1-K800 V23-08 850,773 842,600 821,603 0.96 2.49
Sp1-LaTr V23-02 768,510 759,002 728,992 1.24 3.95
Sp1-PEG1 V23-25 1,063,448 1,059,582 1,053,036 0.36 0.62
Sp1-PEG2 V23-30 944,487 937,396 933,264 0.75 0.44
Sp1-PEG3 V23-28 1,047,572 1,048,511 1,045,822 − 0.09 0.26
Sp1-Sac V23-29 771,872 766,530 745,213 0.69 2.78

Appendix D: Surface and volume changes of the samples 10 years after conservation considering inner structures from µCT

Sample KUR-No Surface Scan3 (mm2) 10 years after conservation Surface µCT (mm2) 10 years after conservation Surface change (%)
Scan 3 and µCT 		Shrinkage (%)
Scan 3 and µCT
Al1-Air V24-23 12,415 35,734 188 28.49
Al1-AlEt V24-45 14,420 34,211 137 5.24
Al1-K800 V24-60 20,390 49,633 143 2.71
Al1-LaTr V24-19 21,314 86,288 305 16.31
Al1-PEG1 V24-13 23,221 80,711 248 2.29
Al1-PEG2 V24-63 14,962 30,272 102 2.05
Al1-PEG3 V24-21 18,914 140,336 642 6.97
Al1-Sac V24-34 16,940 37,182 119 12.79
Al1-Sil V24-56 18,652 38,461 106 4.87
As1-Air V28-27 35,082 119,044 239 10.76
As1-AlEt V28-23 63,506 107,480 69 1.06
As1-K800 V28-30 44,077 87,917 99 0.82
As1-PEG1 V28-08 48,955 155,842 218 1.22
As1-PEG2 V28-10 37,882 96,622 155 2.17
As1-PEG3 V28-36 65,935 148,547 125 0.92
As1-Sac V28-07 44,404 208,448 369 14.16
As1-Sil V28-24 63,532 220,025 246 13.75
Be1-Air V07-23 26,021 84,639 225 13.94
Be1-AlEt V07-18 44,767 54,400 22 0.14
Be1-K800 V07-14 48,788 67,928 39 1.34
Be1-LaTr V07-Exp3 48,653 168,337 246 5.85
Be1-PEG1 V07-09 48,109 119,721 149 0.99
Be1-PEG2 V07-27 49,745 132,591 167 1.68
Be1-PEG3 V07-08 43,384 73,579 70 1.26
Be1-Sac V07-01 39,509 57,242 45 1.39
Be1-Sil V07-37 37,812 70,962 88 2.83
Fi1-Air V22-28 43,576 60,517 39 1.89
Fi1-AlEt V22-09 53,868 66,645 24 0.73
Fi1-K800 V22-33 50,126 86,374 72 1.12
Fi1-LaTr V22-26 50,801 67,624 33 0.24
Fi1-PEG1 V22-22 55,726 59,109 6 0.01
Fi1-PEG2 V22-31 56,394 78,984 40 0.51
Fi1-PEG3 V22-35 57,206 65,082 12 0.16
Fi1-Sac V22-11 53,339 71,043 33 0.60
Fi1-Sil V22-05 53,077 65,926 24 1.03
Fi2-Air V30-Exp1 8635 19,131 122 7.35
Fi2-AlEt V30-33 32,416 51,646 59 0.93
Fi2-K800 V30-09 37,876 86,145 127 0.96
Fi2-PEG1 V30-04 26,653 90,152 238 2.19
Fi2-PEG2 V30-18 26,381 71,852 172 1.45
Fi2-PEG3 V30-15 34,819 100,612 189 2.08
Fi2-Sac V30-17 29,908 71,373 139 1.89
Fi2-Sil V30-24 31,114 38,846 25 0.37
Oa1-Air V10-02 52,255 85,432 63 3.91
Oa1-K800 V10-06 61,457 131,774 114 10.80
Oa1-LaTr V10-33 57,718 102,110 77 0.73
Oa1-PEG1 V10-29 62,422 88,035 41 0.34
Oa1-PEG2 V10-31 58,833 81,532 39 0.27
Oa1-PEG3 V10-39 61,296 122,801 100 2.34
Oa1-Sac V10-03 56,280 68,620 22 0.28
Oa2-Air V16-15 35,080 59,160 69 4.15
Oa2-AlEt V16-09 57,645 89,005 54 0.04
Oa2-K800 V16-25 81,202 198,276 144 15.37
Oa2-PEG1 V16-18 73,083 151,386 107 1.86
Oa2-PEG2 V16-32 80,898 134,853 67 0.40
Oa2-PEG3 V16-36 78,207 131,352 68 1.49
Oa2-Sac V16-17 73,739 163,807 122 0.47
Oa2-Sil V16-13 56,663 76,156 34 0.20
Oa3-Air V27-17 11,681 23,883 104 7.37
Oa3-AlEt V27-40 29,406 41,819 42 1.16
Oa3-K800 V27-07 31,023 62,587 102 3.66
Oa3-LaTr V27-30 31,972 146,691 359 13.95
Oa3-PEG1 V27-12 31,420 49,818 59 1.20
Oa3-PEG2 V27-02 34,300 81,133 137 3.15
Oa3-PEG3 V27-01 32,071 62,450 95 3.25
Oa3-Sac V27-21 37,746 101,695 169 8.15
Oa3-Sil V27-10 36,179 47,156 30 2.05
Pi1-Air V03-01 56,362 70,166 24 3.04
Pi1-AlEt V03-17 52,635 71,872 37 0.66
Pi1-K800 V03-41 69,860 98,177 41 0.48
Pi1-LaTr V03-28 58,601 63,425 8 0.31
Pi1-PEG1 V03-35 59,624 93,104 56 0.38
Pi1-PEG2 V03-32 65,219 94,048 44 0.43
Pi1-PEG3 V03-20 52,700 68,616 30 0.31
Pi1-Sac V03-45 54,560 61,158 12 0.49
Pi1-Sil V03-42 54,128 72,204 33 0.90
Sp1-Air V23-20 45,100 98,004 117 1.48
Sp1-K800 V23-08 60,128 177,153 195 2.49
Sp1-LaTr V23-02 57,273 206,642 261 3.95
Sp1-PEG1 V23-25 68,803 144,463 110 0.62
Sp1-PEG2 V23-30 68,719 95,985 28 0.44
Sp1-PEG3 V23-28 70,485 107,966 35 0.26
Sp1-Sac V23-29 56,646 226,761 300 2.78

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Stelzner, J., Stelzner, I., Martinez-Garcia, J. et al. Stabilisation of waterlogged archaeological wood: the application of structured-light 3D scanning and micro computed tomography for analysing dimensional changes. Herit Sci 10, 60 (2022). https://doi.org/10.1186/s40494-022-00686-6

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  • DOI: https://doi.org/10.1186/s40494-022-00686-6

Keywords

  • Conservation
  • Waterlogged archaeological wood
  • Volume
  • Shrinkage
  • Computed tomography
  • Structured-light 3D scanning