3.6 THE FINAL DRYING PROCESS
The final step of the consolidation process developed in the project should be a drying treatment mild and efficacious, able to eliminate all the free water from the structure avoiding its collapse and favouring the consolidation of the binders introduced. In the project four different drying systems have been compared, in order to select the most useful one:
✔ air drying at room temperature;
✔ hot air drying;
✔ under vacuum drying;
✔ freeze drying;
✔ dehydration by sequence of decompressions (DDS), a new drying process based on DIC technology
The main part of the research work carried out in the project has been devoted to the optimisation of the DDS drying system for waterlogged wood.
3.6.1 Elements for the comparison of the drying processes tested
In this comparative study, we have tried to use, for each test, for each of the four different drying machines available, identical wood samples; these are elements taken from the same treated or non treated variety cut in four equally large pieces.
Table 3.6.1
The parameters adopted in the course of this comparative study have taken into account quality criteria concerning the waterlogged wood objects treated. They include shrinkage and fissuration rates and final colouring. To these drying time must be added. We have not taken into account the destructive potential of micro-organisms during the process and of the mechanical resistance of the dried wood. It must be noted that with the DDS the temperature inside the treatment chamber varies according to pressure; in the highest part of the curve, as high as pressure P. Thus, to carry out the comparative drying tests between the various processes, we have adopted the pressure according to which the temperature of the treatment chamber approaches that of the other systems used (Ph = 0,50 bar). Before coming to the results of the new drying method (DDS) proposed by our laboratory, we have to mention the results of the drying tests carried out with standard processes for a proper comparison.
3.6.2 Standard drying procedures
3.6.2.1 Room temperature air drying
We have shown in the chapter on characterisation methods how, although it is not completely adapted to the preservation of waterlogged wood, room temperature drying has been used to obtain control samples (for ASE – anti shrinkage effect - determination).
Table 3.6.2
The tests thus carried out have allowed us to verify data that has been reported in the course of various researches on the subject. Deformation – micro- or large fissures or cracks and torsion – is sometimes so great that it makes the wood samples completely unrecognisable. To the point that, very often, it is impossible to carry out measures after room temperature drying. A large number of wood samples had thus to be destroyed to establish control samples, which will be our reference in ASE calculations. For this reason, but also for reasons explained in the paragraph concerning the evaluation of the processes for the conservation of waterlogged wood, we have not carried further the search for control samples.
However, a positive side of room temperature drying is the revelation of the natural colour of the various wood samples.
This might be due to the fact that drying has been carried out in the sunlight, which could be the cause of the chemical reactions causing the change of colour. It should be noted, finally, that results as concern shrinkage and deformation of waterlogged wood, in the case of the natural drying method, are almost the same as those of the conventional hot-air system. The only visible differences concern drying time and the rate of residual water, which are higher than in the hot-air system.
3.6.2.2. Hot-air drying
By analyzing the practical results obtained according to the various tests, it is clear that hot-air drying of the untreated or treated samples is not suited to supply the qualities hoped for.
Table 3.6.2-2
In fact, the drying tests concerning untreated or thermally treated samples, before or after impregnation, at 40°C, 30°C or 20°C respectively, or at the higher temperatures of 50, 60, 70 and up to 105°C, have shown various cracks (radial and axial), deformation and shrinkage.
Table 3.6.2-3
Deformation appears even before the samples have barely lost 20 to 25 % of their water rate (as compared to their wet initial weight). At the end of the drying process deformation is so high that the samples break into pieces. Some samples are unrecognizable; not only has their color changed, but, above all, their dimensions had altered following enormous shrinkage. More often, it is impossible to carry out measurements to calculate shrinkage.
3.6.2.3. Continuous-vacuum shrinkage
Vacuum-oven drying is carried out under different temperatures (from 29 to 60) and varying pressure (from 5,6 to 80 mbar).
Regardless of pressure used, drying carried out under temperatures higher than 40°C seems to cause a very bad quality of the finished products, that show cracks and deformations that vary according to the initial deterioration of the samples. Thus, as concerns alburnum wood, that forms the external part of the tree’s wood, the samples are often recovered as pieces at the end of the drying. As concerns central wood, generally less degraded, deformation is slight. It should be noted that the results obtained are relatively better than those obtained by using hot-air drying.
In the case of temperatures between 25 and 30°C, under a vacuum of 7 to 12 mbar, certain samples, usually the less degraded ones, seem well-preserved and keep their shape, dimension and even colour. Drying time is longer if compared to that of the previous tests.
Since temperature is lower than that of equilibrium of free water at this pressure, this system seems not to permit, after 72 hours of testing, the drying of the samples. It turns out, also, that under 16,2 mbar, heating temperature is not efficacious, as it is lower than 20°C. Drying will then be identical to that carried out without any heating; this will allow sometimes a low sensibility to pressure and temperature inside the chamber during the drying.
Table 3.6.2-4
3.6.2.4. Freeze drying
Our aim was not to study freeze-drying as a part of the proposed process; here, it is taken into consideration only as a reference process, especially coupled with PEG impregnation and, more restrictively, with ordinary sugar.
However, beside these purely comparative tests, some samples impregnated with starch (and with a mixture of starch and PEG 400) have been freeze-dried. The results thus obtained have in fact shown that freeze-drying is a very interesting method as concerns the drying of waterlogged wood. It seems, in the particular case of very spongy samples, to be able to make the sample retain its initial dimensions.
However, the mechanical fragility of this type of very spongy wood is a weak point that only impregnation seems to be able to resolve, at the price of a degradation in appearance and quality. Following this, freeze drying of previously thermally treated wood yields much better results.
This improvement can be still higher if this type of wood, before the thermal treatment, is impregnated with granular starch or, preferably, starch-dextrin. Freeze-drying of wood that has been impregnated with starch-dextrin, but that has not undergone a thermal treatment can lead to middle-quality products.
3.6.3 Dehydration process by successive decompression (DDS)
3.6.3.1 Presentation of DDS
The new drying process thus denominated is based on the principle of instantaneous controlled decompression (D.I.C) (Allaf et al. 1992). D.I.C., applied to humid products exposed to the pressure of water vapour (therefore, at a high temperature) induces an instantaneous vaporisation in a vacuum, accompanied by the cooling of the material.
The DDS drying process is based on this system. The same equipment is used for the texturation of a product (D.I.C function) as well as for its drying (DDS function). Under certain conditions of pressure and temperature the DIC process can be used in the field of the extraction of essential oils (Rezzoug et al. 1997).
Figure 1: Block diagram of the equipment used for the lab test with DDS
Drying by DDS consists in subjecting a material placed inside the treatment chamber (Figure 1) to successive alternated phases of compression and decompression forming cycles that are repeated until the desired water content of the material is obtained. Each cycle consists of a controlled pressurisation by means of compressed air, followed by an equally controlled depressurisation, often to a vacuum. To this end, a large container, connected to the treatment chamber and to a vacuum pump allows a constant level of vacuum during the process.
The levels of pressure and vacuum are predetermined according to the operating conditions requested by the material. For drying tests to different vacuum pressures with the same high-plateau pressure, the vacuum tank has been equipped with a manual air-injection valve that allows us to maintain the level of vacuum needed.
The machine is guided by an automatic programme in which operating parameters have been introduced. This avoids the need for human intervention during the process except for the short intervals needed to weigh the samples. It allows us also to preserve the constant operating parameters in the course of the chosen operating cycle on one side, and in the course of the drying on the other.
Figure 2: Theoretical diagram of the evolution of the values of pressure/temperature in the treatment chamber during DDS
The drying tests have highlighted the necessity to follow the evolution of the water content of the material. In experimental practice there is a frequent reference to drying curves.
3.6.3.2 Operating parameters
The preliminary drying tests with DDS have been carried out using all the types of archaeological wood samples. We have also carried out some tests on fresh Douglas fir. The differences in pressure Dp, the values of which were, respectively, 3,0 bar, 2,0 bar, 1,5 bars and 1,0 bar have been analysed in sequence. Dp is the difference between the pressure values inside the chamber with a high plateau (Ph) i.e. after the introduction of pressurised air into the autoclave is over and those of the pressure obtained with a low plateau (Pb) i.e. at the end of the instantaneous decompression.
Figure 3 (left): SEM photo of a sample of wood 6 E impregnated with starch, thermally treated and dried by DDS.
Figure 4 (right): SEM photo of a sample of wood 18 E impregnated with starch, thermally treated and dried by DDS.
The results obtained with the first tests seem to show that this drying system has been systematically connected to cracks which were proportional to the difference in pressure. The fissuration process, already observed in D.I.C., seems also to take place at this level of pressure, although the temperature is relatively low.
Only an important reduction of the difference in pressure Dp, according to experience, is able to reduce or eliminate this phenomenon.
Let us note, moreover, that the DDS tests carried out with a decompression to atmospheric pressure (e. g with Ph = 3,0 bar to Pb = 1 bar of absolute pressure) do not ensure the proper drying of wood. Various other tests have been carried out with a high-plateau pressure level Ph=1bar, carried out on samples impregnated with dextrin. The constant presence of fissuration proved clearly that the DDS drying of waterlogged wood requires particularly «soft» operating conditions.
These observations have led us to privilege, always maintaining a low difference in pressure Dp, the adoption of a domain of low pressure (Pb from 25 to 60 mbar). This is the reason why we have worked with high-plateau levels Ph from de 0,50 to 0,80 bar.
Further results have shown that the DDS, applied in these relatively «soft» conditions (hP = 500 - 800 mbar, and lP = 25 - 80 mbar), is systematically more suitable than the other drying systems, specially in the case of a continuous vacuum.
This has almost never been contradicted; the internal structures of are well-preserved, as it is shown in the photos taken with the SEM (Figure 3 and Figure 4).
3.6.3.3 Impact of the DDS on the quality of waterlogged wood: Comparative experimental analysis.
The DDS drying process is characterised by its very great effectiveness as concerns dehydration in itself. As noted above, the main negative effect as concerns quality in the case of DDS-drying of waterlogged wood is cracking, while the preservation of colour seems to be assured.
The Table 1 provides a summary of the quality of the samples according to the operating parameters of the DDS and wood types.
Table 3.6.3-1
Coupling the DDS with the two previously optimised stages of impregnation (mainly with starch) and thermal treatment has given very interesting results as concerns the preservation of quality in waterlogged wood. The strongest quality point as seen in Table 2 concerns shrinkage. Here, although some negative values betray cases of shrinkage or swelling, the percentage of volume shrinkage R might well indicate, as a first approximation, the dimensional preservation of the wood treated. The colour distribution symbolising quality in Table 2 highlights the enormous possibilities of the DDS. Only in the case of particularly spongy samples (ultra-degraded wood, for ex. ECBL) the results seem not to be of a high quality.
Table 3.6.3-2
Following the results shown in Table 2, we can draw this first conclusion: the quality of the samples treated with the DDS is of the same level of that of freeze-dried samples.
A second conclusion is evident by looking at the table: the importance to proceed starting from a low value of the maximum pressure in the chamber (Ph).
A third conclusion concerns the complementarity between the DDS and the two «pre-treatments», that is impregnation (with starch) and the intermediate thermal treatment. The best quality of the finished product can be achieved through the optimisation of these three steps.
Finally, DDS drying is distinguished, in addition, by a higher drying speed than in other processes, with a much lower final humidity level.
3.6.4 Advantages and unresolved problems of the DDS process
3.6.4.1 Analysis of the experimental results
To begin with, let us note that the experimental results and the kinetics analyses on one side, and quality determination of the finished product on the other, have shown an enormous difference between the DDS process and continuous vacuum drying.
Drying speed seems to be systematically better in the case of the DDS as compared to that of continuous vacuum drying. This is all the more valuable since the level of pressure in vacuum drying ranges from 6 to 16 mbar as compared to 25 to 50 mbar, with the DDS. Other processes must then be introduced to facilitate water removal and the preservation of structure.
Similarly, we have shown how deformation in samples which have undergone a thermal treatment is less pronounced then in samples which have not. This has been verified both in the case of samples taken out of the water as they were (that is, not impregnated) and in that of those taken from an impregnation test. The shrinkage values of dried samples that could be dimensionally measured are shown in the tables shown in this chapter. Improvement, as concerns the reduction of shrinkage brought by the thermal treatment, ranges between 16 and 30%, depending on the type of wood, but may exceed 50% in the case of certain samples. This is a very substantial improvement, since the wood samples analysed in the tests were not subjected to impregnation, and therefore no consolidating material was added.
It must also be noted that these values concern only the samples showing positive shrinkage (expansion). In the case of most wood samples not having undergone a thermal treatment or cracks, or having negative shrinkage values, a thermal treatment would cause higher values. Examples of this were the wood samples named 9 E (137%) and 16 E (265%), for which values would seem too high. The inclusion of these values in that figure hides the values of the other wood types. There are also various other wood samples, in the case of which exaggerated deformation, caused by room temperature or hot air drying, did not permit final shrinkage measurement and thus the evaluation of shrinkage rate.
To conclude the presentation of the results concerning the comparative drying tests, it is worth noting that, among the four drying methods studied, the DDS has yielded the best results. Not only because it is comparable with freeze-drying as concerns dimensional stability for most of the wood types studied, and only slightly less good for other types, but above all for its drying speed and the esthetical quality of the wood samples treated with it.
The DDS always appeared the most rapid among all the considered drying processes while the freeze drying the slowest. So, especially because of the quality offered by DDS, that satisfy the aims of this research, we adopted it as main drying method in the restoring treatment set up for the waterlogged woods.
Moreover, the different results of the drying tests before the impregnation phase brought us to choose a similar procedure. For any developed technical improved here (DDS, freeze drying, continuous vacuum or warm air) the drying, used as pre-treatment for the impregnation phase was excluded by the developed method. The reason is simple: it’s justified because of irreversible structural deformation caused by the initial dehydration of the wood sample. The deformation avoid the successive re-hydration of the dry wood. The tests that we performed, applying the freeze drying of a sample lot before the impregnation, demonstrate that result. It has became very difficult, sometime impossible for the wood sample, to adsorb some liquids after a period in a solution of mixtures of starches (malto-dextrin, granular starch, dextrin of the type of pure dextrose)
The results of these tests showed an irreversible existence of a mechanism induced by the drying operation on the internal structure of the wood. Even if we couldn’t measure the quantity of block or punctuation closed in the dried wood samples, we observed the impossibility of the dried sample to re-assorbe the water contained in the impregnating suspensions, and this corroborate the ending of the experiences performed by other authors on this matter [Tesoro et al. (1974); Wardrop & Davies, (1957)].
It would be interesting to understand better the strength and the performances of this DDS procedure, understanding where these characteristics come from. It’s important to understand the «mother» principle of DDS, as the DIC, to try to answer to these questions.
We can’t answer that, as the DDS is constituted by a series of controlled instantaneous releases, they behave like the process they come from. Only the conditions in the treatment chamber have been modified. The high temperature and the strong presence of steam in the treatment chamber during the DIC treatment have been considerably reduced and nearly eliminated. In the DIC case the water steam at 150°C heats the water contained in the treated alimentary products causing the gelatinization and the fusion of the contained starch. On the contrary, by DDS the temperatures reached by the compressed air when put in the treatment chamber are relatively too low, to bring the water to its evaporation temperature. They don’t allow the starch to reach its gelatinisation and fusion temperature in the short duration of the treatment cycles. As the vaporisation depends on temperature, the same product will result less expanded by DDS than by DIC. We shall remind that, in the working conditions of DDS, some cracking appear very rapidly in the wood specimen when the delivery pressure of pressurized air increases.
3.7 EVALUATION OF THE OBTAINED RESULTS
3.7.1. Analysis of the treated wood quality
We present and comment the different results on quality measurements as the dimensional stabilization, the mechanical measurements, the porosity and the colour analysis.
3.7.1.1 Dimensional stabilization measurements
3.7.1.1.1 Adopted procedure for the measurement of the dimensional stability
Using a centesimal calibre, we measured the dimensions of the samples of wood before and after the treatment.
Figure 1: Adopted method for the dimensional measurement of wood samples.
On these figures the wood show horizontal fibres
The measurements are performed in the way indicated in the above mentioned figure. In each direction - axial (a), radial (r) and tangential (t) – the dimension are taken, at least, in three places, necessary one in the middle and one at the two extremities. For each dimension, the measure is performed doing a complete tour of the sample as indicated in the circulation sense represented by the arrows.
The average if the measured dimensions is taken for each dimension.
3.7.1.1.2 Adopted procedure of the evaluation of the shrinkage
To evaluate the good results of a conservation procedure the various investigators took accordance to adopt a control parameter defined as. «Anti–Shrink Efficiency, abbreviated as A.S.E.». A.S.E is defined as the percentage of shrinkage of the wood that has been eliminated by the applied process. It’s expressed with the following procedure:
in which,
✔ Vc represent the retraction of a control sample obtained drying the same kind of wood in air between 10 and 20°C.
✔ Vt the coefficient of retraction for the treated sample with this conservation method.
As for Vc, the coefficient Vt, is determined by ratio between the final volume of the sample and its initial volume in waterlogged conditions.
Two observation must be signalled about the expression adopted to calculate the A. S. E.
It’s possible to state that, to characterize the success of all the conservation procedure of waterlogged woods would be better to use an evolution of the ASE coefficient. In fact it’s clear that any kind of wood (also fresh wood) when put under waterlogged conditions swells. The reference volume should thus be measured under waterlogged conditions, and can be assimilated to a volume in saturated conditions (Vs).
Afterwards the difference, introduced in the determination of the shrinkage level of the waterlogged wood by comparison to the one of fresh wood, consists in comparing the dimensions measured on treated wood samples to the dimensions of a reference sample simply dried in air between 10 and 20°C.
It’s necessary to underline that, on the contrary of the fresh wood, the shrinkage values of the waterlogged wood in the longitudinal or axial sense are quite elevated, they can’t be easily ignored.
This means that the shrinkage effects, expressed through the definition of ASE, are more accentuated than the fresh wood.
Consequently some investigators consider, in addition to the directional shrinkages, the products of radial and tangential shrinkage to calculate the volumetric shrinkage using this expression:
The choice of the indicator parameter ASE has been justified by Grattan et al. (1980). It’ll be a good system to have successful conservation method because of the under mentioned reasons:
✔ ASE is constant for different types of wood and different levels of degradation even if there is variability in the shrinkage values of control samples (the samples not treated but only dried at open air between 10 - 20°C).
✔ An A.S.E value of 100 means that the treatment is perfect and corrsponds to zero shrinkage. An ASE value of 0 means an absolute failure of the treatment applied;
✔ For a wood sample having an high shrinkages at open air (control sample), the corresponding A.S.E value must be the highest possible to valorise qualitatively the conservation process;
✔ A value of the parameter A.S.E superior to 75 % is generally considered as reasonable. The more elevated is the value A.S.E obtained from a conservation process, the better is its success.
Even though there are many advantages for the use of A. S. E, the main problem of its use is the obtaining of the control sample i.e. the sample dried on open air. Very often, the control samples are difficult to obtain. It’s because of the great quantity of deformations during their desiccation that imply a big loss of wood samples to obtain a dry sample of measurable dimension. Many of the samples studied in this work fall in this situation.
We think that the principle of definition of a quality parameter is surely logic. But, even if the parameter ASE is based on a control sample it seems to lack in unanimity. Some authors use the parameter ASE, but others prefer to calculate the shrinkage directly (Scano et al. 1994). Others express the two parameters (Kazanskaya et al. 1990). The best way would be to take the measurements on the same sample before and after the treatment. In fact the control sample in the ASE calculation is not the one to which the treatment is applied.
Taking into account the heterogeneity of the wood and probably the one induced by the deterioration, the two specimen will not initially show an identical shrinkage if they are treated with the same method (open air ). More, it’s illusory to propose a solution consisting on cutting, for each sample of wood to be treated, its «twin» to be used as control (that should thus be sacrificed). So, it’s better to precise that the dimensions measurement gives relatively good results for the control samples of low degraded wood while it’s surely difficult to obtain reasonable results for the very degraded wood. The excessive deformations resulting from the drying at open air avoid all the measurements and the production of only one useful sample needs the sacrifice of an huge amount of waterlogged wood. Moreover, it’s possible to obtain a swelling of the wood instead of a retraction. In our opinion, the acceptation of ASE as characterization parameter will require some important conditions. The control sample should be completely exempt by fissures and cracks visible with the naked eye both in the external and internal part. A deformation like that can influence the ASE value both in the positive and the negative sense. For this reason, the shrinkage measurements performed directly on the samples before and after the treatment, included in the same ASE expression, are sufficient to evaluate the process. On the basis of the ASE definition and of the lower limit fixed for acceptable ASE (75%), it’s possible to determine the corresponding limit of relative shrinkage: so will be possible to relate it to the shrinkage value characteristic for a fresh wood sample (of the same specie of the waterlogged wood sample) passing from a water saturated state to a dry state, even if it’s very rare to obtain these results with an ancient waterlogged wood sample.
3.7.1.1.3 Note for the evaluation of the results obtained
The results of the test on dimensional stability and shrinkage of treated wood samples have been already shown in the paragraphs related to intermediate thermal treatment and final drying.
We would like to underline that the better way to present the obtained results is the use of tables. We have also to remember that the presence of some negative values for ASE and relative shrinkage is depending from the abnormal deformation of the treated sample or of the control sample caused by cracking or fissures formation.
3.7.1.2. Mechanical properties
3.7.1.2.1 Notes on the measurements
The samples of waterlogged wood impregnated with starch, after thermal treatment and drying, has been tested for the mechanical resistance. Some samples, not treated but simply dried at the open air and some fresh wood samples, have been analyzed.
The tests have been executed with an universal plant of mechanic tests, model TN-MD – 1799. This equipment is connected to a micro computer and is appropriated for this kind of measurements.
We have tried to describe the principle of the two ways of measurement of the mechanical resistance. It consists in the compression test on the axial direction of the fibers – the longest part of the wood fibers- and in the penetration test in transversal direction (perpendicular to the wood fibres).
In the mechanical tests realized on the treated waterlogged wood samples, we measured the resistance of the wood (expressed in terms of maximal resistance) to the axial compression and to the penetration by the conic penetrometer. The kind of rupture noted to these solicitation has been observed.
3.7.1.2.2 Axial compression
The principle of this test consist in the determination of the compression resistance of the sample on the direction of the axis of the fibers of wood till the rupture of the sample.
The sample is placed in a measurement cell formed by the space between two compression lamina of the compression equipment as shown in the following figure:
Figure 1 (left) – Principle layout of axial compression. (Fibers of wood in the vertical direction) and Figure 2 - (right) –Principle layout of the conic penetration test (Fibers of wood in the horizontal direction).
Following the norm NF B 51-007 related to the measurement of the mechanical properties of the fresh wood, to which we refer, the sample must have the shape of a right prism. The base of the prism must be of 20 mm and the height of the prism 60 mm in the direction parallel to the fibers of the wood. Because of some problems on the sampling of the waterlogged woods, the dimension of the samples were the half of the ones required. So the height is reduced to 30 mm instead of 60 and the base is of 10 mm instead of 20 mm. So the ratio between the require dimensions and the samples of the wood to be analyzed have been respected..
The tests have been performed at constant plate speed and lost between one and two minutes, according to the norm .
For some tests the maximal resistance has been determined and the kind of breaking observed indicated with the numbers 1 or 2, according to the norm NF B51-007, that concerns also the dimensions of the samples to be used.
3.7.1.2.3 Pseudo – penetration
It consists in the measurement of the side penetration resistance on a direction perpendicular to the wood fibers axis (Figure 3) that corresponds to a penetration performed in a direction tangential to the growing circles of the wood. The penetration equipment has a conic form, with a top angle of 30°. This value has been determined on the basis of some preliminary test because the pseudo penetration test hasn’t been normalized. In these tests, the wood resistance (= the applied load) has been measured for a penetration of the cone of 4 mm.
We precise that all the kind of mechanical resistance measurements initially foreseen have not been realized because of the important sample dimension required. It’s the case of the bending strenght test that, following the norm, needs pieces of sample of dimensions: a * r * t = 4’’ * 1’’ * 1’’ or: 100 * 25 * 25 mm.
Figure 3 : Principle layout of the bending strength test. (wood fibers in horizontal position).
This sample length is considered too big to be obtained from an archaeological object for the quality control of the final product on a real scale process. Moreover, on the contrary of the compression test in which we have adopted a size reduction maintaining the ratio among dimensions, the minimum dimensions needed for the equipment calibration are really more significant. So this did not allow to test small dimension samples.
It’s anyway necessary to state that the most part of the characterisation tests is destructive for the sample. To find all the information on the modification inside the tested wood samples, we used, as already mentioned, the SEM and magnetic nuclear resonance (RMN).
3.7.1.2.4 Presentation of the measured parameters
Two parameters have been measured: maximal resistance to compression (in MPa) applied by two plane plates and the load applied to allow a penetration of 4mm by a conic drill (in N).
Figure 4: Example of curve strength/distance obtained during the measure of compression and penetration
During both the mechanical tests the distance between the point of application of the load and bearing points of the sample have been measured (this distance is limited to 4 mm in the case of penetration tests).
The strength applied F is equal to the absolute value of the resistance opposed by the sample during the compression and penetration. One of the curves strength-distance (F-d) obtained in these measurements is shown in the Figure 4; it express the energy given (area below the curve) to modify the sample structure.
3.7.1.2.5 Results of the measurements
These quantitative and qualitative measurement are used to find out the increasing of resistance (consolidation) given to the wood structure by the treatment applied. The results are grouped in the following table.
Table 3.7.1.2-1
The comparison of the results in different drying processes from one side, and among the treated samples on the other, must be examined with precaution.
All the hurried conclusion can bring to some interpretation mistakes. We have so extracted some results from this table: An abstract of the comparable results are shown in Table 2.
| Axial compression (MPa) | Conic penetrometry (N) | Wood samples | DDS | FREEZE DRYING | DDS | FREEZE DRYING | 7 E | 42,8 | 11,3 | 330 | 37 | 13 E | 6 | 4,7 | 476 | 341 | 14 E | 25,3 | 10,6 | 247 | 28 | 15 E | 22,5 | 17,1 | 228 | 103 | 15 E | 6,2 | 5,3 | 130 | 73 | ECBL | 0,7 | SPM | 57 | / |
Table 2: Abstract of the obtained results for the mechanical measurements in axial compression and conic penetrometry. [ SPM: Sample microfractured; / :not measurable].
The most part of the soft freeze dried samples (AF9, DN5-BC, I-34, all the samples Cp) are crushed during the mechanical tests in the same way as the samples with code ECBL at the end of the table. Many samples of this kind of wood couldn’t be tested because too friable and inadequate to be measured.
3.7.1.2.6 Discussion of the results
To try to understand the difficulties to find some formal conclusions for the results of these measurements, we have analysed a wood specimen taken from a single sample (in the radial direction) with very short length, in order to minimise the anisotropy of the material. We cut the wood in two identical parts, and we can identify the two samples as 1 and 2 in the following figure.
Let’s suppose that the two specimen have received the same treatment but a different drying process: the sample 1 by DDS and the 2 by hot air till the reaching of the same residual water content.
The Figure 5 gives an indication on the obtained results, with an very big shrinkage for the hot air dried samples compared to the other.
Let’s make the hypothesis that the hot air drying is performed at a relatively low temperature without any chemical evolution and any extraction from the wood. So, only the shrinkage discriminates the two samples.
In these conditions the structure of sample 2 is more compact and well packed because of the drying in hot air: that means a mechanical resistance surely higher than the sample 1. This last one, with a much better dimensional stability, is more porous.
Compared to the samples dried by DDS, the freeze dried samples show this type of behaviour: a very good dimensional stabilization coupled with a low mechanical resistance.
The samples of soft wood, freeze dried and so more porous than the hot air dried, are crushed by simply friction by fingers.
Figure 5: Schematic behaviour of the treated wood after mechanical analysis (explanation model).
Figure 6: Behaviour of some samples of treated wood after axial compression tests. The specimen of the same wood dried with warm air are generally completely deformed, with high presence of big fissures. Nevertheless these samples are better consolidated than those dried by continuous vacuum, DDS and freeze drying, following this order. The analysis of the obtained results on the mechanical tests bring us to the conclusion that the «dimensional stabilisation» and the «mechanical resistance» are two contradictory criteria to determine the quality of the restoring treatment system – conservation of the waterlogged woods. The mechanical resistance can’t be improved maintaining at the same time the shape of the treated sample. The typical example for this argument is the freeze drying that gives, the best results for the parameter of the dimensional stability of the treated samples and the worst one concerning the mechanical resistance both on axial compression and lateral conic penetration.
Figure 7: Behaviour of some sample of treated wood and tested with conic penetration. Regarding the above figure, the question is how the effect of consolidation by impregnation is concretely manifested. To answer we need to consider two identical wood samples differently treated i.e. one of the two samples is impregnated and treated with the intermediate thermal treatment and the other not. Both are freeze dried afterward. The difference is evident: the impregnation has an enormous reinforcing effect. As we announced at the conclusion of the starch impregnation tests from one side, and taking into account the structure of the wall cells from the other side, the material that can assure a complete saturation and that can polymerise will confer an improved dimensional stability and the mechanical resistance. Someone could say that the saturation is a palliative and not a success because the wood at its normal fresh state has some free spaces. These free spaces assure the capillary pressure responsible of the introduction of the nutritive substances from the roots and the leaves to the internal part of the tree. So, for us it’s much more interesting to obtain, by impregnation, a better mechanical resistance, a very low shrinkage effect, a wood aspect more natural, etc. This combination of results represent effectively the final aim of the consolidation process developed in this research.
3.7.1.3 Porosity measurement of the treated wood samples
3.7.1.3.1 Preliminary remarks
Some observation before presenting the results:
The first is related to the a restrict number of comparative measurements performed for the porosity because of the excessive cost of the operation.
The second concerns the samples of wood dried by hot air and continuous vacuum . After the subdivision of each sample in several specimens, according to the experimental plan developed, all the specimens have been utilized in different test and was not possible to find out other specimens to substitute some element damaged during classical drying processes. So the sample (1) in the following table, dried by hot air, has been totally deformed during drying; but, even if it it’s very small, that sample will be examined because it represent very well this drying method. These results are in accordance with all the observation and with the results of the measurements carried out before, also as concerns the presence of some abnormal values, i.e. some negative value of shrinkage.
The third consist in a short comment on the procedure. In fact it’s important to underline that the sample submitted to freeze drying did not have received any intermediate thermal treatment in order to avoid problems in the effective explanation of the results obtained.
The last observation concerns the absence of a wood sample that could be used a real witness according to the fact that the waterlogged one can’t be taken into account because of the technical problem imposed by the mercury porosimeter technique: the maximum moisture content allowed on the material to be analysed.
3.7.1.3.2 Presentation of the measurements results
The results of the performed tests arte collected in Table 1. The wood samples used to this purpose come from the samples coded ECBL (the structure has already been presented). The different treatments of the samples of similar size have been indicated in the legend of the table.
Table 3.7.1.3
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