A Method for Accelerated Natural Weathering of Wood Subsurface and Its Multilevel Characterization

Round 1

Reviewer 1 Report

I have some doubts about the short time available for research. However, the authors pointed out that despite the short time, similar results were obtained as in the case of standard long-term natural weathering tests. Therefore, recognising the knowledge and experience of the authors in this area, I accept this time.
However, one disadvantage is the lack of information on what weather conditions were in July in the place where the research was conducted (average temperature, rainfall, relative air humidity....). The authors only stressed that July, according to the results of previous studies, is the most severe for the process of weathering wood microsegments. Therefore, it would be worthwhile to bring these conditions closer to the reader.

Author Response

Dear Reviewer 1

Thank you very much for your time in reviewing our manuscript as well as for valuable comments and critics. We have deeply discussed your suggestions and modified the manuscript accordingly. Please find below our responses to all points raised in your Review. We do sincerely hope that we correctly interpreted your concerns and our modifications (answers) fulfil your requirements.

I have some doubts about the short time available for research. However, the authors pointed out that despite the short time, similar results were obtained as in the case of standard long-term natural weathering tests. Therefore, recognising the knowledge and experience of the authors in this area, I accept this time.

Thank you very much for your trust. We do understand the doubt of Reviewer regarding reliability of our approach. We would like to declare that we did all our best to assure as objective as possible assessment and interpretation of our results. The only subjective decision was when estimating an approximate acceleration factor, that for several reasons is perhaps impossible to be determined more objectively. We carefully confronted our results (interpretations) with the literature references and on this basis identified an accelerated nature of weathering of ultra-thin samples. It is clear, that the method should be validated by others to confirm its suitability. We revised the manuscript following several comments of Reviewers, including also a critical discussions of the limitations. We sincerely hope that it may improve the reliability of our report.

However, one disadvantage is the lack of information on what weather conditions were in July in the place where the research was conducted (average temperature, rainfall, relative air humidity....). The authors only stressed that July, according to the results of previous studies, is the most severe for the process of weathering wood microsegments. Therefore, it would be worthwhile to bring these conditions closer to the reader.

The comment is important and we addressed it in the revised manuscript. A separate Table 1 was prepared for reporting the weather conditions in month of July (for ultra-thin samples) as well as Table 2 for the whole year average measures (relevant for the thick reference samples). As mentioned in our manuscript, we do intend to develop a dedicated dose-response model for the natural weathering of wood. The detailed climate data are therefore indispensable to achieve that goal. In addition, there are several discussions regarding the specific weather in San Michele All’Adige (Italy) recorded during our test, provided in discussion chapter.

Summarizing, we would like to thank once again for all the kind comments and suggestions. We do believe that we addressed all of these, and the revised manuscript may fulfil your expectations.

With sincerely regards,

Jakub Sandak, on behalf of authors

Reviewer 2 Report

In an attempt to contribute to the improvement of this manuscript, I suggest the following:

In introduction chapter, you refer that several efforts have been implemented aiming to shorten the time of natural weathering, but you do not refer to the established methods of accelerated weathering testing, which are indeed very fast and reliable processes that base on specific standard testing processes and provide comparable results.

In lines 81-83, if you use extra thin specimens, as you propose, I am afraid that you will totally alter the performance of wood. You can not test the performance of the surface of wood by leaving only the layer of surface of wood by itself, since this will have a totally different behaviour than wood (cracks, dimensional changes, anisotropy etc.). You alter mechanical degradation by leaving only some layers of cells and therefore you weaken the specimen. Probably, you could apply such thin specimens only in testing of sliced veneers performance against weathering.

You refer "They entail further internal stresses of material imposed by cyclic wetting and temperature changes.", though it is more likely to use the mentioned dimensions of wood in the structures or even bigger elements, than thin layers of wood.

In methods section, concerning the raw material, you refer only "one piece of Norway spruce wood", without providing any other details such as where did you get it from, density, sap- or heartwood etc. I am not sure if a test and its results could be reliable, when there is only one piece of wood examined and there is not any replicate using other parts of the same tree or different trees. Referring to the piece of wood you used, where exactly did you get the sliced specimens from? how many growth rings did you include? did you include pith?

I feel that the significant information of the mean temperature and mean precipitation values for the period of exposure should be removed from the discussion and placed in the methods section. Lines 389-392 as well. In line 400, venerable or vulnerable? In figure 2, the appearance of the specimens after the exposure reveals that probably this result would not be so useful, since there would not be structures with so thin wooden elements, since even the veneers are applied on surfaces of bigger elements/wood products. The lines 434-447 and the methodology of thick specimens characterization should be analyzed in methods section, providing all the necessary details (mean temperature, precipitation etc.). Please, provide image of the thick specimens used.

You should try to highlight the significance of your results and explain where such thin specimens could be found/used/exposed or what different provide compared to the already established knowledge of weathering impact on wood.

Author Response

Dear Reviewer 2

Thank you very much for your time in reviewing our manuscript as well as for valuable comments and critics. We have deeply discussed your suggestions and modified the manuscript accordingly. Please find below our responses to all points raised in your Review. We do sincerely hope that we correctly interpreted your concerns and our modifications (answers) fulfil your requirements.

In an attempt to contribute to the improvement of this manuscript, I suggest the following:

In introduction chapter, you refer that several efforts have been implemented aiming to shorten the time of natural weathering, but you do not refer to the established methods of accelerated weathering testing, which are indeed very fast and reliable processes that base on specific standard testing processes and provide comparable results.

We do agree with the Reviewer comment and consequently included the following text in the introductory chapter:

“An artificial weathering is an established alternative for testing of material’s resistance to deterioration due to climate related factors. The European standard EN 927-6 [10] describes such method that is frequently used to assess wood coatings. It uses a specially designed device equipped with temperature control, light radiation by means of fluorescent UVA lamps, water spray and water condensation units. Other, equally com-mon methodology relays on testing weathering resistance by simulating the Sunlight irradiance by means of xenon lamps [11]. Podgorski et al. reported relatively good agreement for the development of cracking when compared to results from natural weathering of coated wood, with the estimated acceleration factor of ×5 [12]. Even higher acceleration factor of ×10 was identified there for a loss of surface glossiness. Development of cracking was faster in artificial weathering tests due to higher doses of degrading factors provided in shorter cycles but with precisely controlled exposure conditions. Though, most artificial weathering methods are lacking biological impacts as well as the influence of dirt that are present in typical exterior exposure environments [13]. Özgenç noticed significant differences in the colour alteration as well as in compression strength along fibres for samples subjected to the natural and artificial weathering conditions [14]. It was demonstrated that the reason for such differences laid in the lack of several minor factors that are not included in the artificial weathering protocols. These factors include air pollutions, biological pests and snow as well as difference in the spectral composition of light irradiation, nature of the rain-driven wetting and humidity variations occurring under natural conditions. Similar observations were reported by Ruther and Jelle, where solar radiation and wind-driven rain were identified as the main factor inducing colour changes [15]. However, the surface greying typically observed for the natural wood exposed to exterior conditions is caused by mould growth and is hardly replicated in up-to-data artificial weathering tests. Outdoor exposed samples become usually darker at the first stage of weathering, to turn in to lighter tones afterward. This is supported by both visual assessment of inspectors as well as by objective spectra/colour measurements. On the contrary, samples exposed to the laboratory weathering cycles turn darker to varying extend and do not exhibit initial increase in CIE a* and CIE b* values, typical to samples exposed outdoors. In addition, it was found that samples exposed over a longer period to artificial weathering show a form of unnatural bleaching, not present in wood samples exposed to natural weathering [15]. The overall conclusion is, therefore, that even if several advantages of artificial weathering make this an interesting testing methodology, the results from laboratory weathering can hardly be compared to the real outdoor weathering. The reason lays in different mechanisms and kinetics of the material deterioration, that are especially di-verse in complex biological composites such as wood.”

In addition, following references were also included in the revised manuscript:

BS EN 927-6. Paints and varnishes. Coating materials and coating systems for exterior wood. Exposure of wood coatings to artificial weathering using fluorescent UV lamps and water. Brussels: European Committee for Standardization, 2018.

Arnold, M.; Sell, J.; Feist, W.C. Wood weathering in fluorescent ultraviolet and xenon arc chambers. For Prod J. 1991, 41(2): 40–44.

Podgorski, L.; Arnold, M.; Hora, G. A reliable artificial weathering test for wood coatings. Coatings World, 2003, 2: 39–48.

Grüll, G.; Tscherne, F.; Spitaler, I.; Forsthuber, B. Comparison of wood coating durability in natural weathering and artificial weathering using fluorescent UV-lamps and water. Eur. J. Wood Prod. 2014, 72: 367–376.

Ozgenc, O. Comparing Durability of Wood Material in Natural and Artificial Weathering Conditions. ProLigno 2016, 12(4): 3-11.

Rüther, P.; Jelle, B.P. Color changes of wood and wood-based materials due to natural and artificial weathering, Wood Mater Sci Eng. 2013, 8(1): 13-25

In lines 81-83, if you use extra thin specimens, as you propose, I am afraid that you will totally alter the performance of wood. You can not test the performance of the surface of wood by leaving only the layer of surface of wood by itself, since this will have a totally different behaviour than wood (cracks, dimensional changes, anisotropy etc.). You alter mechanical degradation by leaving only some layers of cells and therefore you weaken the specimen. Probably, you could apply such thin specimens only in testing of sliced veneers performance against weathering.

This is a very important critic, that may tremendously impact the useability of our (proposed) method. In general, we do agree with that comment, but would also like to defend this approach. It is true that we are loosing a complex interaction of different layers within material bulk, not to mention a dramatic change to the diffusion of moisture and heat within bulk material. However, in the context of the (natural) wood weathering it is clear that the most relevant changes occur in the subsurface of the material. All chemical-physical reactions (photolysis, hydrolysis, etc.), deposition of biological and abiotic agents, as well as moisture induced deformations take place on the surface of sample and progress into bulk interior after erosion of the external layers. The light electromagnetic radiation penetrates wood to the extent corresponding to the thickness of our samples (depending on the light wavelength obviously). The only missing aspect in our “accelerated weathering” is a fact that our sample is missing the mechanical support of bulk (constraining moisture-induced deformations) and physical barrier (or buffer) slowing down the moisture and heat transfer. It is important limitation, but even that we do believe that it is a correct approach for as much as possible mimic the real mechanisms of the weathering occurring in “thick samples”. The method reported here was successfully implemented in the round Robin test where similar samples were exposed to the natural weathering conditions in diverse locations within Europe. We were able to differentiate between sites and changes to wood samples allowed determination of different kinetics of deterioration. We do hope to report these results in the follow-up manuscript, assuming that the present paper (focused particularly on suitability of ultra-thin wood samples used for weathering) will be accepted by the research community.

Following the Reviewer’s comment, we included the subsequent additions in the revised document (discussion chapter):

“An important limitation of using ultra-thin samples for natural weathering testing is a fact of loosing a complex interaction of different layers within material bulk. It implicates a dramatic change to the diffusion of moisture and heat within bulk material. However, in the context of the wood weathering it is clear that the most relevant changes occur in the subsurface of the material. Majority of extensive chemical-physical reactions (photolysis, hydrolysis, etc.), deposition of biological and abiotic agents, as well as moisture induced deformations take place on the surface of sample and progress into bulk interior only after erosion of the external layers. The electromagnetic radiation in the range of photodegrading UV light penetrates wood to the extent of 50-100 µm, that corresponds to the thickness samples used in this research. Other important missing aspect in the pro-posed “accelerated natural weathering” approach is a fact that ultra-thin sample is missing the mechanical support of bulk. The support constrains moisture-induced deformations and serves as a physical barrier (or buffer) slowing down the moisture and heat transfer. It affects also the number and extend of microcracks formation. Despite of the above limitations, using ultra-thin wood samples is a correct approach for assuring replication of the real mechanisms of the weathering occurring in “thick samples”. That can not be assured in alternative up-to-data artificial weathering methods. “

You refer "They entail further internal stresses of material imposed by cyclic wetting and temperature changes.", though it is more likely to use the mentioned dimensions of wood in the structures or even bigger elements, than thin layers of wood.

The original sentence referred to the bulk material, but following the comment we revised it to clarify the meaning:

“They entail further internal stresses of bulk material imposed by cyclic wetting and temperature changes.”

We decided also to divide the text to form a separate (from thin samples description) section.

In methods section, concerning the raw material, you refer only "one piece of Norway spruce wood", without providing any other details such as where did you get it from, density, sap- or heartwood etc. I am not sure if a test and its results could be reliable, when there is only one piece of wood examined and there is not any replicate using other parts of the same tree or different trees. Referring to the piece of wood you used, where exactly did you get the sliced specimens from? how many growth rings did you include? did you include pith?

The lack of detailed description of the samples was indeed an important deficiency. We included all that missing info in the revised document:

“Experimental samples were prepared from Norway spruce wood (Picea abies L. Karst.) originated from Italian Dolomite mountains (Trentino Region, Italy). A single log 0.5m long, extracted from the bottom part of the trunk was spitted to assure radial surface opening. A board 30 × 50 × 500 mm3 (radial × tangential × longitudinal directions respectively) was cut out from the sapwood assuring defect free and uniform yearly ring structure. The average yearly ring width was 1.9 mm and latewood width 0.8 mm. No any drying process was used to minimize wood properties alteration due to elevated temperature. Wet board was finally processed on the slicing planner (Super MECA-S, Marunaka Tekkonsho Inc., Shizuoka, Japan) to prepare ultra-thin experimental samples.”

We would also like to clarify that use of a single board for generation of samples, even if apparently seem to be a limitation, was indeed our specific choice. As already mentioned, this manuscript is a pilot report presenting an alternative approach for natural weathering studies. Lack of replica samples may be justified by the overall experiment concept - Round Robin test implemented as an initiative of COST Action FP1006, where we prepared thirty sets of tween samples. Thanks to that, we were able to minimize variance within the test results to only a local climate (excluding all aspects of the natural wood variability).

I feel that the significant information of the mean temperature and mean precipitation values for the period of exposure should be removed from the discussion and placed in the methods section.

The weather data are removed from Table 2 and are forming a separate table (new Table 1), as suggested.

Lines 389-392 as well.

The text was moved to materials and methods section, as suggested.

In line 400, venerable or vulnerable?

The word was corrected, thank you!

In figure 2, the appearance of the specimens after the exposure reveals that probably this result would not be so useful, since there would not be structures with so thin wooden elements, since even the veneers are applied on surfaces of bigger elements/wood products.

The output of the weathering test with ultra-thin samples provides, in our opinion, all the data that are necessary for understanding and modelling of the aesthetical changes to wood induced by the exposure to external conditions. We were able to use these data to simulate the appearance of “thick” samples within a proprietary software tool we developed. However, the wavy structure of ultra-thin samples may provide impression of the incompatibility with other configurations.

The lines 434-447 and the methodology of thick specimens characterization should be analyzed in methods section, providing all the necessary details (mean temperature, precipitation etc.). Please, provide image of the thick specimens used.

We do agree with Reviewer that addition of new figure and table may clarify the research outputs. We did not included it in the original submission to limit the size of already rather long manuscript. Anyway, following the suggestion an additional text, figure and table are included in the revised manuscript.

You should try to highlight the significance of your results and explain where such thin specimens could be found/used/exposed or what different provide compared to the already established knowledge of weathering impact on wood.

It was exactly our intention to present this concept in the Coating journal. Perhaps after modifications our manuscript may better explain the advantages and limitations of our work. We do hope that all the above changes, especially part of the discussion may provide a better explanation of the advantage of our (proposed) approach as well as to confront it with established routines.

Summarizing, we would like to thank once again for all the critics and suggestions. We do believe that we addressed all of these, and the revised manuscript may fulfil your expectations.

With sincerely regards,

Jakub Sandak, on behalf of authors

Reviewer 3 Report

please see my comments attached 

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 3

Thank you very much for your time in reviewing our manuscript as well as for valuable comments and suggestions. We have deeply discussed all of these and modified the manuscript accordingly. Please find below our responses to all points raised in your Review as we extracted from the PDF document. We do sincerely hope that we correctly interpreted all your concerns and our modifications (answers) fulfil your requirements.

please see my comments attached  peer-review-10274173.v1.pdf

Introduction: This is overall excellent introduction. The state of the art of thoroughly presented and the aim was clearly defined. 

Thank you very much for your kind evaluation

Materials and methods: A well described and informative section

The thickness of samples was ~ 100µm – why such a figure was chosen? Have you performed preliminary studies?

The geometrical configuration of our samples was optimized on the basis of several literature references and personal experiences. The thickness of 100µm is a compromise between the mechanical integrity of wood and as thin as possible layer allowing full penetration of light and other degrading factors. Finally, the thickness corresponds to the optimal setting of the slicing machine we used for production of experimental samples.

July (add year)

Thank you, the year of the test is clarified in the revised manuscript.

Figure 2 – enlarge the figures and show your observation with arrow. This will be very helpful for the readers

As requested by other Reviewers, we re-designed Figure 2 and included images of thick samples. We also included arrows indicating occurrence of cracks, as suggested.

Figure 3 – enlarge this. I suggest to use colour instead of dots. This comment also applied to figure 8.

The figure is enlarged, even if we do believe that it is a Journal production Team to finally set the size of figures in the final version of the paper, especially as the template for publication seems to be not corresponding to the printed journal. We would also like to keep the black & white topology for Figure 3 and 8, as from our experiences, most of the colour information disappears when printed or copied on majority of office printers. The gradation of lines with thickness and pattern seems to be optimal compromise, even if far from perfection.

Figure 4 – you did not make a reference in this figure

Thank you, the reference is included in the revised text!

The time related progress of morphological changes of investigated samples is presented in Table 2. Please comment on the data presented in table 2.

We do believe that majority of the discussion chapter 4 is somehow correlated to overall interpretation of the complex system and morphological changes occurring to tested material. We do believe that Table 4 (former 2) is also visually self-explaining and not additional description is indispensable. Moreover, the revised manuscript includes several additions that we do believe contributes to a better transparency and clarity of our work.

Figure 5 please, use arrows inside the figures to show us your findings. The same applies to figure 6.

Both, Figure 5 and 6 are modified as suggested.

Figure 9 this is excellent approach

Thank you very much!

Discussion: A well discussed section

Thank you very much!

Conclusions: Can you estimate how long will require for the experiment of the thicker samples will be selected? Maybe, you can state this as recommendation for future studies. Overall, I like your approach and I found it very interesting

Perhaps, we do not understand the proper meaning of this question, however, we will try to address it according to our interpretation. We would like to declare that this manuscript is a pilot report presenting an alternative approach for natural weathering that was developed for the specific needs of the Round Robin test implemented as an initiative of COST Action FP1006. Within framework of this test we prepared thirty sets of tween samples that were exposed in different location in Europe. Each set included thin and thick samples that were collected by project partners and after sending back, were analysed simultaneously in a single lab. The weathering of thick samples is considered by us as a representation of the typical wood deterioration that may vary (and we evidenced it) at different geographical locations and climate zones. Therefore, the answer to the question is that we experimentally estimated kinetics of the physical chemical changes for all thick samples that were exposed in different places - and that is a basis for any further analysis. The test with thick samples was conducted for 1 year, that is clarified in the revised manuscript. In parallel, we investigated thin samples with intention of understanding early degradation due to weathering. However, we discovered that several aspects of the ultra-thin samples highlight an accelerated nature of weathering that we identified as a worth of further systematic studies. Results of these are presented within this manuscript.

Even if it is impossible to fully integrate results obtained by assessment of thin wood samples with standard wooden samples. This remains a question. How reliable are the results?

This is a very important point that we are asking ourselves. As in every pioneer/pilot studies it is very difficult to identify sources of error and unexpected variance. As mentioned above, we used identical set of samples in the Round Robin test and found our results as rather logical and consistent. Anyway, it would be a sign of arrogance to claim how reliable our method is at this stage, without critical feedback from other researchers adopting this approach for alternative research initiatives.

Summarizing, we would like to thank once again for all the kind comments and suggestions. We do believe that we addressed all of these and the revised manuscript may fulfil your expectations.

With sincerely regards,

Jakub Sandak, on behalf of authors

Reviewer 4 Report

„A Method for Accelerated Natural Weathering of Wood Subsurface and Its Multilevel Characterization” presents the results of an interesting study that aims at better understanding the weathering process of lignocellulosic materials.

Here are some comments on the manuscript:

• Abstract - “Even if the weathering test was conducted for a relatively short time (28 days), the samples exposed to the southern site were slightly more affected by the weathering process.” – slightly more than what? In the next sentence – “Proposed configuration” – configuration of what? Please, specify.
• Introduction, the last paragraph - Please, highlight the novelty in the approach to the accelerated natural weathering of wood (the Authors mentioned above that application of ultra-thin samples has already been used by other researchers). This information could also be given in 2. Weathering test.
• What were the storage conditions of control samples (lines 156-17)?
• Line 195 – there should be “three” instead of “tree.
• Lines 242-243 – “Disconnection of surface cells, being an effect of middle lamella breakdown, is observed early in the weathering process” – what are the basis of this statement? It was not shown in SEM images.
• Lines 310-311 “Positive peaks are highlighted by red colour, in contrast to negative peaks marked in violet.” – please, clarify the meaning of positive and negative peaks. What does it mean that this peak is positive? What information does it give us?
• How were the acceleration factors calculated?
• In the Conclusions – please, highlight the novelty of the research performed.

Author Response

Dear Reviewer 4,

Thank you very much for your time in reviewing our manuscript as well as for valuable comments and critics. We have deeply discussed your suggestions and modified the manuscript accordingly. Please find below our responses to all points raised in your Review. We do sincerely hope that we correctly interpreted your concerns and our modifications (answers) fulfil your requirements.

„A Method for Accelerated Natural Weathering of Wood Subsurface and Its Multilevel Characterization” presents the results of an interesting study that aims at better understanding the weathering process of lignocellulosic materials.

Thank you very much for your kind assessment.

Here are some comments on the manuscript:

• Abstract - “Even if the weathering test was conducted for a relatively short time (28 days), the samples exposed to the southern site were slightly more affected by the weathering process.” – slightly more than what? In the next sentence – “Proposed configuration” – configuration of what? Please, specify.

Indeed, the sentence was not clear enough. The part of abstract was revised as follows:

“Even if the weathering test was conducted for a relatively short time (28 days) the ultra-thin wood samples changed noticeably. The progress of alteration was similar as usually noticed for wood surfaces, but occurred at shorter exposure time. The estimated acceleration factor was ×3, compare to the natural weathering kinetics of wood.”

• Introduction, the last paragraph - Please, highlight the novelty in the approach to the accelerated natural weathering of wood (the Authors mentioned above that application of ultra-thin samples has already been used by other researchers). This information could also be given in 2. Weathering test.

It is very relevant and important comment. It is clear that our research can be hardly considered as a ground-breaking contribution to the Science. However, we do believe that it is at least a valuable confirmation of the state-of the art knowledge that has been systematized and properly tested in the experiment. Moreover, we hope that specific focus on ultra-thin samples as used for carefully designed natural weathering campaign is an innovation, even if similar samples were used in the past for diverse purposes. The revised manuscript includes in several points additional content that may support our thesis as well as provide an objective assessment of the limitations. In addition, considering suggestion of Reviewer, the definition of the research goals evolved as follows:

“The goal of this research was to investigate the feasibility of the novel approach for accelerated natural weathering of wood. An extent of the microstructure degradation and other alterations on the molecular, microscopic and macroscopic levels recorded in ul-tra-thin wood samples were confronted with thick references. As a result, an acceleration factor relating weathering kinetics of ultra-thin samples with references was estimated. An additional objective was to develop a novel testing protocol resulting in at least com-parable or even more detailed characterization, with simplified sample preparation, but still assuring significant shortening of the natural weathering procedure.”

• What were the storage conditions of control samples (lines 156-17)?

The description of chapter 2.2 was modified following Reviewer 3 and other Reviewers’ suggestions. The storage conditions of samples after collection and measurement are clarified.

• Line 195 – there should be “three” instead of “tree.

Thank you, the error is corrected

• Lines 242-243 – “Disconnection of surface cells, being an effect of middle lamella breakdown, is observed early in the weathering process” – what are the basis of this statement? It was not shown in SEM images.

The statement was following literature references but it was not evidenced on our images. The sentence was removed since the corresponding SEM image was not presented in the manuscript.

• Lines 310-311 “Positive peaks are highlighted by red colour, in contrast to negative peaks marked in violet.” – please, clarify the meaning of positive and negative peaks. What does it mean that this peak is positive? What information does it give us?

2D spectral correlation is an established technique for identification of the hardly visible, but systematic changes in the spectra that are associated to the disturbance. As stated in the text, the disturbance in our experiment was a time of sample exposure to natural weathering. The meaning of peaks in the graph indicates as follows:

• The positive peak on the synchronous spectrum means that spectral bands corresponding to coordinates of that peak change continuously in the way that direction of changes is identical for both wavelengths. It means that synchronous increase or decrease of both wavelengths is present.
• The negative peak on the synchronous spectrum means that spectral bands corresponding to coordinates of that peak change continuously in the way that direction of changes is opposite for both wavelengths. It means that one wavelength increases, while the second wavelength decreases.
• The positive peak on the asynchronous spectrum means that the sequence of changes at the wavelength 1 occurs in advance to the sequence at wavelength 2.
• The negative peak on the asynchronous spectrum means that the sequence of changes at the wavelength 2 occurs in advance to the sequence at wavelength 1.

We decided, by following comment of the Reviewer, to include a brief explanation of the meaning of 2D spectral correlation peaks as follows:

“The positive peak on the synchronous spectrum indicates that spectral bands corre-sponding to coordinates of that peak change continuously in the way that direction of changes (increase or decrease) is identical for both wavelengths. Conversely, the negative peak appears when one wavelength increases, while the second wavelength decreases. The sign of asynchronous spectrum allows identification the sequence of changes occur-ring at both confronted wavelengths [49].”

• How were the acceleration factors calculated?

The acceleration factor was determined empirically by adjusting the time scale for diverse deterioration kinetics measured on ultra-thin and thick samples. A simple magnification factor was adjusted manually to overlap both trends. We are aware of the limitation and subjectivity of this approach, but we were not able to find another more objective procedure. Anyway, the acceleration factor is only an indicative quantifier that can be highly useful for rough approximation.

The following sentence was included in the revised text to clarify our methodology for Readers:

“Assuming that the sole difference between both sample types is kinetics of deterioration, it become possible to identify an acceleration factor. This factor was determined empirically by adjusting the time scale for diverse deterioration kinetics curves measured on ultra-thin and thick samples to assure an overlap of both trends.”

• In the Conclusions – please, highlight the novelty of the research performed.

The conclusion chapter was revised following comment of Reviewer. The following text is included to summarize the novelty of our approach:

“Even if the weathering test was conducted for a relatively short time (28 days), the progress of ultra-thin wood samples alteration was similar as usually noticed for thick wood surfaces. However, such extent of changes occurred at shorter exposure time. The estimated acceleration factor was ×3. The research methodology presented here can be directly used for determination of the weather dose-response models essential to predict the future service life performance of timber elements. It may also serve as a screening technique for a rapid estimate of the material’s resistance to deterioration. In that case, all the natural weathering factors are fully considered, in contrast to artificial weathering procedures limiting the real exposure conditions.”

Summarizing, we would like to thank once again for all your critics, comments and suggestions. We do believe that we addressed all of these and the revised manuscript may fulfil your expectations.

With sincerely regards,

Jakub Sandak, on behalf of authors

Round 2

Reviewer 2 Report

Dear authors,

most of the suggestions of changes of reviewer were applied in the text of the manuscript. Generally, the manuscript is much improved and the significance of the work is highlighted, while some explanations were provided in the text in order to be clear to the readers your objectives and incentive.

In my opinion, the manuscript can be published as it is.