Analysis of the Natural Aging of Silver Fir (Abies alba Mill.) Structural Timber Using Dendrochronological, Colorimetric, Microscopic and FTIR Techniques
Round 1
Reviewer 1 Report
In order to explore the natural aging properties of wood, the paper compared some aging parameters of historic and recently felled silver fir wood by using dendrochronological, colorimetric, microscopic and FTIR techniques, and the trends of color, lignin content, hemicelluloses content , cellulose content and crystallinity were discussed. But there have been many similar studies, even using more test means, so the paper is not innovative enough. From the reviewer’s point of view, this paper needs further modification. The following revisions and comments are suggested to be considered:
(1) As a biomass material, wood itself has great discreteness. Especially, to study the aging properties of materials, a large number of samples are needed. However, the number of samples provided in this paper is seriously insufficient.
(2) There are many ways to evaluate natural aging properties, and this paper only makes comparative analysis from color, lignin content, hemicelluloses content, cellulose content and crystallinity. In the comparison of chemical components, in addition to the photodegradation of cellulose, hemicellulose and lignin, the photochemical reaction of wood extracts also has a great impact on its aging properties. In addition, in terms of physical properties, only the color changes under the influence of light were considered, without considering the influence of temperature and humidity. Finally, the paper should further explore the effect of aging properties on mechanical properties, after all, is to study the aging properties of structural wood.
(3) As for the changing trend of color, only qualitative analysis has been carried out, so whether the quantitative prediction model of wood color can be further established.
(4) The sharpness of the pictures needs to be further improved. Many of the pictures are fuzzy.
(5) There are artificial aging and natural aging methods to study wood aging properties. Please compare the differences between the two, and whether the change rule obtained in the article can be verified by using the artificial aging test method.
In conclusion, wood aging is a complex process, and any single evaluation method can not fully explain the aging behavior. In order to better elucidate wood aging behavior, we should integrate different evaluation methods to make up for the missing and deficiency.
Author Response
We thank the editor and the reviewers for their constructive criticism and valuable comments, which were of great help in revising the manuscript. Please find attached document with the response.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The manuscript focuses on an important subject: the changes in servicing wood as it ages.
In general, the manuscript appears to be of interest, but the analysis of chemical alterations using FTIR spectroscopy requires substantial revision.
Some important information is missing, like: what longitudinal thin sections (radial or tangential) were used to obtain the FTIR spectra?
Why utilize a resolution of 1 cm-1? This is only used for gases; the standard for solids and liquids is 4 cm_1. In addition, the extremely small number of sans (16) does not guarantee a decent signal-to-noise ratio. Please note that the minimum number of images for a 4 cm-1 resolution is 32, with 64 being recommended.
The baseline procedure used is not often used for spotting differences in composition or for quantitative analysis.
The FTIR analysis based on the average absorbance of the lignin peak ratio, "Lignin ratio=lignin average/fingerprint average" has no practical meaning and cannot be used as an estimate of the lignin content. The same can be said for hemicelluloses.
Due to baseline procedure-induced distortions, it is impossible to determine the overall absorbance of the two spectra in Figure 4, but it appears that the absorbance of the blue spectrum (recent fire) is much lower than that of the red spectrum (ancient fire).
This indicates that the spectra were not normalized (as the authors claimed). This could be the result of varying section thicknesses or a combination with the baseline procedure. The thin portions of the obtained spectra may have originated from distinct longitudinal planes.
Whatever the case, no conclusion can be retrieved from the FTIR analysis, even if the anatomical analysis seems to show a relative increase in the lignin content.
Comments for author File: 
Comments.pdf
Author Response
We thank the editor and the reviewers for their constructive criticism and valuable comments, which were of great help in revising the manuscript. Please find attached document with the response.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The authors did improve on some points raised. However, they did not address the most important question regarding the ratios as indicators of the relative amounts of lignin and hemicellulose. Regarding the ratios, the authors should provide convincing evidence that the calculated ratios are roughly proportional to the lignin or hemicellulose contents; I am not convinced.
The authors stated in response to reviewer questions that a similar multipoint-level baseline correction was also used by Faix 1991 [23]. According to Faix (1991), the baseline correction in the fingerprint region was between 1860 and 780 cm-1. This is completely different from the authors baseline with two additional points at 1538 cm-1 and 1186 cm-1, and earlier ending at 918 cm-1, which affects the lignin peak and particularly the main peak, that are less distorted in the Faix method.
The authors claim that the baseline correction is also a normalization, which is also incorrecas evidenced by their citation [23] (Faix 1991). The spectra are normalized by further dividing them by the absorbance of the highest peak.
In fact, the authors should explain why normalization was not performed.
As for lignin, the average absorbance is between 1504 cm-1 and 1513 cm-1 (please note that it is not even centered around the maximum of 1595!) and the average absorbance of the region between 1586 and 1186 cm-1. The authors claimed that the entire fingerprint region was used, which is not correct. The entire fingerprint region should include the main peak and at least extend to around 800 cm-1. Why was it not used for the entire fingerprint region?
In fact, the authors should explain why normalization was not performed.
As for lignin, the average absorbance ranges between 1504 and 1513 cm-1 (notice that it is not even centered on the maximum of 1595!) and the fingerprint average absorbance between 1586 and 1186 cm-1. The authors falsely asserted that the entire fingerprint region was utilized. The complete fingerprint region must contain the principal peak and extend to at least 800 cm-1. Why wasn't it utilized throughout the overall fingerprint region?
Please double-check Figure 4, which shows that the absorbance of the hemicellulose peak (1786), is very similar between recent- and historic fir. This would imply that lignin (less) and cellulose are disproportionately lower in young fir, which contradicts the stated conclusions. The authors should clarify this matter
Author Response
Dear Sir or Madam,
Thank you for reviewing this article again and for all your comments and suggestions. We address your comments in the attached document.
On behalf of the authors,
Assoc. Prof. Ales Straze, PhD.
Author Response File: Author Response.docx
