Optimized Method for Mapping Inorganic Pigments by Means of Multispectral Imaging Combined with Hyperspectral Spectroscopy for the Study of Vincenzo Pasqualoni’s Wall Painting at the Basilica of S. Nicola in Carcere in Rome

Round 1

Reviewer 1 Report

I read the paper titled "Optimized method for mapping inorganic pigments by means of multispectral imaging combined with hyperspectral spectroscopy for the study of Vincenzo Pasqualoni’s wall painting at the Basilica of S. Nicola in Carcere in Rome"
I found the research interesting. In fact, although the topic is well known, results are interesting because provide a good non-invasive approach for the identification of inorganic pigments and their distribution on a painting. The paper is well written and the goal is focused and well-realized.
The manuscript is potentially interesting for the readers of Minerals. As a reviewer, I still have some comments and suggestions.

1.            Figure 4: Please insert also the ordinate axe label

2.            Figure 5: Some of the points are out of the ellipse, could you justify this strong variance along with the PC? 

3.            Authors perform the PCA on both series of measurements, before conclusion it could be interesting to compare the obtained results in order to evaluate the impact of the loading on the pigment discrimination. I suggest performing this comparison to improve the overall quality of the considerations. 

4.            references. please check the years, some of them are not in bold

Author Response

The authors wish to thank the Reviewer for the revision and the suggestions.

the Authors responses and the revised text are reported in the attached file. The integrations are reported inside the manuscript as tracking changes in green colour.

Author Response File: Author Response.pdf

Reviewer 2 Report

General comments

This paper describes the application of multispectral and hyperspectral imaging for spatially mapping the distribution of inorganic pigments.

The aims and scope of this journal state:

Minerals (ISSN 2075-163X) is an international open access journal that covers the broad field of mineralogy, economic mineral resources, mineral exploration, innovative mining techniques and advances in mineral processing.

This might indicate that this paper is better suited for publication in a different journal, however a review of recently published articles in this journal (e.g. archaeological ceramics) indicates that this paper could be published in ‘Minerals’.

The study or wall painting and artwork is quite topical and of interest to many readers. The examination of wall paintings or other historical works of art are always of interest and the application of new techniques or methodology is always of interest. Multispectral and hyperspectral imaging are mature techniques and certainly are not new approaches for mapping the distribution of pigments that have common spectral features in the visible/near IR. The application of PCA for classification of spectral data as well as data in hyperspectral imaging data cubes is also well known. It does not seem that the authors have solved a new problem and I don’t see that they have developed a novel approach to advance the methodology for mapping the distribution of pigments.

In this paper the authors ‘propose a multi-modal approach for the pictorial analyses of a painted abse of a basilica’. (I believe ‘abse’ is a typo). The authors likely mean ‘Apse’. Nevertheless, if this is the stated objective of their project, the authors probably need to re-write their objective to make it consistent with the title of the paper.

One aspect that would be critical to include in this paper are details of the analytical work that was performed previously. Providing a reference [12] and vague statements about the previous work is not sufficient. I think the authors should state explicitly the pigments (perhaps in a table) that were identified and a description (1 paragraph?) of the methodology used to identify the pigments.

Overall, the quality of the writing in this paper certainly needs improvement in order to be considered for publication. At this stage there are simply too many typos or misused terms. I have highlighted a few in my specific comments. The quality of the figure are generally very good. The reference are also v. good.

Specific comments

P1 line 40 – The authors should state what they mean by ‘ancient times’

P2 line 50 – Typo? ‘… due to the fact that the basic ambient of the binder…’

P2 line 53 – Typo? ‘… that could be determined alteration procedures of pigments.’

P2 line 61 – Typo ‘in situ analysis that returns imaging output allowing to achieve..’

P2 line 70 and elsewhere – I don’t feel that the authors need to consider the cost of instrumentation unless they are speaking about commercially available instruments. Imaging systems can be lab built for a relatively low cost.

P3 lie 110 ‘Restauration’ probably should be spelled as ‘restoration’.

P6 Figure 2 & 3 – Showing the individual spectral as well as the average spectra is not necessary. In Figure 2 they are presented as spectra of points in Figure 3 they are presented as spectra of pigments.

1. 8 Figure 4 (Heading) Typo? ‘Resulting of preprocessed spectra…’

Conclusions

Hyperspectral and multispectral imaging was not used for the identification of mineral pigments. They were used to map their spatial distribution?

The authors evaluated the possibility to detect the same colour variability with a minor number or wavelength. Do they mean number of wavelengths? It is not clear why it would be important to achieve the goal of minimizing the number of wavelengths.

I don’t see that the authors demonstrated that they were able to achieve a ‘map of a pictorial layer with a high accuracy level of pigment recognition’. 

How to know if the results are accurate?

Author Response

The authors wish to thank the Reviewer for the revision and the suggestions.

the Authors responses and the revised text are reported in the attached file. The integrations are reported inside the manuscript as tracking changes in green colour.

Author Response File: Author Response.pdf

Reviewer 3 Report

The article proposes and describes an analytical approach for non-invasive study of pigment compounds. The authors use the combination of hyperspectral spectroscopy and multi-spectral imaging in order to perform the preliminary pigment identification. The case study itself is interesting and important, however, since the focus of the article is on methodology, some important aspects need to be improved. In general, the described method has potential for cultural heritage diagnostics, therefore, the presented topic has value for the scientists in the field.

The general comment to the text is to improve the language - grammar, spelling and use of words/phrases - throughout the entire article. Regarding the scientific content, the authors should present reference spectra of the suggested pigments along with the measured spectra obtained from the wall painting. Below more suggestions for improvement of the article.

1. Lines 49-55 – Grammar/spelling/rephrasing
2. In-situ in cursive
3. Line 88 – “get”, non-academic language
4. Line 94 – “punctual” or maybe better “point” hyperspectral analysis
5. Line 100 – 4) goes to another paragraph, similar to 1)-3)
6. Lines 90-101: “In this paper, we propose a multi-modal approach for the pictorial analyses of a painted abse of the basilica of S. Nicola in Carcere (Rome) by performing…”. The main purpose of the article needs to be better emphasized. In the way it is written, it is not clear, if the new methodology or the pigment identification was the main focus. Based on the Conclusions, it is possible to assume that the authors propose a new approach and test it on the example of a specific wall painting. Also the authors analyse pigments, not pictorial analysis (more general).

7. Lines 108-109 – the purpose of the sentence in not clear about the use of natural and synthetic pigments in the XIX century. Both natural and synthetic pigments have been in use since millennia ago (e.g. lead white, Egyptian blue are synthetic) up to the present day. So what do authors try to say by this sentence?
8. Line 110 – “the results was” – were, grammar/spelling
9. Lines 112-114 – the authors present areas of analysis. Explain, why exactly these areas were selected, but not others or the entire wall painting? It would also be useful to present a picture of the entire wall painting for the readers to know, where the selected areas are located. Also add dimensions or scales to the presented images in figure 1 in order to understand the scale of the performed analysis.
10. Line 135 – Finally. – comma, not a dot
11. Line 192 – grammar. As – maybe Is performed?
12. Check the indent of new paragraphs – it is different throughout the article.
13. Line 94 – again about natural and synthetic pigments used in the 19^th The authors should focus not on the distinction “natural-synthetic”, but rather on possible pigment options available and commonly used in the 19^th century.
14. Figure 2 – the inscriptions on the images (such as S, SR etc) are not of the same size. Unify the size and style of these marks for all images at least within one figure.
15. Line 208 – reasons to use red ochres and other iron pigments are not limited only to their durability, but also due to their abundance and low cost compared to other red pigments.
16. Figure 3 – How do authors make attribution of the recorded spectra to specific pigments? The authors must present reference spectra – taken from known pigments. Then compare these spectra with the measured ones in order to conclude on the pigments present on wall paintings.
17. Line 206 – “…an inflection point at 580 nm and an absorption at 869 nm in the visible range” – 869 nm is near infrared, not visible range. In general, since authors indicate the specific nm, it is redundant to indicate, in which part of spectrum this feature occurs. The same in line 212: “…an inflection point at 480 nm in the visible range”. Thus, such words as “in the visible range” can be omitted.
18. Lines 237-241 – the authors should present reflectance spectra of gypsum, reference and of the base material of the analysed wall paintings.

Author Response

The authors wish to thank the Reviewer for the revision and the suggestions.

the Authors responses and the revised text are reported in the attached file. The integrations are reported inside the manuscript as tracking changes in green colour.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Abstract (line 23)  '...allows the characterization...'

p.2 line 45 'Mineral pigments have been used in paintings...'