Likely Technology Making the Ancient Cham Bricks Lightweight, Carvable, and Durable for Constructing Big Engraved Towers Lasting Thousands of Years: A Case Study of the Po Nagar Towers, Nhatrang, Vietnam

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

After reading this article carefully, I consider it publishable in its current state.

Author Response

Comments: After reading this article carefully, I consider it publishable in its current state.

Response: Many thanks for your kind words.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript presents a topic of interest to the field of cultural heritage, focusing on the characterisation of old bricks and their behaviour under water and mechanical stress. The aim of the study is clearly defined, but there are important aspects that need to be substantially improved in order to consider the work as scientifically sound. The main observations are detailed below:

Methodology and testing:

1. As this is a study on cultural heritage materials, logically, it is not possible to extract a large number of samples to be tested, but it should be explained and justified.
2. The methodology followed in the laboratory is not adequately described. The authors need to clearly explain the whole experimental process from the reception of the bricks to the obtaining of the results.
3. There is no information on how many brick samples were used and their exact location in the towers. This point is fundamental for the validity of any experimental work.
4. It is not clear whether one or more specimens were tested per brick, nor how many bricks were used.
5. In the compression tests, it is essential to indicate the number of specimens, their size and the criteria for their selection. An illustrative image of the test should also be included.
6. In the analysis of the core of the material, it is not detailed whether dust was collected from a single part or from several parts, nor the criterion used to choose it. It is necessary to specify the number of tests performed, as the results may vary depending on the part analysed.
7. In the open porosity test, the technique used is not defined, nor is the number and dimensions of the specimens, both black core and red crust, reported. This point should be clarified.
8. Regarding the graphs, it is not specified whether the values presented are averages or if any statistical treatment was performed. In the compressive stress curves, was the modulus of deformation assessed by another method?
9. In section 3.5 it is mentioned that imitations have been made based on a technology described in a patent. If this research is not novel, explicit reference should be made to this patent in the introduction and its use should be contextualised.
10. The conclusions include claims that are not adequately supported by the results presented (e.g. the alleged ability to prevent the growth of moss and mould). In addition, some sentences are not conclusions as such (lines 324 and 325), and should be deleted or relocated.

Additional content and form:

- The inclusion of modern bricks laid in the year 2000 does not add value to the main research, and may even divert the reader's attention.

- Figures are adequate, but somewhat sparse, especially with regard to the samples analysed and the tests. It would be useful to incorporate images of both samples and tests.

- The tables and graphs are well presented but lack statistical data to validate the values.

- As for the bibliography, although many references are prior to 2010, the most relevant and specific references on brick manufacturing with sustainable criteria are recent, so that the whole can be considered acceptable. However, the bibliography should be more current.

A thorough review of these aspects is recommended before considering a new version of the manuscript.

Author Response

Comments and Suggestions for Authors:

The manuscript presents a topic of interest to the field of cultural heritage, focusing on the characterisation of old bricks and their behaviour under water and mechanical stress. The aim of the study is clearly defined, but there are important aspects that need to be substantially improved in order to consider the work as scientifically sound. The main observations are detailed below:

Response: We thank the Reviewer for appreciating our work and providing insightful comments. This work is preliminary and is strongly constrained by the low number of samples. Not mentioning this issue clearly was a mistake. The mechanical analyses are only illustrative and do not allow for the construction of a Weibull line to characterize the bricks. However, the main goal of this work is to show that the combination of various appropriate analyses can provide useful insights into approaching ancient brick technology. The text has been amended and highlighted in the revised manuscript. We believe that the comparison with modern bricks is educational for historical readers.

Below, we respond to your comments point by point.

Methodology and testing:

1. As this is a study on cultural heritage materials, logically, it is not possible to extract a large number of samples to be tested, but it should be explained and justified.

Response: Yes, you are absolutely right. The original ancient Cham bricks used in our study were officially provided by the Khanh Hoa Relics Conservation Center, which is in charge of conserving the Cham Po Nagar Towers. The OCBs include an intact brick and smaller pieces. This clarification has been included in the revised manuscript.

2. The methodology followed in the laboratory is not adequately described. The authors need to clearly explain the whole experimental process from the reception of the bricks to the obtaining of the results.

Response: We apologize for assuming that the characterization techniques used in this study are familiar to researchers in the field of materials science. Therefore, we initially thought it might not be necessary to provide detailed descriptions in order to keep the paper concise. Instead, we briefly provided the full name of the equipment and its manufacturer to help clarify the methodology used. Nonetheless, in the revised manuscript, we have added experimental procedures and more detailed descriptions for specific characterizations, including sample sizes.

3. There is no information on how many brick samples were used and their exact location in the towers. This point is fundamental for the validity of any experimental work.

Response: The main aim and important meaning of our work is to understand the technology applied in making OCBs, rather than focusing strictly on exact historical work with the identified location where bricks were located. Therefore, we studied samples that have an official biography and were kindly provided by the Khanh Hoa Relics Conservation Center.

4. It is not clear whether one or more specimens were tested per brick, nor how many bricks were used.

Response: We have added the number of tests in the text. Typically, multiple samples were cut from a single brick, and several bricks were used in our study. This information has been incorporated into the revised manuscript.

5. In the compression tests, it is essential to indicate the number of specimens, their size and the criteria for their selection. An illustrative image of the test should also be included.

Response: The sample sizes have been added to the text, specified as cubes of 3 cm x 2 cm x 2 cm. The compress measurements aim to compare the compression behavior of OCBs and NRBs, to show the ‘elastic’ behavior of OCBs compared to the NRBs.

6. In the analysis of the core of the material, it is not detailed whether dust was collected from a single part or from several parts, nor the criterion used to choose it. It is necessary to specify the number of tests performed, as the results may vary depending on the part analysed.

Response: You are right that when cutting the brick to prepare the core material, some dust could be collected from several parts of the brick, including from the crust. However, we believe these dusts have not significantly affected the results, as the amount collected is very small compared to the entire sample. The most meaningful difference between the core and the crust of OCB, which has been clearly analyzed, is that the core contains a significantly higher amount of carbon and exhibits elasticity, allowing it to accommodate compression more effectively.

7. In the open porosity test, the technique used is not defined, nor is the number and dimensions of the specimens, both black core and red crust, reported. This point should be clarified.

Response: Thank you for pointing out this issue. We have added in the revised manuscript a description of the measurement, which we specifically designed to study the water sorption rate of bricks.

8. Regarding the graphs, it is not specified whether the values presented are averages or if any statistical treatment was performed. In the compressive stress curves, was the modulus of deformation assessed by another method?

Response: The graphs show values from a single measurement, not averaged, although, as mentioned above, at least three measurements were performed for each sample, and several samples were measured for each characterization. We did not assess the modulus of deformation by other methods. In fact, the graphs may not fully represent the variations observed in each compressive stress curve, but they provide a general understanding of the trends. These results have been discussed in the revised manuscript.

9. In section 3.5 it is mentioned that imitations have been made based on a technology described in a patent. If this research is not novel, explicit reference should be made to this patent in the introduction and its use should be contextualised.

Response: Based on our primary research on ancient Cham bricks conducted a few years ago, we made imitations and described the proposed technology in our patent (Ref 40 in the revised manuscript). The patent review process took almost three years, and it was recently granted by the National Office of Intellectual Property of Vietnam (Sept 2024). In the patent, the relevant science was not presented in detail, while the technology for making bricks hard, yet porous and carvable, was proposed, and samples imitating the originals had to be provided as examples. The data presented in this research are novel, detailed, and systematic. We have made appropriate revisions in Section 3.5 to address this issue.

10. The conclusions include claims that are not adequately supported by the results presented (e.g. the alleged ability to prevent the growth of moss and mould). In addition, some sentences are not conclusions as such (lines 324 and 325), and should be deleted or relocated.

Response: Thanks. These have been deleted.

Additional content and form:

- The inclusion of modern bricks laid in the year 2000 does not add value to the main research, and may even divert the reader's attention.

Response: We believe that the comparison with modern bricks is educational for historical readers.

- Figures are adequate, but somewhat sparse, especially with regard to the samples analysed and the tests. It would be useful to incorporate images of both samples and tests.

Response: The samples are simple cubes with the sizes/dimensions specified in the text.

- The tables and graphs are well presented but lack statistical data to validate the values.

Response: You are right about the statistical data. However, in this paper, the most important message we want to convey for science and technology is how OCBs were made to be lightweight, sculptable, and durable, meeting the requirements for constructing big engraved towers that have lasted for thousands of years. Therefore, a combined dataset with representative main values is more meaningful. In the future, if we have a set composed of a number of OCBs from a tower, measurements should be conducted to derive statistical data including error estimates.

- As for the bibliography, although many references are prior to 2010, the most relevant and specific references on brick manufacturing with sustainable criteria are recent, so that the whole can be considered acceptable. However, the bibliography should be more current.

Response: Thank you for this comment. This study focuses on ancient bricks, which is why many of the references are old. A few relevant and specific references are recent, possibly driven by the requirements of specific applications.  

Reviewer 3 Report

Comments and Suggestions for Authors

This report presents a technically sound comparative analysis of ancient Cham bricks (OCB) from the Po Nagar Towers in Nhatrang, Vietnam, and modern restoration bricks (NRB). The study meticulously investigates their materials, structural phases, porosity, and thermal processing, yielding valuable insights for cultural heritage conservation and materials science. The robust methodology and clear data presentation support the findings' practical implications for historically informed restoration. However, the report requires minor revisions before publication, including language editing for clarity and conciseness, restructuring the "Results and Discussion" section for better readability, adding a schematic of the proposed ancient manufacturing process for accessibility, providing definitions for technical terms, and expanding the literature review with comparative examples from other ancient brickmaking traditions.

Author Response

Comments and Suggestions for Authors: This report presents a technically sound comparative analysis of ancient Cham bricks (OCB) from the Po Nagar Towers in Nhatrang, Vietnam, and modern restoration bricks (NRB). The study meticulously investigates their materials, structural phases, porosity, and thermal processing, yielding valuable insights for cultural heritage conservation and materials science. The robust methodology and clear data presentation support the findings' practical implications for historically informed restoration. However, the report requires minor revisions before publication, including language editing for clarity and conciseness, restructuring the "Results and Discussion" section for better readability, adding a schematic of the proposed ancient manufacturing process for accessibility, providing definitions for technical terms, and expanding the literature review with comparative examples from other ancient brickmaking traditions.

Response: We thank the Reviewer for the kind appreciation of our work and the insightful comments. The English writing has been revised for clarity and conciseness. In the "Results and Discussion" section, we have cited the patent (Ref 40 in the revised manuscript) to enhance accessibility. Moreover, a work on other ancient bricks has also been cited. We note that only a few publications are available that provide insights into similar ancient bricks. We believe the current work can serve as a valuable reference in the field.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

1. Number of samples tested

There is still a lack of clarity regarding the total number of samples tested. Although the cut of specimens is mentioned, it is not specified how many specimens were tested and the values obtained. This information is essential to assess the statistical validity of the results.

2. Visual documentation of the specimens

Although the dimensions of the specimens have been provided, it is still necessary to include pictures of the specimens and the experimental process. A visual documentation would contribute significantly to the understanding of the study and would allow the physical characteristics of the tested pieces to be appreciated.

3. Comparative study of the bricks

Although the previous review questioned the relevance of the comparative study between old and new bricks used in the restoration, the authors insist on its importance. In such a case, it is suggested to extend the analysis to include other contemporary bricks manufactured in the same region. This would provide a more robust and enriching framework for comparison.

4. Testing methodology

Possibly, the recommendation on the description of the methodology was not expressed clearly enough in the previous review. It is not necessary to detail the standard procedure of a porosity test, which is well known. What would really enrich the work is the inclusion of specific information on absorption and desorption times, the behaviour of water inside the material (from inside to outside and vice versa), and whether the tests were performed on whole pieces or fragmented (extracted cubes).

5. Inaccuracy in the wording (page 155)

Page 155 uses the ambiguous expression ‘certain dimension’, which should be replaced by a concrete and precise description.

6. Images and description of the tests

There is still a need to include images of the samples and the testing process to clarify which elements have been worked on. This would help to reinforce the methodological transparency of the study.

7. Number of bricks studied

It should be clearly specified whether the study is based on a single brick and how many subsamples were extracted from each part (core and reddish crust). This information is essential to assess the scope of the analysis.

8. Revision of the first conclusion

The first conclusion in its present form is not a conclusion as such, but a description of an action taken. It should be reworded so that it communicates an idea derived from the results obtained or, alternatively, it should be deleted.

9. Response to other comments

It is acknowledged that the authors have adequately responded to and expanded on many of the comments made in the first review.

Author Response

1. Number of samples tested

There is still a lack of clarity regarding the total number of samples tested. Although the cut of specimens is mentioned, it is not specified how many specimens were tested and the values obtained. This information is essential to assess the statistical validity of the results.

Response: We obtained one almost intact ancient brick, from which more than 10 samples were prepared as cubes of 3 cm x 2 cm x 2 cm for compressive, mass density, and hydric tests; and rods of 0.7 x 0.7 x 2 cm for thermal expansion/shrinkage measurement. Additionally, we obtained 4 pieces from different ancient bricks of the same type, from which we also cut more than 2 samples from each piece for comparative tests to assess the statistical values.

2. Visual documentation of the specimens

Although the dimensions of the specimens have been provided, it is still necessary to include pictures of the specimens and the experimental process. A visual documentation would contribute significantly to the understanding of the study and would allow the physical characteristics of the tested pieces to be appreciated.

Response: We have included photos of the samples in Figure 2, and described their shape and size in the corresponding experiments.

3. Comparative study of the bricks

Although the previous review questioned the relevance of the comparative study between old and new bricks used in the restoration, the authors insist on its importance. In such a case, it is suggested to extend the analysis to include other contemporary bricks manufactured in the same region. This would provide a more robust and enriching framework for comparison.

Response: This is a very interesting suggestion but outside the scope of this preliminary work. This work primarily focuses on determining the likely technology used to make ancient Cham bricks lightweight, carverable, and durable for constructing big engraved towers. The restoration bricks were specifically made by a brick company under the Ministry of Culture, Sports, and Tourism for the conservation of Cham towers in Vietnam. In light of our study, we have identified some critical issues causing the newly made bricks to deteriorate faster. We plan to examine this problem in detail later. 

4. Testing methodology

Possibly, the recommendation on the description of the methodology was not expressed clearly enough in the previous review. It is not necessary to detail the standard procedure of a porosity test, which is well known. What would really enrich the work is the inclusion of specific information on absorption and desorption times, the behaviour of water inside the material (from inside to outside and vice versa), and whether the tests were performed on whole pieces or fragmented (extracted cubes).

Response: Thank you for this clarification. Yes, in the recently revised manuscript, we have included a detailed description for the water sorption measurement from the outside to the inside of the bricks. The tests were performed on samples as cubes of 3 cm x 2 cm x 2 cm.

5. Inaccuracy in the wording (page 155)

Page 155 uses the ambiguous expression ‘certain dimension’, which should be replaced by a concrete and precise description.

Response: We have revised it with the size of samples used in experiments.  

6. Images and description of the tests

There is still a need to include images of the samples and the testing process to clarify which elements have been worked on. This would help to reinforce the methodological transparency of the study.

Response: As responded above, we have included photos of the samples in Figure 2, and mentioned their shape and size relative to the experiment.

7. Number of bricks studied

It should be clearly specified whether the study is based on a single brick and how many subsamples were extracted from each part (core and reddish crust). This information is essential to assess the scope of the analysis.

Response: Yes, we have clarified in the text that the samples were prepared from one nearly intact ancient brick and other pieces from different ancient bricks of the same type.

8. Revision of the first conclusion

The first conclusion in its present form is not a conclusion as such, but a description of an action taken. It should be reworded so that it communicates an idea derived from the results obtained or, alternatively, it should be deleted.

Response: We believe the first sentence is necessary to clarify the samples that were analyzed. However, following your opinion, we have deleted it.   

9. Response to other comments

It is acknowledged that the authors have adequately responded to and expanded on many of the comments made in the first review.

Response: Thank you!  

Author Response File: Author Response.docx