# 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

Nguyen Thu Loan, Ung Thi Dieu Thuy, Luong Van Duong, Tran Thi Thu Huong, Ba Trung Toan, Maria Luisa Saladino, Francesco Armetta, Philippe Colomban, Dariusz Hreniak, Nguyen Quang Liem

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Nguyen Thu Loan, Ung Thi Dieu Thuy, Luong Van Duong, Tran Thi Thu Huong, Ba Trung Toan, et al.. 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. Heritage, 2025, 8 (5), pp.173. 10.3390/heritage8050173. hal-05089470

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Article

# 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

Nguyen Thu Loan <fnref n="1" />, Ung Thi Dieu Thuy <fnref n="1" />, Luong Van Duong <fnref n="1" />, Tran Thi Thu Huong <fnref n="1" />, Ba Trung Toan <fnref n="2" />, Maria Luisa Saladino <fnref n="3" />,*, Francesco Armetta <fnref n="3" />, Philippe Colomban <fnref n="4" />,*, Dariusz Hreniak <fnref n="5" /> and Nguyen Quang Liem <fnref n="1" />,*

Academic Editor: Massimo Lazzari

Received: 21 March 2025

Revised: 3 May 2025

Accepted: 5 May 2025

Published: 15 May 2025

**Citation:** Loan, N.T.; Thuy, U.T.D.; Duong, L.V.; Huong, T.T.T.; Toan, B.T.; Saladino, M.L.; Armetta, F.; Colomban, P.; Hreniak, D.; Liem, N.Q. 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. *Heritage 2025*, *8*, 173. https://doi.org/10.3390/heritage8050173

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** The Po Nagar Towers (Thap Ba) complex, an iconic heritage site of Cham culture and a nationally recognized special relic, has stood in Nhatrang, Vietnam, for over a thousand years. We report here a preliminary analysis of original ancient Cham bricks from the Po Nagar Towers using a combination of appropriate characterization techniques, including X-ray fluorescence (XRF), X-ray diffraction (XRD), Raman micro-spectroscopy, thermal dilatometry, compressive strength testing, and water sorption. Mechanical properties and firing temperatures of the ancient bricks have been determined to support the discussion on the likely technology used to make them. Specifically, they were made from clay, sand, plagioclases/feldspar, and grog mixed with intentionally added carbon precursor (charcoal powder), then fired at temperatures between 800 °C and 1000 °C to form lightweight bricks with a mass density of 1.3–1.6 kg/dm<fnref n="3" /> and an open porosity of 18–25%. The ancient Cham bricks have their texture and porosity to meet the requirements of the thin rubbing joint technique in tower construction and to contribute to the carvability and durability of Cham towers. A comparison is made with the bricks for tower restoration during the 2000s.

**Keywords:** Po Nagar towers; lightweight brick; microstructure; carverability; durability; charcoal powder; chaine opératoire

## 1. Introduction

Vietnam has a long history of potteries, including making bricks, with relics found dating back to well before the 10th century BC [1–3]. Along with the Dong Son culture in

<footnote n="1">¹ Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi 100000, Vietnam; loannt@ims.vast.ac.vn (N.T.L.); dieuthuy@ims.vast.vn (U.T.D.T.); duonglv@ims.vast.ac.vn (L.V.D.); huongttt@ims.vast.ac.vn (T.T.T.H.)</footnote>

<footnote n="2">² Khanh Hoa Relics Conservation Center, 54 Sinh Trung, Van Thanh District, Nhatrang City, Vietnam; batoan179@gmail.com</footnote>

<footnote n="3">³ Biological, Chemical and Pharmaceutical Science and Technology - STEBICEF Department, University of Palermo, Viale delle Scienze, Bld. 17, 90128 Palermo, Italy; francesco.armetta@unipa.it</footnote>

<footnote n="4">⁴ Sorbonne Université, CNRS, MONARIS UMR8233, Campus Pierre-et-Marie Curie, 4 Place Jussieu, 75005 Paris, France</footnote>

<footnote n="5">5 Institute of Low Temperature and Structural Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland; d.hreniak@intibs.pl</footnote>

<footnote>* Correspondence: marialuisa.saladino@unipa.it (M.L.S.); philippe.colomban@sorbonne-universite.fr (P.C.); liemnq@vast.vn (N.Q.L.)</footnote>
North Vietnam [4], the Sa Huynh culture in the central region of Vietnam had developed since the 8–7th century BC and several centuries AD [5]. One of the well-known relics of the Sa Huynh culture is terra-cotta jars used to bury the dead [6]. From the late 2nd century AD until the early 19th century, there existed the Cham (or Champa) Kingdom in this central region of Vietnam [7,8]. One of the tangible elements of the Cham cultural heritage is the beautiful Cham towers. There remain about 50 Cham brick towers located along the central-to-southern central coast of Vietnam, representing the history of Cham temple and tower construction from the end of the 7th century to the early 18th century [8–11]. These relics have unique cultural and architectural values and have received the great attention from the international conservation community, especially since 1999 UNESCO has recognized the My Son temples complex as a World Heritage Site. The Cham towers of the ancient Champa Kingdom are influenced by the Hindu architecture and have served religious activities. A typical Cham tower was constructed as a single architectural block made of reddish-brown bricks, built on a solid base with meter-thick brick walls, extending at the top with floral and tapered figures, and usually featuring a single door opening to the east (towards the rising sun) [12]. Inside the tower, only a rather narrow space is available for setting up a stone altar with the linga-yoni, a hermaphrodite symbol of the God Shiva in Hinduism. The exterior brick walls are decorated with meaningful and beautiful carved reliefs, elaborate sculptures of flowers, birds, animals, dancers, etc. Among the ancient Cham towers, those at the My Son site have been investigated for many years through various international joint research projects involving Polish [8], Indian [9], Italian [10], and Vietnamese [11] researchers. A major technological discovery arising from these projects is the identification of the thin rubbing joint technique in tower construction, in which tree latexes were used to aid in initial grinding and rubbing and binding of the bricks together during construction [8,10–13]. This is the important answer to the long-standing question as to why in the construction of Cham towers, the bricks are joined together very closely with almost no grout joints visible. A similar rubbing joint technique in tower construction has been proposed for Khmer towers at Sambor Prei Kuk, a World Heritage Site in Cambodia [14].

After the My Son site, the Po Nagar Cham Towers complex, located in Nhatrang City, Khanh Hoa Province, Vietnam, is considered the second most famous among Cham tower relics. The Po Nagar Cham Towers, with the tallest one reaching 23 m, were first built with bricks during the late 8th century (Figure 1) [15]. So far, only a few studies have examined the materials and construction technology used for the towers. Specifically, a ¹⁴C dating study reveals that the bricks were made during the 13th–14th centuries, possibly the time of one of the tower reconstructions [16]. An optical microscope study reported the use of rice husks or ground straws mixed into the starting materials to make the bricks porous [17]. Additionally, Abdrakhimov et al. analyzed ancient bricks collected from the Po Nagar Towers, showing the high contents of silicon, calcium, and aluminum, which contribute to the material’s durability [18]. Over time, Cham towers have been partially damaged due to natural weathering and disasters from human wars, and they require restoration. During the 2000s, the Po Nagar Cham Towers were restored using specially made bricks. After about 20 years, the restoration bricks have partially deteriorated and rotted, changing color to a bright red very different from the original, as shown in Figure 1c. We should note that the deterioration of restoration bricks has occurred in the restoration of other Cham towers in Vietnam.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Map of Vietnam, showing the My Son Heritage Site (15°46'27" N, 106°6'34" E) and the Po Nagar Towers complex (12°15'55" N, 109°11'43" E); (b) the Po Nagar Towers at the top of a hill near the coast of Nhatrang Bay [12]; (c) the Northeastern and Southern Towers, with a magnified view (inset) of the wall restored during the 2000s, showing damages of the restoration bricks. Map of Vietnam is taken from https://earth.google.com/web/@16.40602111,106.87769294,793.44594506a,3564183.51090431d,35y,0.0008772h,0t,0r/data=CgRCAggBOgMKATBKDQj8BEAA (accessed on 13 January 2025).</figcaption></figure>
The aim of this preliminary study is to understand how the ancient Cham bricks were made to possess the valuable characteristics suitable for constructing large towers using the thin rubbing joint technique, decorated by engraving or sculpting delicate figures on their wall surfaces and top roofs, and why the Po Nagar Cham Towers have remained durable for thousands of years although tropical conditions. The samples, having been taken several years ago, are very limited, and in this exploratory work we have to limit invasive analysis as much as possible.

Appropriate characterization techniques, such as XRF, XRD, Raman micro-spectroscopy, and other methods commonly used in ceramic technology [19–21], were utilized to analyze the original Cham bricks in detail, supporting the discussion on the likely technology and key factors involved in making them. The optimal behavior with regard to the evacuation of the large quantities of rainwater specific to the tropical climate leads to measuring the open porosity. Mechanical and physical properties are also crucial for buildings of significant height and carvability. Comparative analyses of the newly made bricks for the tower restoration during the 2000s were, thus, performed.

## 2. Experimental

### 2.1. Bricks and Samples

Two types of samples used in our study were kindly provided by the Khanh Hoa Relics Conservation Center, which is in charge of the conservation of the Cham Po Nagar Towers. One type of samples consists of original ancient Cham bricks used in the construction of the Po Nagar Cham Towers. These include intact bricks (36 × 19.5 × 5.5 cm³) and pieces taken from the tower wall, as well as those collected during the tower's restoration (Figure 2a). They were sourced from the same group of ¹⁴C-dated bricks (13th–14th centuries), hereafter referred to as OCB [16].

<figure><img src="image_4.png" /><figcaption>**Figure 2.** View of a 13th–14th century OCB (a) and an NRB (b) used for tower restoration in the 2000s, both with their cross-sections, respectively, shown in the right-hand-side of each one; the cut traces are from the sample preparation. Cubes and rods from the reddish crust (c,d) and black core (e,f) of the OCB, and from the NRB (g,h), used for hydric, compressive, and thermal expansion tests.</figcaption></figure>
The other type of samples consists of newly made bricks (31 × 17 × 5.7 cm³) produced specifically for restoration of the Po Nagar Cham Towers during the 2000s, hereafter called NRB (Figure 2b). Their major difference in appearance is that the OCB shows a reddish crust and black core, while the NRB is rather homogeneously reddish throughout the cross-section. Depending on the specific experiments, the appropriate samples were prepared from the aforementioned bricks, either by cutting them into cubes of 3 cm × 2 cm × 2 cm (Figure 2c,e,g) for mass density, hydric, and compressive tests, and rods of 0.7 × 0.7 × 2 cm (Figure 2d,f,h) for thermal expansion/shrinkage measurement, or grinding them into fine powder for XRF, XRD, and Raman micro-spectroscopy. Logically, it is not possible to extract a large number of samples from cultural heritage relics for statistical measurements. However, it is important to demonstrate that a combination of various appropriate analyses on the available samples can yield useful insights for understanding ancient brick technology, enabling more suitable approaches for future restoration efforts.

## 2.2. Characterization Techniques

Images of the cross and thin sections of the samples were taken using an optical microscope (OM) interfaced with a digital camera (MC 170 HD, Leica, Germany), and the Leica Application Suite v4.3 software was used for investigating the petrographic composition. They were also observed with a Quanta 200 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA), coupled with an energy dispersive detector (EDS) for the elemental analysis.

Carbon (C) content in the samples was determined via CO₂ generated during burning samples in an oxygen atmosphere at temperatures up to 1550 °C, using a CHS-580 analyzer (ELTRA, Oststeinbek, Germany).

XRF spectra were taken to determine the mineral compositions, using a portable Tracer III SD AXS instrument spectrometer (Bruker, Billerica, MA, USA) equipped with a Rhodium Target X-Ray tube operating at 11 mA, 40 kV to detect elements Z-number greater than 11.

XRD patterns were taken using a Philips PW 1050/39 powder diffractometer or Equinox 5000 (Thermo Fisher Scientific, Waltham, MA, USA) using Cu Kα radiations.

Raman spectra were recorded using the green laser excitation of XploRA PLUS and HR800 micro-spectrometers (Horiba Scientific Jobin Yvon, Kyoto, Japan) within the spectral range of 50–4000 cm⁻¹ to determine the mineral phases present in the samples. This is a powerful tool to distinguish between charcoal and coal or different structural phases with the same chemical formula [19,20].

The compressive strength of brick was tested using a Tensile Tester Supper L120 (USA, maximum capacity of 600 kN) to assess the stress until the brick failed. Samples for measurement were cut into cubes (3 × 2 × 2 cm³) from the bricks shown in Figure 2 and from other pieces provided by the Khanh Hoa Relics Conservation Center.

The mass density was determined by weighing the dried sample of certain dimension, divided by the calculated volume of the sample. In addition to the samples cut from the bricks shown in Figure 2, pieces of other OCBs and NRBs were collected to determine their mass density for comparison.

The open porosity and the water sorption rate were measured via continuous real-time weighing the samples over time, starting from the dry state until reaching stable water-saturated sorption after immersion in water. Initially, the dried brick sample is suspended on the balance and weighed. Then, a water bath is introduced to contact the brick sample, causing it to adsorb water and increase in weight. When the sample immerses in water, the weight decreases due to Archimedes' force, and this weight is considered the net dry weight for subsequent calculations. The measurements stop at a certain time when the weight is fully stabilized. The weight difference between the dry and water-saturated
bricks was determined, presenting the total sorbed water, which corresponds to the open porosity.

Thermal expansion/shrinkage was measured using a dilatometer L75VS1600C (Lindeis, Selb, Germany) with the working range from room temperature to 1600 °C to determine the sintering temperatures used for firing the bricks. Samples for measurement were cut into rods of 0.7 × 0.7 × 2 cm from the bricks shown in Figure 2, with a clear distinction between the crust or core of the OCB and the near-external part of the NRB. This measurement is based on the shrinkage that occurs during the firing process as a result of the reaction of solid components and then stops at the highest temperature applied when making the brick. Therefore, measuring the change in the brick sample size as a function of temperature can give information about structural phase changes and help determine the highest sintering temperature used when firing the brick. In other words, as a general phenomenon in solids, with increasing temperature, the brick expands until the temperature approaches the highest firing point, at which it has already undergone changes; then, it begins to shrink again as a continuation of the past firing process.

## 3. Results

### 3.1. *Black Core of OCB and Morphology*

It is well known that various technological factors, such as starting materials, their grain sizes, with certain additions of specific minerals, coal or plant residues, firing temperature and time, redox environment during firing, all result in the different characteristics of the bricks, including color, porosity, and durability [22–27]. Bricks are usually made from clay, which contains Fe³⁺ ions in Fe₂O₃, giving them a reddish-brown color. After firing, most bricks appear reddish-brown on the outside, but the inside can be different colors depending on the materials used and the manufacturing technology. The first significant feature of the OCBs is that the inner part (core) looks fine black like a black velvet; this serves as a fingerprint (Figures 2a). We emphasize that the fine and smooth black color of the OCB core resembles the black of charcoal powder, not the metallic reflected dark gray of FeO or Fe₃O₄ [28,29].

To document the black core of OCBs, the carbon (C) contents (in wt%) in different samples were determined. The C content (~0.6 wt%) detected in the black core of OCB is about 8 times higher than that in NRB (0.08 wt%). The C in the OCB black core may arise from the deliberate addition of carbon-rich raw materials or from the high humic acid content [30–32]. Traditionally, in Vietnam, the mass production of fine charcoal powder could be easily collected from the slow smoldering of piled rice husks or straws for various uses, including as an additive material in brickmaking. A much lower C content (0.22 wt%) was determined in the reddish crust of OCB due to oxidation during firing (in an oxidizing atmosphere).

It is interesting that the Raman spectra taken from the OCB black core clearly demonstrate the presence of charcoal (Figure 3a) and its disappearance after firing at 800 °C in open air (Figure 3b). Correspondingly, the sample colors changed from black to reddish-brown due to the burning of charcoal into CO₂. The signature of disordered carbon [doublet lines at 1350 cm⁻¹ (sp³-type C-C bond stretching mode) and ~1600 cm⁻¹ (sp² C-C bond stretching mode)] is present in the black core but disappears in the burnt sample. Quartz is easily identified with its strong ca. 465 cm⁻¹ bending Si-O mode [19,20].
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Raman spectra taken from the OCB black core (a) and after it was fired at 800 °C in open air (b). The insets show that the black from charcoal in the core (a) was burnt, changing the color to reddish-brown (b).</figcaption></figure>

Figure 4 compares representative OM and SEM images of the OCB black core and the NRB core, respectively. The OM image of the OCB shows a dispersion of different phases formed by grains of 50 µm to 200 µm, even 500 µm (Figure 4a), including homogeneously distributed fine sand grains with high sorting in a visual estimation of the 20–25% (area) packing and also angular grains of quartz, feldspar (plagioclase-orthoclase), and very small mica plates. In addition, grog grains are visible, most reasonably from crushing and mixing of some piece of fired bricks with the clays and sand mixture, following the well-known brickmaking technology.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** OM images (taken with plain-polarized light) of the OCB black core (a) and the NRB core (b); SEM images of the OCB black core (c) and of the NRB core (d).</figcaption></figure>
In contrast, the OM image of the NRB presents the much smaller fine and very fine grains (<50 µm, Figure 4b), with packing of 2–3%, which consist mainly of quartz, feldspar, and rare mica. Additionally, medium coarse-sized grains (1–2 mm) are observed, comprising quartz, potassium feldspar, xenoplastic polycrystalline quartz, and high grade metamorphic rocks. From the SEM images, comparing the textures, one can see the OCB higher porosity with µm-sized pores and structure consisting of different shaped materials (Figure 4c), while the NRB looks denser and smoother (Figure 4d).

## 3.2. Compositions and Structural Phases

Table 1 presents the oxide compositions of typical OCB and NRB, determined using XRF and EDX methods. As XRF is not sensitive to the light elements, C could, therefore, only be detected in the OCB black core using EDX analysis. A relatively high C content was identified in a specific pot under analysis (sample number 5 in the table). After firing the OCB black core at 800 °C in open air, only a very low C content was detectable (sample 6). Other main elements (Si and Al) are determined to be similar in the black core and reddish crust of the OCB, when the C content is accounted for in the calculation. Iron oxide of around 5 wt% in the samples and the addition of calcium, magnesium, and titanium oxides are normal for most clay-based bricks. The SiO₂ composition determined from the ancient bricks at Sambor Prei Kuk, Cambodia, is very high at 81–92 wt%, which seems unreasonable for bricks [14].

<table><caption>**Table 1.** Compositions (wt%) in oxide forms (element for C) of OCB and NRB determined using XRF and EDX methods. The results obtained from the OCB core fired at 800 °C in open air (sample 6) are shown for comparison with those from its black core (sample 5). n.m, not measured; low, close to limit of detection.</caption><thead><tr><th>Sample</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>Fe<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>MgO</th><th>K<sub>2</sub>O</th><th>TiO<sub>2</sub></th><th>MnO</th><th>ZnO</th><th>C</th></tr></thead><tbody><tr><td>1. Reddish crust OCB (XRF)</td><td>58.6</td><td>25.0</td><td>7.8</td><td>1.6</td><td>1.5</td><td>3.9</td><td>1.2</td><td>0.13</td><td>0.02</td><td>n.m</td></tr><tr><td>2. Reddish crust OCB (EDX)</td><td>60.3</td><td>31.4</td><td>3.3</td><td>0.6</td><td>1.1</td><td>2.6</td><td>0.4</td><td>0.12</td><td>0.12</td><td>low</td></tr><tr><td>3. Crust NRB (EDX)</td><td>58.9</td><td>23.5</td><td>8.9</td><td>1.2</td><td>2.2</td><td>3.6</td><td>1.3</td><td>0.14</td><td>0.02</td><td>low</td></tr><tr><td>4. Black core OCB (XRF)</td><td>61.5</td><td>22.8</td><td>7.0</td><td>1.7</td><td>1.9</td><td>3.8</td><td>1.0</td><td>0.08</td><td>0.02</td><td>n.m</td></tr><tr><td>5. Black core OCB (EDX)</td><td>58.3</td><td>24.9</td><td>2.5</td><td>0.8</td><td>0.6</td><td>2.2</td><td>1.9</td><td>0.03</td><td>0.06</td><td>8.7</td></tr><tr><td>6. Fired core OCB (EDX)</td><td>61.9</td><td>22.9</td><td>6.3</td><td>1.7</td><td>2.0</td><td>3.8</td><td>1.1</td><td>0.08</td><td>0.02</td><td>low</td></tr></tbody></table>

XRD and Raman scattering spectroscopy were used to identify the structural phases in the OCB and NRB. The XRD patterns of the OCBs (Figure 5a) indicate the presence of quartz (Q), orthoclase (O), potassium feldspars (F), and illite (I). The presence of illite in all OCBs suggests that the bricks were fired at temperatures below 950 °C [33]. If fired at higher temperatures, illite would transform into other minerals, such as muscovite, or even form a glassy phase. The quartz phase is a major constituent, originating from sand, which is one of the starting materials. Additional small peaks around 2-theta of 22.9°, 27.9°, and 28.1° are assigned to parent calcium-based silicoaluminate, and those around 10°, 18°, and 35° are from magnesio-silico-aluminate (richerite), likely due to addition of grog. The XRD patterns of the NRB (Figure 5b) indicate the presence of quartz (Q), potassium feldspars (F), and hematite (H). The presence of hematite (H) phase is an indication of a higher firing temperature for the NRB in more oxidative conditions [34].
<figure><img src="image_7.png" /><figcaption>**Figure 5.** XRD patterns (c,d) taken at different parts as indicated in the photos of OCB (a) and NRB (b), respectively, showing different mineral phases (Q: quartz, F: feldspar, O: orthoclase; I: illite, H: hematite).</figcaption></figure>

Raman scattering spectra were taken from various spots of the black core and reddish crust of OCB to confirm the presence of various crystalline phases: quartz, with its strong ~465 cm⁻¹ Si-O bending mode; feldspars and plagioclases, with the 745 and 508 cm⁻¹ Si-O bending modes; hematite, with bands at 215, 280, 400, and 1330 cm⁻¹ [35]; calcite, with narrow peaks at 1085, 711, 276, and 152 cm⁻¹ [36]; anatase (a TiO₂ phase typically observed in ceramic fired below 1000 °C), with the 144 cm⁻¹ peak; and AsO₄⁻ or CrO₄⁻-based disordered phases, with a strong broad band at ~835 cm⁻¹ [37]. In some spots, a spectrum characteristic of a glassy phase was also recorded.

### 3.3. Open Porosity, the Rate of Water Sorption, Mechanical and Physical Properties

Besides the black core noted, it can be readily recognized that the OCBs are significantly lighter compared to the NRBs of similar size. This results from their lower mass density, which is directly related to the porosity. Specifically, the open porosity, resulting from the randomly interconnected fine voids of various volumes and distributions within the OCBs, essentially control the water sorption and drainage rates. As mentioned above, the open porosity of brick was determined via the increase in the weight of the water-sorbed sample compared to that of its dry. Figure 6 represents the water sorption rate of the OCB and NRB.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Water sorption rate as a function of immersion time in water of a typical OCB (a,b), and NRB (c).</figcaption></figure>

From the mechanical point of view, the open porosity of bricks is reasonably related to their compressive behavior. Figure 7 compares the representative stress-displacement curves by the OCB (for both the reddish crust and black core) and NRB samples. The compressive stresses were measured along with the reduction in sample size due to compression, which corresponds to the measured displacement of the actuator plate along with compression to deform the sample. The stress curves also give information regarding the elastic modulus, showing that the OCB core accommodates compression much more effectively. Obviously, series of measurements are required in order to extract the Weibull modulus of the different bricks.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Curves for the compressive stress measured on OCB (a,b), and NRB (c).</figcaption></figure>

The open porosity and its relevant value, the mass density, and the compressive failure strength of typical OCBs and NRBs are summarized in Table 2. The OCB presents the porosity of 18–25% and the mass density of 1.3 to 1.6 kg/dm³, while NRB shows the corresponding values of 14% and 1.8 kg/dm³, demonstrating much denser brick. The open porosity is crucial fact of the OCBs, enabling them to sorb and drain water fast. In fact, the amount of moss and mold is much lower on original bricks than on restoration ones (moss
is a rootless plant, and mold is a fungus; both thrive only in high humidity surfaces). The highly porous OCB facilitates rapid water drainage, effectively drying the brick surface to prevent the moss and mold grow) and to self-cleaning salts (from daily sea winds) that essentially make the Cham towers durable for thousands of years. Adding fine charcoal powder as a starting material could give the OCBs a high open porosity, with fine grogs to make their texture similar to sandstone suitable for rubbing construction and sculpting fine decorations afterwards. Similar bricks were made using clay-added biomass to make changes in the porosity and mass density [23–25]. Alternatively, adding spent coffee grounds improve buildings’ insulation [26]. The OCB and NRB show the compressive failure strength values above 20 MPa (Table 2 and Figure 7), sufficient for construction of big towers. One should remember that in civil engineering, normal bricks for housing usually have the compressive strength of 10 MPa, corresponding to the destructive compressive pressure of 104 kg/cm² [38].

<table><caption>**Table 2.** Open porosity, mass density, and compressive failure strength of the OCB and NRB.</caption><thead><tr><td>Sample</td><td>Open Porosity (%)</td><td>Mass Density (kg/dm³)</td><td>Compressive Failure Strength (mpa)</td></tr></thead><tbody><tr><td>Reddish crust OCB</td><td>18–25</td><td>1.30–1.60</td><td>23–25</td></tr><tr><td>Black core OCB</td><td>18–25</td><td>1.32–1.61</td><td>20–22</td></tr><tr><td>NRB</td><td>7–14</td><td>1.80–1.83</td><td>36</td></tr></tbody></table>

### 3.4. Firing Process

Figure 8 presents the thermal expansion/shrinkage of the OCB and NRB. The beginning of the release of adsorbed water above ~100 °C is obvious for the OCB that is reasonably related to the adsorbed water in a porous body, as is usually the case with ancient pottery. The characteristic ⊕-⊕ phase transition of quartz at 573 °C is clearly seen in all samples. For the OCB, the shrinkage curve decreases in a more complex shape, starting at ~820 °C and gradually shrinking until 1200 °C, possibly resulting from the complex starting materials including clay, sand, feldspar, grog, and charcoal powder. This is consistent with the phenomenon that liquid phases with silicates were formed under reducing atmosphere created by burning of charcoal, particularly in the core with much less oxygen, to form wustite (FeO) [6,39]. For the NRB, the shrinkage curve shows a step down starting at ~1050 °C and changing shape at ~1180 °C, which demonstrates the liquid phase mechanism as usual for a terra cotta in oxidizing firing [28].

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Thermal expansion/shrinkage of the OCB (black, dotted curve) and NRB (blue curve) with temperature.</figcaption></figure>
## 4. Discussion: Likely Technology for Making Original Cham Bricks

Based on the detailed analyses and discussion above, the likely technology for making OCBs can be proposed with the two most important key points as follows:

(1) The raw materials were locally mined clay and sand and charcoal powder derived from rice husks or straws. All starting materials must be ground fairly fine and well mixed together. The charcoal/clay ratio could be 10–15 wt% for making OCBs that have light mass densities ranging 1.3–1.6 kg/dm³ and high open porosities.

(2) The moderate firing temperature of bricks was between 800 °C and below 1000 °C, with a relatively long duration cycle to provide the conditions for efficient solid-state reaction, to ensure that the brick had adequate strength and open porosity.

At the early stage of our study, we presented a technological idea for producing bricks that are sufficiently hard, yet porous and carvable, specifically designed as imitations of Cham bricks for restoration work. This was recently granted as our patent [40]. The imitation bricks, utilizing 10–15 wt% charcoal powder as an additive and followed by firing at 800–900 °C for 5 h, exhibit the mechanical and hydric properties most similar to those of the ancient Cham bricks. The data obtained from our study on the original ancient Cham bricks, along with the imitation brick examples, provide validation for this proposed technology for making bricks suitable for the proper restoration of Cham towers. Moreover, practically oriented toward specific applications, recent studies have also explored similar approaches for producing lightweight bricks to maintain archeological heritage, by adding plant residues or rice husk into the green paste [23–25] or enhancing porosity for improved thermal-acoustic insulation by incorporating charcoal [41].

## 5. Conclusions

Based on the results obtained from various appropriate analyses, we propose the likely technology for making the original ancient Cham bricks by mixing clay, sand, grogs, and a carbon-rich precursor, shaping, and then firing at temperatures between 800 and below 1000 °C. As a result, the original Cham bricks possess the low mass density of 1.3 kg/dm³ to 1.6 kg/dm³ and the high open porosity of 18–25% for significant water sorption and fast drainage. The deliberate use of charcoal powder to make the bricks highly open-porous, combined with firing at moderate temperatures for sufficient-but-not-excessive hardness, are proposed to be the key technological factors promoting a unique hydric behavior and desirable mechanical and physical properties, to self-clean salts (from daily sea winds) through rainwater. Cham towers were constructed using the rubbing joint technique, then were beautifully decorated by engraving or sculpting, and they have sustained for thousands of years. The restoration bricks are too dense, resulting in a much lower rate of water sorption and drainage, which could be the main reason for their worse performance. It is important to note that our findings may provide a technological basis for producing modern bricks suitable for the proper restoration of Cham towers and similar architectural structures.

**Author Contributions:** Conceptualization, N.Q.L., P.C. and M.L.S.; sampling, B.T.T. and N.Q.L.; data curation, N.Q.L., N.T.L., T.T.T.H. and F.A.; formal analysis, N.Q.L., U.T.D.T., N.T.L., T.T.T.H. and L.V.D.; methodology, N.Q.L., P.C., N.T.L., U.T.D.T., F.A. and L.V.D.; validation, N.Q.L., P.C., D.H., M.L.S. and B.T.T.; writing—original draft, N.T.L., N.Q.L. and P.C.; writing—review and editing, N.Q.L., P.C., M.L.S. and D.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Data are available upon request.
**Acknowledgments:** The authors thank Le Quang Huy at IMS for his help in XRF measurement, and Ngo My Chau and Nguyen Tuan Dung at the Relics Conservation Center of Khanh Hoa Province for their helpful discussions. The Objective Lab for Agriculture-Bio-Medicine and Energy of VAST/IMS is kindly acknowledged for allowing us to use the lab's equipments. Thanks are also due to R. Giarrusso of GeoLab s.r.l. (Italy) for the support in the petrographic study.

**Conflicts of Interest:** The authors declare no conflict of interest.

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