Performance Assessment of Acrylate Metal Complex (AMC) and Conventional Consolidants for Fragile Bone Artefacts

Highlights

What are the main findings?

• AMC forms an organic-inorganic hybrid reinforcement network via coordination with Ca²⁺ in bone matrix.
• AMC exhibits superior penetration, mechanical enhancement, and aging resistance compared to B72 and Remmers 300.
• AMC demonstrates excellent compatibility and stability in real archaeological bone artifacts.

What are the implications of the main findings?

• Theoretical significance: This study breaks through the traditional “surface film-forming” reinforcement model and proposes a strategy of constructing an internal reinforcement network through organic-inorganic synergy, providing a new theoretical path for the deep reinforcement of porous bone artifacts.
• Methodological significance: This study established a simulated sample model based on structure matching and applied it to real unearthed bone artifacts for verification. This formed a systematic research path from mechanism research and performance evaluation to practical application, providing a research paradigm that can be promoted for the scientific evaluation and technical verification of cultural relic protection materials.
• Application significance: AMC exhibits comprehensive advantages in mechanical reinforcement, aging resistance, and appearance compatibility, providing a new solution for the long-term stable preservation of fragile bone artifacts.

Abstract

Archaeological bone artifacts frequently exhibit diminished mechanical integrity as a result of organic matrix degradation. Under adverse environmental conditions, such artifacts are particularly susceptible to surface cracking and disintegration into powder. It is urgently necessary to develop protective materials that possess high permeability, strong reinforcing power and good compatibility. This study evaluated the protective performance of a novel Acrylate Metal Complex (AMC) and two conventional commercial consolidants (acrylic resin Paraloid B72 and ethyl silicate-based material Remmers 300) on fragile bone artifacts. Using simulated samples resembling bone artefacts, a systematic evaluation was conducted to assess the penetration, mechanical reinforcement efficacy, microstructural modifications, chromatic impact, and aging resistance of three consolidants. The results indicate that AMC demonstrates optimal permeation capability and can significantly enhance the surface hardness of bone specimens, achieving an increase of 7.7%. The colorimetric changes observed in all three reinforced materials following treatment remained within acceptable limits (ΔE* < 1.5). Accelerated aging tests—including 300 h of UV irradiation and 30 cycles of alternating dry-wet conditions—demonstrated that bone-mimetic composites reinforced with AMC exhibited significantly superior aging resistance relative to those treated with B72 and Remmers 300. In the actual application verification of the archaeological bone relics, the surface hardness of the reinforced AMC increased by 10%, the wave velocity increased by 14.8%, and there was no glare or crust on the surface. Comprehensive comparison shows that AMC outperforms traditional commercial materials in key performance indicators, demonstrating great potential as a next-generation bone relic conservation material.

1. Introduction

Bone artefacts, as vital material carriers of human civilisation’s development, bear significant historical information concerning ancient dietary culture, craftsmanship techniques, and social etiquette, possessing irreplaceable historical and artistic value. Bone artefacts, as a biological material, comprise a composite system primarily composed of hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂, HA] and collagen. Bone artefacts are highly susceptible to environmental factors during burial, including fluctuations in soil pH, microbial erosion, and groundwater dissolution. These processes lead to the gradual degradation and loss of collagen, disruption of the hydroxyapatite crystal structure, and weakening of intergranular bonds, ultimately resulting in a significant reduction in overall mechanical strength. Under such circumstances, excavated bone artefacts become highly sensitive to environmental changes, particularly under fluctuating humidity conditions. They are extremely prone to exhibiting typical ‘powdering’ deterioration, characterised by surface powdering, delamination, and a sharp decline in mechanical strength [1,2,3,4,5,6,7,8,9,10]. This category of powdering disease not only severely compromises the visual integrity of bone artefacts, making it difficult to preserve their original form and historical appearance, but also significantly weakens their mechanical properties. This increases structural fragility, thereby elevating the risk of secondary damage during subsequent conservation, display, and research processes. Consequently, how to implement effective reinforcement and protection for excavated powdered bone artefacts has become a critical issue requiring urgent resolution within the field of cultural heritage conservation.

Early reinforcing agents used for bone artefacts primarily consisted of natural polymeric materials such as gelatin and agar. Although these materials show good compatibility with the bone matrix, they generally exhibit low mechanical strength, poor water resistance, and susceptibility to microbial degradation [11,12,13]. With the development of conservation materials, various polymer-based and inorganic consolidants have been introduced for the reinforcement of bone artefacts. However, these materials may still present several potential drawbacks, including moisture-induced swelling and cracking, insufficient toughness that may affect the fragile structure of bone remains, the possible introduction of harmful components during treatment, limited penetration into porous substrates, as well as long-term aging and discoloration [14]. Currently, acrylic resins and silicone-based compounds are among the most commonly used reinforcement materials for bone artefacts. Acrylic ester resins, particularly Paraloid B72, are widely applied due to their relatively mature application techniques and good reversibility [15,16,17]. Nevertheless, their penetration depth is often limited, and they tend to form surface films that may influence the appearance and long-term stability of the artefacts [18,19]. In contrast, silicone-based consolidants such as Remmers exhibit superior penetration ability but become brittle after curing, which may lead to microcracking. Therefore, the development of new reinforcing materials that combine good compatibility, adequate mechanical strength, and excellent aging resistance remains an important objective in the conservation of fragile bone artefacts.

Acrylate metal complex (AMC) represents a novel composite material formed through the coordination reaction between acrylate monomers and metal ions such as Ca²⁺, Zn²⁺, and Al³⁺. This process endows the material with ‘organic-inorganic’ hybrid properties, conferring the flexibility characteristic of organic polymers alongside the high strength and stability typical of inorganic materials. Research has demonstrated that AMC exhibits favourable performance in the reinforcement of inorganic cultural artefacts such as stone and ceramics [20,21,22,23,24,25]. Its unique ‘organic-inorganic’ hybrid structure confers excellent interfacial bonding capability with the inorganic matrix of bone artefacts. Concurrently, the presence of organic segments effectively mitigates internal stresses following material curing, thereby reducing the risk of cracking. For bone artifacts, which constitute a complex system primarily composed of an inorganic mineral phase (hydroxyapatite) and an organic matrix (collagen), AMC can penetrate into the interior of the bone and interact with the hydroxyapatite, thereby forming an internal supporting framework that bridges the powdered particles and ultimately achieves effective reinforcement of the bone artifacts. The supporting framework constructed through this solidification effect not only significantly enhances the mechanical strength of powdered bone, improving its overall structural stability, but also mitigates the detrimental impact of external environmental factors, such as fluctuations in humidity and temperature. This dual functionality of AMC theoretically enables it to overcome the limitations of conventional reinforcement materials in terms of compatibility, mechanical performance, and aging resistance, thereby offering new possibilities for the effective reinforcement and long-term preservation of bone artifacts.

This study addresses the powdering deterioration of bone artifacts by focusing on AMC and comparing its reinforcement performance with those of conventional commercial materials, Remmers 300 and Paraloid B72. By systematically evaluating the effects of the three reinforcing agents on the physical properties, chemical stability, microstructure, and aging behavior of powdered bone samples before and after treatment, the reinforcement effectiveness for powdering bone artifacts was comprehensively assessed. The results aim to provide a scientific basis for the selection and optimization of reinforcement materials for bone artifacts, and to further verify the practical application potential of AMC in the field of bone artifact conservation, ultimately offering a feasible technical approach to addressing the challenge of powdering deterioration in bone artifacts.

To facilitate a clearer comparison among the consolidants,

Table 1. Comparative characteristics of AMC, Paraloid B72, and Remmers 300 used for cultural heritage consolidation.

Reinforcement Materials	Strength	Weakness	Penetration	Optimal Substrate
Acrylate Metal Complex (AMC)	Hybrid organic–inorganic network; enhanced mechanical strength; good aging resistance	New material; limited long-term field data	Deep	Highly porous and severely degraded cultural heritage materials, particularly fragile bone artefacts
Remmers 300	Strong penetration	Increased brittleness	Deep	Porous materials with moderate to slight deterioration
Paraloid B72	Excellent reversibility	Surface film formation	Moderate to shallow	Slightly degraded porous materials; also suitable for surface protection or sealing after consolidation

summarizes their main characteristics, including strengths, limitations, penetration behavior, and suitable substrates.

2. Materials and Methods

2.1. Materials and Preparation

2.1.1. Bone Artifact Materials

In this study, an ivory artifact excavated from the Bashan site (original inventory number: 2022BS⑬:188) was selected as the research object. The artifact is in poor overall preservation condition, with severe local fragmentation, and exhibits widespread deterioration phenomena, including powdering, cracking, and flaking on the surface, representing typical characteristics of fragile bone artifacts (Figure 1). Ivory and bone share similar basic compositions, both primarily consisting of hydroxyapatite minerals and collagen-based organic components, and exhibit comparable porous microstructures. However, ivory generally possesses a more compact structure and distinctive Schreger lines, whereas bone contains more developed vascular pores and structural heterogeneity. Despite these differences, both materials undergo similar deterioration processes in burial environments, such as organic component degradation and mineral structure weakening. Detached fragments were used for compositional, structural, and microstructural analyses, and the analytical results served as an important basis for the preparation of simulated samples.

2.1.2. Preparation of Reinforcing Agents

Relevant information on reinforcement materials is shown in

Table 2. Reinforcement materials used in this study.

Reinforcement Materials	Main Components	Solvent	Manufacturer
AMC	Metal acrylate complex	Deionized water	Self-developed by the research group
Remmers 300	Tetraethyl silicate	Ethanol	Remmers GmbH (Löningen, Germany)
Paraloid B72	34% Ethyl methacrylate 66% Methyl acrylate	Acetone	Dow Chemical Company (Midland, MI, USA)

. The preparation procedure of the AMC reinforcing agent was as follows. Acrylic acid was reacted with calcium hydroxide to synthesize calcium acrylate, which was subsequently dissolved in deionized water. A specific amount of crosslinking agent and catalyst was then added, and the pH of the system was adjusted by the addition of ammonia solution. Finally, the AMC reinforcing agent was prepared at a concentration of 7 wt% [23].

Remmers 300 is a typical ethyl silicate–based inorganic reinforcing agent that has been widely applied in the conservation of porous inorganic cultural heritage materials. To improve its penetration performance in highly porous bone materials and to avoid surface enrichment caused by the direct application of the undiluted product, ethanol is commonly used as a diluent [23,25]. In this study, Remmers 300 was mixed with anhydrous ethanol at a volume ratio of 1:1 and applied for reinforcement treatment.

Compared with high-concentration systems, low-concentration B72 solutions exhibit lower viscosity, which is more favorable for penetrating into internal pores through capillary action, and have therefore been widely used for the reinforcement of loose and porous cultural heritage materials [15]. Considering both the reinforcement capability and penetration performance of B72, acetone was selected as the solvent, and a 5 wt% B72–acetone solution was prepared and used as the reinforcing agent [26].

2.2. Performance Evaluation

2.2.1. Experimental Equipment

FA1204 electronic balance (Shanghai Tianshang Precision Instrument Co., Ltd., Shanghai, China), CR-400 color difference meter (Konica Minolta, Tokyo, Japan), Sigma360 field emission scanning electron microscope (Zeiss, Oberkochen, Germany), D8ADVANCEX X-ray diffractometer (Bruker, Billerica, MA, USA), STA449F3 synchronous thermal analyzer (Naichi Scientific Instrument Trading Co., Ltd., Toyama, Japan), LUMOS_II ultra-fast imaging infrared microscope (Bruker, USA), AutoPore V 9600 automatic mercury intrusion porosimeter (Micromeritics, Norcross, GA, USA), universal testing machine (Instron, High Wycombe, UK), etc.

2.2.2. Characterization Methods

• Surface color:

The L*, a*, and b* values of the sample surfaces before and after reinforcement were measured using a color difference meter, and the color difference (ΔE) was calculated accordingly. Five samples were tested for each group, and each sample was measured five times, with the average value reported. The calculation formula is shown below [23]:

$$∆\mathrm{E} = {[{(∆\mathrm{L}*)}^2+{(∆\mathrm{a}*)}^2+{(∆\mathrm{b}*)}^2]}^{1/2}$$

(1)

• Penetration performance:

The specimens were divided into three groups, with five specimens in each group. Each group was treated with one of the three reinforcing agents. The masses of the specimens before reinforcement (m₀) and after curing (m₁) were measured using a balance, and the effective penetration rate of the reinforcing agent was calculated based on the mass change in the specimens before and after reinforcement. The calculation formula is presented below [23]:

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}}\times 100\%$$

(2)

• Morphological analysis:

Macroscopic morphology: An ultra-depth-of-field microscope was used to observe the surface powdering and material loss of the simulated samples before and after reinforcement, as well as the reinforcement effect on the real artifact samples after AMC treatment.

Microscopic morphology: Scanning electron microscope (SEM) was employed to examine the microstructure of fracture cross-sections of both simulated samples and real artifact samples before and after reinforcement. Prior to observation, the samples were coated with gold to a thickness of 9 nm. SEM observations were performed at an accelerating voltage of 10 kV, with magnifications ranging from 100× to 5000×.

• Elemental and phase analysis:

Energy-dispersive X-ray spectroscopy (EDS) was used to analyze the changes in elemental composition and distribution on the cross-sections of the samples before and after reinforcement.

X-ray diffraction (XRD) was employed to investigate the phase composition of the samples before and after reinforcement. The measurements were carried out at a scanning rate of 5°/min over a 2θ range of 10–70°.

Fourier transform infrared microspectroscopy (FT-IR) was used to characterize the changes in functional groups of the samples before and after reinforcement. The spectra were recorded in the range of 600–4000 cm⁻¹.

• Compactness:

A non-metal ultrasonic testing instrument was used to evaluate the compactness of the simulated specimens before and after reinforcement. The compactness was assessed by measuring the ultrasonic wave velocity propagating through the specimens, which reflects changes in the internal structural density.

• Structural and mechanical property tests:

Flexural strength test: The three-point bending strength of the reinforced simulated samples was measured using a universal testing machine. The flexural strength was calculated according to the following equation:

$$\mathsf{\sigma }={\displaystyle \frac{3\mathrm{F}\mathrm{L}}{2\mathrm{b}{\mathrm{h}}^2}}$$

(3)

where σ is the flexural strength (MPa), F is the maximum load at fracture (N), L is the span between the two supporting points (mm), b is the specimen width (mm), and h is the specimen height (mm).

Surface hardness test: The surface hardness of simulated samples and real cultural relic samples was measured using a Leeb hardness tester. Five different positions were tested for each sample, and the average value was calculated.

Porosity test: Mercury intrusion porosimetry (MIP) was employed to evaluate the changes in pore size distribution and average pore diameter of the simulated samples before and after reinforcement.

• Durability and stability evaluation:

Dry–wet cycling test: The dry–wet cycling test was conducted using a constant temperature and humidity chamber. The temperature was set at 40 °C with a relative humidity of 75%. The specimens were placed in the chamber for 16 h, then removed and dried in an oven at 40 °C for 8 h. This procedure was defined as one cycle, and a total of 30 cycles were performed. After cycling, the specimens were examined for cracking, and changes in mass loss rate and flexural strength were evaluated.

Ultraviolet aging test: Ultraviolet aging was carried out using a UV aging chamber, with the distance between the specimens and the lamp maintained at approximately 500 mm. The surface temperature of the specimens was controlled at 40 °C ± 2 °C. The specimens were exposed to UV radiation at an irradiance of 0.68 W/m² in the UVB-313 wavelength range for 300 h. The mass loss rate, color difference, and changes in functional groups were measured during the aging process.

Salt resistance test: The salt resistance test was performed using a saturated sodium chloride solution. The specimens were immersed in the salt solution for 240 h, and the simulated samples were observed for the occurrence of cracking.

Thermal stability analysis: Thermal stability of the samples before and after reinforcement was analyzed using a simultaneous thermal analyzer. The temperature range was set from 30 to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere.

2.2.3. Statistical Analysis

All experiments were conducted using at least three parallel samples. Each measurement was repeated five times, and the average value was calculated. The experimental results are presented as mean values. Data processing and statistical analysis were performed using Origin 2021 software.

2.3. Preparation of Simulated Powdered Bone Samples

2.3.1. Composition and Microstructure of Excavated Archaeological Bone Artifacts

Following cleaning of the excavated samples, the microscopic morphology of the artefacts was examined using a scanning electron microscope (SEM). The composition and phase structure of the excavated bone artefact samples were analysed using an energy dispersive spectrometer (EDS), an X-ray diffractometer (XRD), and a Fourier transform infrared microspectrometer (FT-IR). The mechanical properties of the excavated bone artefact samples were tested using a universal testing machine.

SEM image (Figure 2a) reveals that the overall structure of authentic bone artefact samples exhibits distinct porosity, with numerous irregularly distributed pores and canal structures visible within the bone matrix. EDS analysis indicates the bone samples contain primary elements including C, N, O, Si, P, and Ca, wherein Ca and P constitute the principal components of hydroxyapatite, while the presence of Si relates to the infiltration of soil particles from the burial environment. FT-IR analysis (Figure 2b) indicates the presence of carbonate (CO₃²⁻), phosphate (PO₄³⁻), and carbonyl (C=O) groups within the samples, suggesting the retention of organic constituents [27,28,29].

The XRD results (

Table 3. Phase composition of the excavated bone sample.

No.	Phase Name	Chemical Formula	Content (%)	Source Description
1	Hydroxyapatite (HA)	Ca10(PO4)6(OH)2	74.8	Major inorganic component of bone
2	Carbonated hydroxyapatite (CHA)	Ca10(PO4)3(CO3)3(OH)2		Formed through partial carbonate substitution during burial
3	Calcium carbonate	CaCO3	8.2	Carbonate deposits formed by the reaction of external Ca2+ and Mg2+ with CO32−
4	Magnesium carbonate	MgCO3	9.0	Carbonate deposits derived from the burial environment
5	Silicon dioxide	SiO2	8.0	Introduced by soil or groundwater infiltration during burial

and Figure 2c) indicate that the main crystalline phases present in the samples include hydroxyapatite (HA), carbonated hydroxyapatite (CHA), silicon dioxide (SiO₂), and minor amounts of carbonates, reflecting that mineral replacement and recrystallization processes occurred to a certain extent during burial. The presence of nitrogen (N) and carbonyl groups confirms that trace amounts of organic matter are still preserved in the samples. Furthermore, JADE 9.0 software was used for qualitative phase identification and semi-quantitative analysis of the XRD peaks to determine the major phase composition and their relative contents.

2.3.2. Preparation of Simulated Samples

Based on the analytical results in Section 2.3.1, 7.48 g of nano-hydroxyapatite, 0.82 g of calcium carbonate, 0.90 g of magnesium carbonate, and 0.80 g of sieved soil were accurately weighed and mixed to prepare 10 g of simulated bone aggregate (Figure 3a). A certain amount of menthol was ground in an agate mortar to obtain a homogeneous powder for later use. Subsequently, 1 g of menthol was added to the 10 g of simulated aggregate and thoroughly mixed, followed by the addition of 2 mL of a 2% gelatin solution. The mixture was stirred until homogeneous and then reserved for further use. (Figure 3b). The introduction of menthol was intended to simulate the pore structure of real bone tissue, while the addition of the gelatin solution served to introduce an organic component and to improve the moldability and mechanical properties of the simulated samples.

The thoroughly mixed simulated bone aggregate powder was compacted into disk-shaped specimens with a diameter of 40 mm and a thickness of (5.5 ± 0.2) mm using a hydraulic press under a pressure of 40 Mpa (Figure 3c,d). To ensure complete volatilization of menthol and the formation of a porous structure, the molded specimens were dried in a forced-air drying oven at 80 °C until a constant mass was achieved (Figure 3e,f).

2.3.3. Comparative Analysis of Simulated Samples and Samples

To enhance the similarity between simulated samples and authentic bone artefacts samples, comparisons were conducted in terms of macrostructure, composition and mechanical properties.

Figure 4a and Figure 4b depict the cross-sectional macrostructures of authentic bone artefact samples and simulated samples respectively. Under prolonged burial and complex environmental influences, the organic constituents within the authentic bone artefact samples gradually leached away, forming a porous structure characterised by varying degrees of mineral filling within the pores. In this study, menthol was introduced during the preparation of the simulated samples. Taking advantage of its sublimation behavior, a large number of pores with similar sizes and good interconnectivity were artificially generated within the samples, resulting in a simulated system with a porous skeletal structure. Comparative results indicate that the simulated samples exhibit a high degree of similarity to authentic bone artefacts in terms of pore morphology, distribution characteristics, and overall structural porosity. This provides a comparable structural framework for subsequent investigations into the penetration, diffusion, and reinforcement behaviour of reinforcing agents within the material.

In terms of phase composition, the authentic bone artefact samples are mainly composed of hydroxyapatite (HA) and carbonated hydroxyapatite (CHA), accompanied by minor amounts of carbonate minerals and soil-derived particulate components introduced from the burial environment. Elemental analysis results show that the dominant elements are Ca, P, and O, with certain proportions of exogenous elements such as Si and Mg, indicating material exchange between bone minerals and the surrounding soil during burial. Combined with the elemental composition comparison shown in Figure 4d, it can be observed that the simulated samples exhibit elemental proportions that are generally consistent with those of the real samples. Moreover, the raw material ratios of the simulated samples were designed based on the semi-quantitative XRD analysis of the real samples. Therefore, a high degree of consistency between the simulated and authentic bone artefacts samples is achieved in terms of both elemental composition and phase constitution.

In terms of mechanical properties, the results show that the flexural strength of the authentic bone artefact samples is approximately 0.8–1.0 MPa, indicating a pronounced deterioration in mechanical performance. The prepared simulated specimens exhibit a compressive strength of 1.0 ± 0.2 MPa, which is comparable to that of the authentic bone artefact samples. This similarity suggests good mechanical comparability between the simulated specimens and the authentic bone artefact samples.

2.4. Reinforcement Process

The simulated samples were placed in a vacuum drying oven and dried at 80 °C for 24 h. The dried samples were randomly divided into four groups: a blank control group, an AMC-treated group, a B72-treated group, and a Remmers 300-treated group. Reinforcement was carried out using the dropwise impregnation method, in which 7 wt% AMC, 5 wt% B72, and Remmers 300 were respectively applied dropwise onto the dried simulated samples. During the impregnation process, the dripping rate was carefully controlled to ensure uniform penetration of the reinforcing agents into the interior of the samples. For each specimen, 0.5 mL of reinforcing agent was applied per treatment at 1 h intervals, with a total of five applications. After impregnation, the samples were cured at room temperature for 7 days to allow complete reinforcement.

3. Results

3.1. Surface Color Variation

According to the Chinese color tolerance system, the allowable color difference (ΔE) is defined as ΔE ≤ 1.5 for achromatic colors and ΔE ≤ 3.0 for chromatic colors [30]. As shown in Supporting Information Table S1 and Figure 5a, after reinforcement with AMC, Remmers 300, and B72, the ΔE values of all treated samples were within 3.0. Among them, the sample reinforced with Remmers 300 exhibited the smallest color difference, with a ΔE value of 0.95, while the ΔE values for the AMC-and B72-treated samples were 1.25 and 1.05, respectively, both within the acceptable range.

Overall, all three reinforcing agents meet the requirement of ‘minimal color alteration’ in cultural heritage conservation. Remmers 300 shows the best performance in preserving the appearance of the samples, whereas AMC maintains a relatively low color difference while ensuring effective structural reinforcement, indicating its good applicability in terms of color stability.

3.2. Penetration Performance

The penetration performance of reinforcing agents directly affects the reinforcement depth and the overall effectiveness of bone artifact treatment. Among the evaluation indicators, the mass change rate is an important parameter for assessing penetration ability. As shown in Supporting Information Table S2 and Figure 5b, the mass change rate of the AMC reinforcing agent is significantly higher than that of the B72 solution and Remmers 300, indicating that AMC is able to penetrate into the internal structure of the simulated samples in larger amounts.

From a microscopic perspective, the superior penetration performance of AMC is closely related to its unique molecular structure. In solution, AMC exists as flexible coordination structures prior to full curing, which favors their diffusion and penetration into pores and microcracks within the bone matrix. In addition, during the dropwise impregnation process, both AMC and Remmers 300 were observed to spread rapidly on the sample surface and penetrate quickly into the interior, suggesting good fluidity and wettability that facilitate effective contact with the bone surface and inward penetration. In contrast, the B72 solution showed a relatively slower spreading rate during application, and part of the solution tended to accumulate on the sample surface, forming a dense film that hindered subsequent penetration.

Overall, the AMC reinforcing agent demonstrates a clear advantage in penetration performance, enabling more effective infiltration into the internal structure. This provides a strong foundation for improving the microstructure and mechanical properties of bone cultural relics.

3.3. Morphological Analysis

The unreinforced simulated specimens exhibit a loose internal structure, in which particles are mainly arranged in a loosely packed manner. Well-developed pores with high interconnectivity are observed, and the particle boundaries remain clearly distinguishable, presenting typical characteristics of powdering deterioration (Figure 6a).

After treatment with AMC, a continuous and uniform network-like reinforced structure is formed within the specimens. The reinforcing agent is able to fully penetrate the pore spaces of the skeletal framework and construct an organic-inorganic hybrid three-dimensional network between particles, effectively bonding the originally loose mineral grains. No surface film formation is observed, indicating good penetration capability and structural compatibility (Figure 6b).

Following treatment with Remmers 300, the internal structure of the specimens becomes denser compared with the untreated samples, and the interparticle bonding is enhanced to some extent. However, due to the limited penetration capacity of Remmers 300, the reinforcing agents are mainly distributed in localized regions and fail to form a continuous reinforcing network (Figure 6c).

In contrast, treatment with B72 results in the formation of an evident dense film-like layer on the specimen surface, with a large number of pores being covered or blocked. The reinforcing agent is primarily concentrated in the surface layer of the specimens (Figure 6d).

3.4. Elemental and Phase Composition Analysis

EDS analysis revealed that there were no significant changes in the elemental species or their relative contents before and after reinforcement (Figure 7a). Notably, Zn was detected in the specimens treated with AMC, which originates from the AMC formulation, indicating that the reinforcing agent successfully penetrated the interior of the samples (Figure 7b). Furthermore, a comparison of the FTIR and XRD patterns of the samples before and after reinforcement shows that the characteristic peaks are generally consistent, with no new absorption bands or diffraction peaks observed after treatment (Figure 7c,d). This indicates that no new crystalline phases were formed during the reinforcement process.

3.5. Compactness Evaluation

The non-metal ultrasonic testing instrument primarily evaluates the compactness and integrity of materials by measuring the propagation velocity of ultrasonic waves within the material. In general, a denser material with fewer internal defects allows ultrasonic waves to propagate more rapidly. In contrast, the presence of pores, cracks, or a loose internal structure leads to elongated propagation paths and increased energy attenuation, resulting in a reduced wave velocity.

As shown in Supporting Information Table S3 and Figure 8, the ultrasonic wave velocity of the samples treated with different reinforcing agents increased to varying degrees. Based on the changes in average wave velocity before and after reinforcement, all three reinforcing agents improved the internal structure of the samples to different extents. Among them, the AMC-treated samples exhibited the most significant increase in wave velocity, rising from 0.506 km/s to 0.91 km/s. This indicates that AMC can effectively fill internal pores within the bone structure, enhancing the overall continuity and compactness of the material. Consequently, ultrasonic wave scattering and attenuation during propagation are reduced, leading to an increased wave velocity. In comparison, Remmers 300 and B72 showed relatively smaller increases in wave velocity, although they still demonstrated a certain degree of reinforcement effectiveness.

3.6. Porosity Characteristics

To quantitatively evaluate the densification effect of different reinforcing agents on the internal pore structure of bone materials, mercury intrusion porosimetry (MIP) was employed to determine the porosity and pore size distribution of unreinforced and reinforced samples.

The unreinforced sample exhibited a porosity of 37.3434% with an average pore diameter of 123.24 nm (

Table 4. The result of porosity characteristics.

Item	Unreinforced	AMC	Remmers 300	B72
Porosity (%)	37.3434	26.7837	27.7483	29.4995
Average pore diameter (nm)	123.24	27.29	24.86	24.24

). Its pore size distribution curve showed a pronounced peak in the region above 100 nm, indicating the presence of a large number of highly interconnected macropores and irregular pores within the simulated partially fossilized bone material. This high porosity and broad pore size distribution are key factors contributing to the loose structure and poor mechanical properties of the sample (Figure 9a).

After reinforcement treatment, the pore structures of all samples changed significantly. Compared with the unreinforced sample, the AMC-treated sample showed a substantial reduction in porosity to 26.7837%, the lowest among the three reinforcement systems, with an average pore diameter of 27.29 nm. The pore size distribution curve reveals that the originally dominant macropores and irregular pores were markedly reduced after AMC treatment, and the overall pore size distribution shifted toward the mesopore range, exhibiting a more concentrated and continuous peak (Figure 9b). These results indicate that AMC does not simply block pore openings by forming a dense surface film; instead, it preferentially penetrates and fills numerous micropores and microcracks, thereby significantly reducing the total pore volume. Meanwhile, during reinforcement, some of the original small pores with complex morphologies and widely distributed sizes were integrated into a smaller number of more stable mesopores, resulting in a relative increase in the statistically averaged pore diameter. This pore structure evolution demonstrates the superior penetration and effective filling capability of AMC at the micro- and mesopore scales, enabling the formation of a continuous and stable reinforcement network within the bone material.

The porosities of the Remmers 300-and B72-treated samples were 27.7483% and 29.4995%, respectively, both higher than that of the AMC-treated sample, with average pore diameters mainly concentrated in the range of 24–25 nm. Their pore size distribution curves indicate that, although these reinforcing agents can reduce porosity to some extent, their ability to regulate the original pore structure is relatively limited (Figure 9c,d).

3.7. Flexural Strength and Surface Hardness

3.7.1. Flexural Strength

Based on the flexural strength test results of the samples treated with the three reinforcing agents (Supporting Information Table S4 and Figure 10a), significant differences can be observed in their mechanical reinforcement effects on partially fossilized bone. The samples reinforced with AMC exhibited the highest average flexural strength, reaching 1.924 MPa, representing the most pronounced improvement compared with the untreated samples. This indicates that AMC is able to form an effective reinforcement network within the pore structure. The average flexural strength of the samples treated with Remmers 300 was 1.37 MPa, showing a certain strengthening effect, although the improvement was relatively limited. In contrast, the samples reinforced with B72 showed the lowest average flexural strength, at only 1.274 MPa. Overall, AMC demonstrated the best performance in enhancing the load-bearing capacity of partially fossilized bone artifacts, indicating its superior reinforcement effectiveness.

The flexural strength of a material is closely related to its pore structure. In general, an increase in porosity usually indicates a higher number of internal voids and defects, a reduction in the effective load-bearing cross-sectional area, and a greater tendency for stress concentration under external loading, all of which are detrimental to the material’s resistance to bending failure. In contrast, the influence of permeability on flexural strength is more indirect, as it primarily affects the migration, diffusion, and distribution of the consolidant within the material, thereby influencing the final reinforcement performance. For fragile bone artifacts, relatively high porosity and strong pore connectivity not only reflect a more deteriorated structure and lower initial flexural strength but also provide accessible pathways for low-viscosity and highly penetrative consolidants such as AMC to enter the interior of the material. When the consolidant is able to penetrate sufficiently into pores and microcracks and form a relatively continuous supporting or consolidation network within the internal structure, the bonding between particles is enhanced and the initiation and propagation of cracks are effectively restrained, resulting in an increase in the overall flexural strength of the material. By contrast, if the consolidant remains mainly on the surface and fails to penetrate into the internal pore system, its effect is largely limited to superficial consolidation, and the improvement in flexural strength is usually restricted.

3.7.2. Surface Hardness

Surface hardness is an important indicator for evaluating the resistance of bone artifacts to surface wear and exfoliation. After reinforcement, the surface hardness of all simulated samples increased to varying degrees, with AMC showing the most pronounced improvement, as shown in Supporting Information Table S5 and Figure 10b. Compared with the untreated samples, the surface hardness increased by 7.7%, further demonstrating that the AMC reinforcing agent can effectively enhance the surface mechanical properties of friable bone materials and reduce surface powdering and flaking.

The change in hardness after reinforcement is mainly associated with the penetration behavior of the consolidant, its mode of consolidation, and its interaction with the bone matrix. When the reinforcing material is able to enter the internal pores and interparticle contact regions of the bone sample, it can not only fill voids but also bond loose particles and strengthen the connections between them. As a result, the compactness of the surface and subsurface regions is improved. With the enhancement of local structural stability, the sample becomes more resistant to indentation deformation, leading to an increase in surface hardness.

The extent of hardness improvement varies among different reinforcing materials, which is closely related to their distribution within the bone matrix and the structural state formed after curing. Materials with better penetration and the ability to form a relatively continuous consolidation structure inside the pores can enhance interparticle bonding and improve the stability of pore wall regions, and therefore usually produce a more pronounced increase in hardness. In contrast, if the consolidant is mainly concentrated near the surface, or if the cured supporting structure is discontinuous and exhibits limited interaction with the substrate, the resulting improvement in hardness is often less significant. In addition, the compatibility between the reinforcing material and the bone matrix, the interfacial bonding ability, and the volume change during curing may also affect the final hardness variation.

In this study, AMC showed relatively good penetration and internal consolidation ability. It was able to form a comparatively uniform organic–inorganic hybrid supporting structure within the pore system of the bone sample, thereby enhancing the bonding between loose particles and improving the overall stability of the surface region. As a consequence, AMC produced a more evident increase in hardness.

3.8. Durability and Stability Evaluation

3.8.1. Dry–Wet Cycling and Ultraviolet Aging

After hygrothermal aging and UV aging, the flexural strength of samples treated with all three reinforcement systems decreased to varying extents; however, clear differences were observed in the degree of degradation and overall stability. Under both aging conditions, AMC-treated samples consistently exhibited higher flexural strength than those treated with Remmers 300 and B72, while the latter two showed larger strength fluctuations after aging, indicating relatively poorer mechanical stability, as shown in Supporting Information Tables S7 and S8.

Under hygrothermal aging, the AMC-treated samples showed a comparatively higher mass loss; nevertheless, their flexural strength remained at a relatively high level, without a pronounced mechanical deterioration corresponding to the mass loss. This suggests that, as a hydrophilic reinforcing agent, the mass loss of the AMC system during hygrothermal aging mainly originates from the desorption of absorbed water and the migration or volatilization of a small amount of weakly bound organic components, rather than from damage to the main reinforcing framework. The organic-inorganic hybrid cross-linked network formed by AMC within the bone matrix exhibits good structural stability and resistance to hygrothermal conditions. Even when part of the absorbed water is lost during aging, the main structure of the reinforcing agent remains intact and continues to provide mechanical support. In addition, the effective filling and reinforcement of micropores and microcracks by AMC enhance the flexural performance of the samples from a structural perspective. Therefore, although a certain mass loss occurs after hygrothermal aging, the AMC-treated samples retain relatively high mechanical performance, demonstrating good durability and structural stability.

A comprehensive analysis of the mechanical properties, mass changes, and color differences under both hygrothermal and UV aging conditions indicates that the AMC reinforcement system exhibits a higher retention of flexural strength and better color stability after aging, effectively mitigating the degradation of bone materials caused by environmental aging (Figure 11). Compared with Remmers 300 and B72, AMC is more capable of maintaining the structural integrity and mechanical performance of bone materials under complex aging conditions, while exerting minimal impact on the appearance of the artifacts. This highlights its advantages in terms of durability and overall stability, making it a more suitable reinforcing agent for bone cultural heritage.

3.8.2. Salt Resistance

To evaluate the stability of different reinforcement systems under salt erosion conditions, unreinforced and reinforced simulated samples were immersed in a 5%NaCl solution, and their macroscopic structural changes were continuously observed. The results indicate that samples subjected to different treatments exhibited significant differences when exposed to the salt solution.

The unreinforced samples showed obvious cracking after only 4 h of immersion (Figure 12a), indicating a loose internal structure with high pore connectivity. Under the combined effects of salt solution penetration and crystallization pressure, structural damage readily occurred. The salt resistance of the B72-reinforced samples was improved compared with that of the unreinforced samples; however, cracking was still observed after 48 h of immersion (Figure 12b), suggesting that its reinforcement effectiveness is limited under prolonged salt erosion conditions.

In contrast, no obvious cracking or structural damage was observed in the AMC-and Remmers 300-treated samples after 240 h of immersion, and the samples maintained good structural integrity (Figure 12c). The superior performance of AMC may be attributed to its effective penetration and reinforcement of the meso- and microporous structures within the bone material, which reduces pore connectivity and the migration pathways of the salt solution. In addition, the formation of a relatively stable internal reinforcement network helps to alleviate the destructive effects of salt crystallization pressure, resulting in enhanced resistance to salt erosion.

3.8.3. Thermal Stability

Figure 13a and Figure 13b show the TG–DSC curves of the unreinforced simulated sample and the AMC-treated simulated sample under a nitrogen atmosphere, respectively. In the low-temperature region (room temperature to 150 °C), the mass loss of both samples is mainly attributed to the removal of adsorbed water, bound water within the pore structure of the simulated samples, and residual solvent from the AMC reinforcing agent. Compared with the unreinforced sample, the AMC-treated sample exhibits a slightly higher mass loss in this temperature range, indicating that the reinforcing agent and its associated solvent successfully penetrated and were distributed within the internal pores of the sample.

In the intermediate temperature range (150–500 °C), the observed mass loss corresponds to the thermal decomposition of minor organic components in the simulated sample as well as the organic constituents of the AMC reinforcing agent. In the high-temperature region (500–800 °C), both samples show a pronounced mass loss, which is associated with the decomposition of carbonates (CaCO₃ and MgCO₃), involving the release of CO₂ from CO₃²⁻ groups.

The onset temperatures of mass loss in this high-temperature stage are approximately 490 °C for the unreinforced sample and 545 °C for the AMC-treated sample. This shift indicates that the formation of an organic–inorganic composite structure after AMC reinforcement delays the decomposition of carbonates, thereby enhancing the overall thermal stability of the material system.

3.9. Reinforcement Mechanism Discussion

In terms of the reinforcement mechanism, the superior reinforcing performance of AMC can be mainly attributed to its unique organic–inorganic hybrid structure. Owing to its excellent penetration capability, AMC can infiltrate the microporous and mesoporous structures within the bone matrix and form a continuous and stable organic–inorganic hybrid consolidation network within the pores during the curing process. This network reinforces the pore walls and the interfaces between particles in a coordinated manner, effectively strengthening the bonding between particles and thereby significantly improving the overall structural stability and mechanical properties of the bone material. Meanwhile, this reinforcement approach avoids the formation of a dense surface film that is commonly observed with traditional resin-based materials, which helps preserve the original pore structure and air permeability of the bone material. In addition, it improves the stability of the material under hygrothermal conditions to a certain extent, demonstrating clear advantages in terms of material compatibility and the long-term preservation of cultural relics.

3.10. Bone Artifact Reinforcement Experiment

3.10.1. Pre-Treatment of Bone Artifacts

Prior to reinforcement treatment, essential surface pretreatment was carried out on the artifact. A soft-bristle brush combined with ethanol was used to remove adhered dust, soil, and biological residues from the surface in order to minimize their interference with the penetration of the reinforcing agent. This pretreatment ensured sufficient contact between the reinforcing agent and the artifact substrate, thereby providing favorable conditions for subsequent penetration and reinforcement.

3.10.2. Reinforcement Method

The AMC reinforcing agent was applied to the artifact surface using a dropwise method. During application, a slow and uniform dripping rate was maintained, and subsequent drops were applied only after the previous liquid had been fully absorbed, ensuring that no surface accumulation occurred. This allowed the reinforcing agent to naturally penetrate into the internal structure along the bone pores. As the solvent gradually evaporated, the pre-crosslinked acrylate metal complexes progressively formed a stable micro-network within the pore structure of the bone material, thereby achieving effective reinforcement of the powdering areas. After application, the artifact was left to cure under a constant-temperature environment.

3.10.3. Characterization of Reinforcement Effects

• Morphological structure

After reinforcement of the bone artifact with AMC, the pore wall surfaces exhibited a relatively uniform and continuous reinforced state. The originally loose particles along the pore walls were effectively bonded, and the pore boundaries became clearly defined, indicating that the pore walls were thoroughly infiltrated by the reinforcing agent and supported by a stable internal structure. Within some pores, thin and continuous reinforcing phases could be observed; however, no obvious thick film coverage was present. This suggests that AMC is capable of penetrating deeply into the bone pore system and forming a uniformly distributed reinforcement network within the internal structure (Figure 14a). Scanning electron microscopy (SEM) further confirmed these findings: unreinforced samples displayed a highly porous, structurally disintegrated surface morphology—characterized by flocculent or granular aggregates and minimal interparticle cohesion (Figure 14b). In contrast, AMC-treated samples developed a continuous, uniform consolidation layer; particle aggregation was significantly enhanced, pore boundaries were attenuated due to matrix infilling, and the overall microstructure demonstrated pronounced densification (Figure 14c).

• Characterization of reinforcement effects

To comprehensively evaluate the effectiveness of the AMC reinforcing agent on authentic bone artifacts, systematic characterization was conducted in terms of macroscopic appearance, mechanical properties, and surface stability, including color difference, surface hardness, and ultrasonic pulse velocity tests. The results show that the color difference (ΔE) after reinforcement was 2.15, which falls within the generally accepted range in the field of cultural heritage conservation, indicating that AMC treatment has a minimal impact on the artifact’s appearance. The surface hardness increased from 324 HL before reinforcement to 356 HL after treatment, demonstrating that AMC effectively enhances the surface mechanical properties of the artifact and improves its resistance to wear and exfoliation. Meanwhile, the ultrasonic pulse velocity increased from 610 m/s to 700 m/s, reflecting an improvement in the overall compactness of the internal structure. These results indicate that AMC not only enhances surface strength without significantly altering the appearance of the artifact but also effectively improves internal structural integrity, exhibiting a favorable reinforcement effect on real bone artifacts (Figure 15).

3.10.4. The Reinforcement Effect of the Archaeological Bone

Through practical consolidation treatments on archaeological bone, the effectiveness and applicability of the AMC reinforcement agent in the conservation of bone cultural heritage were further verified. After AMC treatment, the overall mechanical strength and internal structural integrity of the bone material were significantly improved, transforming the original loose and exfoliation-prone condition into a stable state. On this basis, the treated samples were able to effectively avoid secondary damage caused by mechanical disturbance or environmental changes during subsequent conservation procedures, such as cleaning, gap filling, adhesion, and surface finishing. Therefore, AMC reinforcement agent not only serves as a prerequisite structural reinforcement but also provides the necessary material and structural foundation for the smooth implementation of subsequent restoration processes and the long-term stability of conservation outcomes (Figure 16).

4. Conclusions

This study systematically prepared and evaluated the application performance of an Acrylate Metal Complex (AMC) for the reinforcement of friable bone artifacts. The results demonstrate that AMC exhibits excellent penetration ability, enabling effective infiltration into the internal pore structure of bone materials and achieving deep reinforcement. Compared with conventional reinforcement materials such as Paraloid B72 and Remmers 300, AMC significantly improves the mechanical properties of bone materials, with the surface hardness of simulated bone samples increasing by 7.7% after treatment.

AMC establishes a continuous organic–inorganic hybrid consolidation network throughout the bone matrix, thereby enhancing structural integrity without inducing surface film formation—thus preserving the original aesthetic and morphological characteristics of the cultural relics. Colorimetric analysis further indicates that the color variation after treatment remains within an acceptable range (ΔE < 1.5).

Accelerated aging experiments show that AMC-treated samples maintain stable mechanical and structural properties after 300 h of UV irradiation and 30 humidity cycles, demonstrating superior aging resistance compared with samples treated with Paraloid B72 and Remmers 300. In practical conservation tests on excavated bone relics, AMC treatment increased surface hardness by approximately 10% and ultrasonic wave velocity by 14.8%, confirming its effective reinforcement capability.

In conclusion, the Acrylate Metal Complex (AMC) demonstrates excellent overall performance in the reinforcement of friable bone artifacts, combining effectiveness, compatibility, and durability. It provides a reliable solution for the conservation of friable bone cultural heritage.