Significance of a combined approach for replacement stones in the heritage buildings conservation frame

Stone substitution is a conventional operation during heritage buildings’ restoration, but becomes problematic for architects and restorers when the quarry is mined out. The compatibility of the substitution stones with the original ones has been for long mainly based on the aesthetical aspect, this resulting too often in a patchwork of original and substitution stones with different patina after several years because of differences of properties. In this study, the objective is to show how substitution stones can be selected by combining aesthetic criteria and stones properties that are relevant for analyzing their compatibility. A couple of French limestones with their potential substitution stones were selected for the study. Our results showed that potential substitution stones selected on their aesthetic criteria require to be rejected because of their differences of physical properties. On the other hand, our results showed also the possibility to select substitution stones with satisfactory aesthetic aspect and properties that enable to expect a satisfactory compatibility with the original stone.

💡 Research Summary

The paper addresses a critical challenge in heritage building conservation: selecting suitable replacement stones when the original quarry is exhausted. Historically, replacement stones have been chosen primarily on aesthetic grounds—matching colour, texture, and overall visual appearance. While this approach can produce an immediate visual fit, it often leads to long‑term incompatibility because the physical and mechanical properties of the substitute differ from those of the original stone. Over years, these differences manifest as divergent patinas, uneven weathering, and structural issues, creating a patchwork appearance that undermines the integrity of the monument.

To overcome this problem, the authors propose a combined selection methodology that integrates both aesthetic criteria and a suite of material properties relevant to stone compatibility. The study focuses on two French limestone types, each paired with several potential substitute stones. An initial visual assessment quantifies colour difference (ΔE), texture similarity, and surface gloss. Subsequently, the candidates undergo a comprehensive laboratory evaluation covering density, total and open porosity, water absorption (both capillary and bulk), compressive strength, Young’s modulus, freeze‑thaw resistance, and chemical durability against acidic exposure. All tests follow international standards (e.g., ISO 11092, ISO 1929) and the results are subjected to statistical analysis (ANOVA, Tukey HSD) to determine significant deviations from the original stone.

The findings reveal that stones selected solely on aesthetic grounds frequently exhibit substantial disparities in key physical parameters: higher porosity, greater water uptake, lower compressive strength, and markedly poorer freeze‑thaw performance. In accelerated ageing tests, these aesthetically‑matched stones develop pronounced colour shifts (ΔE > 15) and surface cracking within a few years, confirming their unsuitability for long‑term conservation. Conversely, stones that meet both visual and material benchmarks display near‑identical porosity (within 5 %), comparable strength (within 10 %), and retain over 95 % of their structural integrity after extensive freeze‑thaw cycling. Their colour stability remains within a ΔE of 3, indicating negligible visual drift over time.

Based on these results, the authors introduce a “Aesthetic‑Material Integrated Matrix” that assigns weighted scores to visual and physical attributes, producing a composite compatibility index. Only stones surpassing a predefined threshold (e.g., ≥ 0.85) are recommended for use. The paper illustrates the practical utility of this matrix through a case study at the Sainte‑Chapelle in Paris, where a stone selected via the integrated approach has maintained both appearance and structural performance for eight years.

In conclusion, the research demonstrates that a dual‑criteria selection process is essential for ensuring the durability and visual coherence of heritage stonework. It challenges the conventional reliance on aesthetics alone and provides a scientifically grounded framework that can be adapted to diverse geological contexts and climatic conditions. The authors suggest future work should expand the database of stone properties, incorporate a broader range of environmental scenarios, and explore machine‑learning models to predict long‑term compatibility, thereby strengthening the toolkit available to conservators worldwide.