Coupling Micro-Scale Stone Decay Measurements with Bedrock Fracture Analysis at the Castle of Mykonos

Quantifying stone decay rates in coastal heritage sites remains a major challenge, owing to strong spatial variability in environmental exposure and material properties. This study presents a combined micro-scale and macro-scale investigation of stone deterioration processes at the Castle of Mykonos, a medieval coastal fortification founded directly on crystalline bedrock and exposed to intense marine forcing.

An in situ monitoring experiment was implemented to quantify stone surface recession over time. Corrosion-resistant reference plates (marine-grade 316 stainless steel) were installed on selected stone ashlars of different lithologies, and the surrounding surfaces were replicated using high-precision silicone moulds. Non-contact 3D optical profilometry was applied to the moulds to generate high-resolution surface models at time zero and after one year of exposure. Surface roughness parameters and elevation differences between stone surfaces and reference plates were computed to determine material loss rates with sub-millimetre accuracy.

Petrographic analyses show that the castle masonry consists mainly of granitic and tonalitic orthogneisses, with subordinate crystalline marbles. These lithologies display markedly different deterioration behaviours. After one year, marble surfaces show negligible changes in roughness and elevation, indicating high resistance to salt-related decay. In contrast, gneissic stones exhibit severe surface recession and textural degradation, including preferential detachment of feldspar porphyroclasts. Quantitative measurements indicate average material losses of approximately 1.6 mm, locally reaching up to 2.7 mm, accompanied by a significant increase in surface roughness.

Ion chromatography analyses of soluble salts reveal a strong marine signature, dominated by chlorides with subordinate sulphates. Salt concentrations are systematically higher at stone surfaces than in near-surface layers, but their spatial distribution does not correlate straightforwardly with proximity to the shoreline, highlighting the complexity of salt transport and accumulation processes in coastal masonry.

At the macro-scale, photogrammetric surveys were conducted to assess the structural condition of the bedrock underlying a seawards-facing wall of the Church of Sotira (Panagia Paraportiani complex). Mapping of discontinuities reveals a dense network of steeply dipping conjugate joints and subordinate foliation-parallel planes, which subdivide the bedrock into decimetric blocks. Salt-enhanced joint opening, combined with the load of overlying masonry, promotes block detachment, progressive undercutting, and local instability of the foundation.

The integration of quantitative micro-scale decay measurements with structural analysis of the supporting bedrock provides a robust framework for assessing deterioration rates and stability risks in coastal heritage sites, with direct implications for long-term monitoring and conservation planning.

 

Acknowledgement:

This research has been funded by European Union’s Horizon Europe research and innovation programme under THETIDA project (Grant Agreement No. 101095253) (Technologies and methods for improved resilience and sustainable preservation of underwater and coastal cultural heritage to cope with climate change, natural hazards and environmental pollution).