Experimental definition of microclimatic conditions based on water transfer and porous media properties for the conservation of prehistoric constructions: Cueva Pintada at Galdar,Gran Canaria,Spain

Microclimatic parameters and natural materials were studied in order to assess conservation of the cave complex at Galdar, Gran Canaria. Based on the microclimatic data, experimental simulations were carried out to quantify water retention capacity and water vapour transport kinetics under continuously changing extreme temperatures and relative humidity values. The behaviour of natural construction materials is greatly influenced by changes in thermo-hygrometric conditions and is linked to pore structure. The host rock has a complex porous media: high porosity and polymodal pore size distribution, where the smallest pores contribute to water condensation, whilst large pores lead to high water absorption rates. The axial compressive strength of the host-rock decreases between 30 and 70% at water saturation. Stuccos covering cave wall paintings are formed by clay minerals, mainly smectites. These clay minerals cover a large specific surface area, which leads to a high capacity for water condensation and retention. It is also shown that neither water condensation nor vapour transport are noticeably modified by the presence of stucco on the host-rock when rapid, highly variable changes occur in environmental conditions if large shrinkage cracks are present. Results show that safe threshold microclimatic conditions can be found below 75% RH in the natural temperature range and that slight variations in temperature and relative humidity do not modify durability properties.