A new method for making artificially weathered stone specimens for testing of conservation treatments

The application of new consolidating products on the surface of weathered materials is a common intervention technique in conservation practice. Due to the difficulty of producing artificially weathered substrates in a reproducible way, the effect of consolidating products in laboratory is generally assessed on sound substrates. However, the properties of a weathered substrate largely differ from that of the original sound material; this might make the results of laboratory tests unreliable or hamper their interpretation. In this research, a new method for the production of weathered specimens in a reproducible way has been developed and validated on three types of limestone with different total porosity, pore size and petrographical characteristics: Maastricht, Savonnières and Euville. The aim was to develop a substrate on which the effectiveness, compatibility and durability of consolidating products can be tested in laboratory in a more reliable way than when using fresh stone. The method consists of grinding and sieving the stones in a grain size largely similar to that of the sound material and re-aggregating the particles by the use of air lime: a lean “mortar” is obtained which is applied as a layer on the sound stone to simulate the decayed surface of a material showing granular disintegration. The grain size and the binder to aggregate ratio are chosen in such a way as to reproduce those characteristics typical of weathered stones showing loss of cohesion (i.e. sanding or powdering): i.e. increased pore size and open porosity and lower cohesion and strength in comparison to the sound substrate. The properties of the obtained weathered substrates have been studied in comparison to that of the fresh stone: pore size and pore size distribution have been measured by Mercury Intrusion Porosimetry; Polarized and Fluorescence Microscopy has been carried out to study the petrographical characteristics of the assemblage sound stone/re-aggregated layer; the water absorption behavior and hardness (by means of Drilling Resistance Measurement System, [DRMS]) have been measured as well. The results of the research show that with this method it is possible to obtain specimens reproducing the higher and coarser porosity and lower mechanical strength, typical of stones suffering loss of cohesion.