Stone Matching for Historic Buildings

Tower, Glasgow University – highlighting both decayed and replaced stonesStone Decay – A Major Concern for Historic Buildings

Historic sandstone buildings throughout Scotland are decaying rapidly as a consequence of their age, together with the impact of long-term pollution from domestic coal burning and the industrial revolution era. This decay manifests itself either in the slow disintegration of the stone or by the catastrophic failure and collapse of parts of a facade.

The optimal option is to completely replace the original stone, but unless the new stone is closely matched to the old one, repair may be harmful to the structure itself and the building’s appearance can be compromised. Unfortunately, many of the original quarries are no longer in operation. Meanwhile, inappropriate interventions, such as stone cleaning, and poor quality repairs, such as mixing impervious, less porous cement materials with much softer sandstones, have already caused widespread damage to historical buildings that are now in urgent need of attention.

Stone Matching – A Difficult Challenge

Experience has shown that the specific characteristics of each stone are crucial to their performance on a facade. These critical properties include the grain sizes and specific minerals in the stone, as well as the amount of pore space between grains, all of which must be taken into account when selecting a suitable replacement. Some sandstone is more prone to decay than others. In stones where their properties differ, permeability often varies. Permeability barriers are created between the stones. Water penetrates at different rates and collects at points that become a focus for decay.

The University of Glasgow's School of Geographical and Earth Sciences (GES) has decades of experience in the study of rocks and minerals.

Sandstone Analysis with Electron Microscopy

GES has fully equipped laboratories dedicated to the characterisation of a wide range of stone types,  highly-skilled sample preparation capabilities, which are no longer widely available in the UK.

The Imaging Spectroscopy and Analysis Centre (ISAAC) within GES can undertake more sophisticated analysis. Using high-resolution field-emission scanning electron microscopy (SEM), those minerals in pre-cut thin section samples, which are of a grain size too fine to study by light microscopy, can be identified from their chemical composition via X-ray microanalysis.

High-resolution images can also reveal the size and shape of mineral crystals such as calcite, kaolinite, and illite that may block pore spaces and so impact the rate of stone decay.

‎Download the full Stone Matching for Historic Buildings case study.