Characterization of Irreversible Black Soiling Layer Formation on Historic Unglazed Terracotta Substrates Using Analytical Scanning Electron Microscopy (ASEM) with Energy Dispersive X-Ray (EDX) Analysis

Abstract

The increased use of architectural terracotta in the smoke-ridden cities of Victorian Britain was largely attributed to its ‘self-cleaning’ nature. However, it quickly became apparent that use of the material in urban areas often resulted in its affliction with certain soiling characteristics. Previous research has largely overlooked thorough investigation of the very mechanisms by which the soiling becomes so tenaciously bonded to the facade of terracotta in the first place. The manufacturing process and the properties of architectural terracotta have been reviewed along with current theories surrounding soiling and adhesion mechanisms. Both soiled and unsoiled samples were characterized using reflected light optical microscopy and analytical scanning electron microscopy (ASEM) with energy dispersive x-ray (EDX) analysis. The soiling layer was a uniformly thin amorphous layer that appeared to be brittle in nature and formed in-situ by external influences. It contained crystalline gypsum inclusions and submicron iron-rich particles.