Digital image processing of weathered stone caused by efflorescences: A tool for mapping and evaluation of stone decay

This paper shows the usefulness of the digital image processing in the study of black and white images of stone damage caused by efflorescences and sub-efflorescences. The research has been undertaken using photographs from the Crypt of the Cathedral of Cádiz (Spain). The treatment of the digital images obtained allows the separation of the damage indicators (efflorescences and sub-efflorescences) and lets us quantify the actual damaged stone surface. This technique also shows the different degrees of stone damage, which depends on the thickness of salt deposits. These results, together with those obtained from other traditional analysis, explain the sequence of the damage process. Finally, the validity of the method is established by the preparation of very precise maps showing the superficial damage on a wall of the Crypt.

Research highlights
► This study shows that digital image processing is a very useful technique for the study of cultural heritage deterioration caused by efflorescences and sub-efflorescences.
► This non-destructive technique, based on the analysis of grey level, allow distinguishing between different degrees of stone damage and the thickness of salt deposits.
► This methodology is able to quantify the extension of the weathering and the damage degree.
► The technique is fast and precise to build weathering maps of walls.