Salt Decay and Salt Mixtures in the Architectural Heritage: A Review of the Work of Arnold and Zehnder

ABSTRACT

Salt decay is one of the main decay mechanisms of the porous building materials but due to its complexity progress has been slow, both in terms of understanding the mechanism itself and defining mitigation solutions. Research has so far privileged a reductionist approach, focusing on the behavior of a few individual salts. However, the typical reality of these constructions is the presence of multi-ionic solutions from which, depending on the conditions, different types of salts can precipitate in different crystalline forms. The originality of Arnold and Zehnder was that they did not embrace reductionist approaches, maintaining the focus on salt mixtures and the way these behave in real constructions. This path has led to approaches to the problem and possible mitigation solutions that have not yet been adequately discussed and explored. In this review, I seek to identify the most interesting contributions of the two authors and the directions they suggest for future research.

KEYWORDS:

• Andreas Arnold
• architectural heritage
• efflorescence
• Konrad Zehnder
• monitoring
• preventive conservation
• salt
• salt decay
• salt mixtures

Acknowledgments

I thank Konrad Zehnder and Andreas Kueng for giving permission to reproduce the images in Figures 1 to 3 and Table 2. I am also grateful to Konrad Zehnder for the comments he was kind enough to make on my original text.

Disclosure statement

No potential conflict of interest was reported by the author(s).