Cleaning Methods for Historic Building Facades: A Systematic Review and Decision-Making Framework

This journal article presents a comprehensive Systematic Literature Review (SLR) of cleaning methodologies for historic building facades, synthesizing 78 peer‑reviewed sources (1990–2025) across six linguistic traditions. The study critically evaluates aqueous, chemical, abrasive‑mechanical, and advanced cleaning technologies, demonstrating—based on cross‑cultural evidence—that no universal cleaning method exists and that effective intervention requires material‑specific and climate‑specific diagnostic assessment. The article introduces an executable multi‑criteria decision‑making framework that operationalizes the principle of minimum intervention. As stated in the paper, “cleaning is not an automatic or self‑justifying operation” and must only proceed when diagnostic evidence confirms active deterioration or significant loss of legibility. The framework consists of sequential diagnostic gates: necessity assessment, substrate and porosity characterisation (including intra‑lithotype variability), contaminant‑type diagnosis, climatic and expertise constraints, spot testing, and post‑intervention monitoring. Drawing on Persian, English, Russian, Chinese, German, and Turkish scholarship, the review provides a cross‑linguistic synthesis with direct relevance for practitioners working in arid and semi‑arid climates, where earthen and brick heritage is highly vulnerable to moisture ingress and salt crystallisation. The article further identifies femtosecond laser cleaning, biocleaning, and nanostructured controlled‑release gels as the most substrate‑compatible emerging technologies for sensitive historic materials. A qualitative vignette based on field observations at Golestan Palace (Tehran; UNESCO World Heritage Site) illustrates the application of the framework to a materially heterogeneous facade comprising unfired mud brick, fired brick, glazed tile, and limestone. The study concludes by proposing three strategic axes for sustainable conservation: pollutant control at source, targeted use of advanced cleaning and consolidant technologies, and intelligent environmental/structural monitoring. This work contributes a unified, field‑ready protocol that integrates minimum‑intervention ethics, cross‑linguistic evidence, and petrographic variability into a single decision‑making tool for heritage conservation professionals.