Ancient Chinese City Wall Bricks: An Integrative Review of Materials, Provenance, and Conservation
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Abstract
Ancient Chinese city wall bricks are engineering units, archaeological artefacts, and epigraphic records, yet the evidence is dispersed across architectural history, archaeology, materials science, building physics, and conservation engineering. This integrative critical review organizes 93 retained sources by evidential role: 40 direct studies of Chinese city walls or Great Wall materials (E1), 11 contextual studies of related Chinese heritage materials (E2), and 42 transferable mechanistic or normative sources (E3). Synthesis-generated propositions are explicitly labelled as hypotheses (E4). The evidence is arranged along a whole-life sequence from raw material and firing to circulation, masonry, weathering, diagnosis, and repair. Three conclusions emerge. First, pore-network and transport characteristics mediate moisture-, salt-, and frost-related durability, but must be evaluated with strength, soluble salts, drying behaviour, and interface compatibility. Second, published property values are only partly commensurable because studies mix specimen roles, porosity definitions, geometries, moisture states, and direct and indirect tests; the corpus supports a comparability audit, not pooled material constants. Third, the record is historically selective: 19 of 25 direct analytical or diagnostic studies are primarily concentrated on Nanjing, Xi’an, Beijing/Linqing, or major Great Wall sectors, while only 56 of 319 seats (17.6%) in the three Nanjing brick-levy provinces had brick cladding by the early Ming. Reuse can further separate present find-spot from production and exposure history. System-level evidence remains sparse; one Ming Great Wall study reports a 100–200 μm brick–mortar transition zone. The review proposes an evidence-calibrated provenance framework, distinguishes established mechanisms from testable hypotheses such as atmospheric sulfation, and defines six priorities for comparable data, source-clay and refiring references, interface and repaired-assembly testing, long-term monitoring, provenance analysis that treats reuse explicitly, and information continuity across conservation cycles.
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