Early-Stage (10-Cycle) Freeze–Thaw Damage Sensitivity and Multi-Metric Conservation Assessment of Historic Blue Bricks from Beijing

Previous characterisation of historic blue bricks (qingzhuan) from Beijing identified compositional and physical differences across periods, but their effect on early-stage freeze–thaw behaviour was untested. An adapted laboratory wetting–freezing–thawing procedure was applied to four Ming-attributed and four Qing-attributed Great Wall bricks from the Miyun section as the primary cohort, with Lingyue Temple and Guanyin Chanlin bricks as supporting cases. The 10-cycle endpoint is below the 15 cycles GB/T 2542-2012 prescribes for a frost-resistance rating, so the results index early-stage damage sensitivity, not freeze–thaw durability. Dry mass loss, colour difference (ΔE*ab), gloss change and visible damage were evaluated. The lower-density, higher-absorption (Ming-attributed) group lost 1.22 ± 0.84% of its dry mass versus 0.37 ± 0.12% for the denser, lower-absorption (Qing-attributed) group, with complete rank separation that persists after normalising by estimated coupon surface area. At four bricks per group, the exact Mann–Whitney result (U = 16, p = 0.029) sits at the smallest attainable p-value, so the comparison is exploratory and hypothesis-generating and establishes no dynastic difference in frost resistance. Bulk properties predicted neither damage magnitude nor mode: a low-mass-loss coupon fractured through, and the metrics ranked specimens differently. Conservation assessment should report material loss, structural integrity and surface preservation as separate endpoints, and match repair material by water absorption, pore structure and freeze–thaw behaviour rather than colour and composition alone.