Architecture and function of salt-producing kilns from the 8th to 18th century in the coastal Sundarbans mangrove forest, Central Ganges-Brahmaputra Delta, Bangladesh

Subsidence along the central coast of the Ganges-Brahmaputra Delta and a subsequent rapid burial by mangrove mud have preserved a hundred earthen kilns at 11 localities, exposed due to recent coastal erosion. The associated pottery indicates that the kilns were used for salt crystallization in brine pots mounted on perforated hotplates. Four kiln varieties were found at successive elevations. The kiln chamber walls and the basal charcoal layers were dated using thermoluminescence and radiocarbon dating respectively. The oldest kilns with a round outline, found at the lowest elevations, were operating in the early 8th to middle 9th century. Oval-shaped kilns, found at a 1-m higher elevation, were used from the mid-9th to early 10th century. After a 600-year long period without documented kilns, rectangular kilns with a lateral slot were built again at a 1-m higher land surface in the 16th and 17th century. The youngest kiln type consists of rectangular twin chambers dating from the late 16th to mid-18th century and occurs at 1.5 m below the present sediment surface. Architecture and functioning of these rectangular kilns are reconstructed, showing a multi-step procedure. This technique had been improved in the 19th century when pyramidal kilns were described in historic records.