Chemical and microscopic investigation of historical wall bricks collected from the City Wall of Xi'an, China

Abstract

The Xi'an City Wall was built in the early Ming Dynasty not only as a traditional defensive facility in the past but also as a landmark of Xi'an City at present. This huge building was a kind of brick-soil composite structure with a rammed earth wall core wrapped by large blue bricks approximately 45.0 cm long, 22.5 cm wide, 9.5 to 10.0 cm thick. In this paper, a brick sample stamped with “Qianwei” (Named the Ming brick), a government agency of the Wei-so system of the Chinese Ming Dynasty and four brick samples of the Chinese Qing Dynasty (Named the Qing brick) from the Xi'an city wall were analyzed by X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), optical microscopy (OM), the TESCAN integrated mineral analyzer (TIMA), mercury intrusion porosimetry (MIP) and other detection methods to study their composition, structure and properties, etc. Through analysis of the microphase species, the main components of alumosilicates-mixture and quartz including small amounts of albite, plagioclase, calcite and other minerals, were confirmed in historical brick samples. Meantime, the fine mineral particles were detected in brick samples resulting from the refined process of the clay raw material not only to reduce the mineral particle size but also to remove the organic material, soluble salts, etc., with the purpose of making wall bricks with high density and small porosity. This study provides important references about the composition, structure and property of historical wall bricks, which is conducive to recognizing their manufacturing technology and protection.