Soil may be a key reason for the preservation of Terracotta Army bronze weapons

We now proceed to assess other hypotheses, leading to the conclusion that the traces of chromium found on some weapons are the product of post-depositional contamination from Cr-rich lacquer. We hypothesised that a chromium compound could have been added to the lacquer by Qin artisans, rendering it Cr-rich and thus a potential source of chromium contamination for the weapons. The presence of chromium in these weapon parts would explain the consistent patterns of chromium presence/absence on bronze surfaces, depending on their proximity to lacquer, i.e. highly probable in sword grip fittings, shaft ferrules and those trigger parts bolted to the stock, and highly improbable in blades or arrowheads.

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