 Therefore, the actual pure copper content in "red cash coins" is usually around 98%, the remaining 2% usually being lead, zinc, and other impurities which were all beyond skills of the local mint technicians in Xinjiang to remove.Sometimes due to circulation or due to corrosion the slag kernels or organic impurities in the body of a particular "red cash coin" would decompose or wear out and would form a comb-like cluster of very small see-through openings which the Chinese refer to as "sand eyes".At the introduction of red cash system in Southern Xinjiang in 1760, the exchange rate of standard cash (or "yellow cash") and "red cash" was set at 10 standard cash coins were worth 1 "red cash coin".