The casting of coins during the Tang

In the Tang dynasty there were written rules for alloys. The materials for one furnace that produced 3.300.000 coins in the Kai Yuan and the Tian Bao period (713-756) were 21220 jin of copper, 3709 jin of tin and 540 jin of lead. On the basis of the average weight of a coin, Peng calculated a waste of 23,5 %. So these Kai Yuan coins were an alloy of 83% copper, 15 % tin and 2% lead. Some of the Kai Yuan coins however, are almost as white as silver and some are bluish, these must be a different alloy. In the same period there were 99 mints in the country and Peng cites two historical texts that 327.000 strings were minted annually (Peng p. 257. Kaplanís translation of pewter and black tin is a mistranslation of tin and lead, xi and heixi, Yang p. 37 also refers to this, but has tin and lead confused). 

Mother coins.
A different technique was used in Sui and Tang to produce moulds. So-called mother coins were cast in molds which were made from an engraved master coin. The mother coins were used to impress sand moulds used for the actual casting. This method was used in all later periods, and the old mother moulds were only rarely used again (Tang p. 74). My own guess is that minters shifted to use mother coins, because more coins, several hundreds, could be arranged on the mould using single mother coins. Mother moulds that large would have been too difficult to handle.

Lost wax.
Another technique was also used in Sui-Tang,
the so-called "lost wax method". It was used to cast the Kai Yuan Tong Bao coins (Tang p. 75). Mother coins of wax were made in large numbers. They were easy and cheap to make. The wax coins were used in the same way as the metal mother coins, but they were not removed from the molds. When the clay mold was heated up, the wax melted and ran out, thus leaving a cavity (A Xiang p. 66 and Peng p. 554). This technique had been used for other types of bronze casting, but the Sui-Tang was the only period this technique was used for casting of coins. We don't know why it was discontinued, but it indicates that there were disadvantages using this technique for mass production of small items like coins.

Mother moulds were not seen again after Tang, only mother coins of various materials were used.