The casting of coins during the
 Spring-autumn and Warring States

The earliest coin moulds which have been found, are spade- and knife coin moulds from the Spring-Autumn and Warring States period. The earliest coin moulds were carved in clay, stone or bronze. The inscriptions were in intaglio and they were used for the actual casting. The clay moulds could be used only one time, the stone- and bronze moulds could be used several times. Since the clay moulds were destroyed after the casting, this has been a slow an expensive way of producing coins. Soft stone types were used for stone moulds since hard stone would crack, because of the high shift in temperatures during the casting. The type of mould shown below, had to be destroyed after the casting, in order to release the coins. Stone moulds were not destroyed, and had a shape, which allowed the mould to be opened and used again. The early carved bronze moulds could be used for the actual casting and were not mother moulds. The ant nose money of the Chu state is an example of direct casting in carved bronze moulds (A Xing p. 67). I have not been able to find an explanation to why direct casting in bronze moulds were only used in this period and not later on. If Bronze could be used for direct casting, one should think it would have been preferred to clay and stone, since it was more durable. My own guess is, that there must have been problems releasing the coins after casting. The ant nose money are very simple and crude coins with inscriptions in intaglio, maybe it was possible to cast this shape in bronze moulds, but not the more sophisticated designs with rims, holes and inscriptions in relief. 

Later moulds from this period were usually made from clay. The mould used for the actual casting was called a child mould (fig. 2). The child mould was made with a so-called mother mould (fig. 1). The mother moulds could be made of clay, stone or bronze. The clay child molds had tenons, that fit into the opposite part of the mould, to keep the two parts aligned. This kind of moulds had half the impression on each side, but some types of coins were cast in moulds where only the obverse was impressed. In that case, the reverse side of the mould was a completely flat polished stone or brick. All the round coins shown on the Warring States page are cast with this method.
The child moulds were piled and covered with a clay sheath to keep the assembly together. The moulds of the Han were baked before casting, and moulds were likely also baked in earlier times, since the clay would probably crack during the casting, if they were only sun-dried. 

      Fig. 1. Mother mould                       Fig. 2. A pile of child moulds      Fig. 3. The cast knife coins before 
                                                                                                                                    they were broken apart.

Illustrations from Tang, Wenguang: Wo Guo Gudai Jizhong Huobi de Zhuzao Jishu.
Zhong Yuan Wenwu, 1983 nr. 2, p. 75, 76.