Another eleventh-century medieval Chinese coin found in England

This is sort of half-true. Non-fiat coins trade slightly above the value of their weight for a few reasons, primarily the convenience they offer. And there certainly is a theory that minted coins could be expected to contain a standardised weight and metal mixture.

But they couldn't really. Just look at that extract -- the Tang dynasty published laws detailing the metal mixture its coins contained. It did not bother to follow or even approximate those laws.

Here's a description of the bronze currency of Seleucid Persia:

> coin weights were not controlled to nearly the same extent as for gold and silver. An analysis of the bronze coinage of Seleukos I from Seleukia-Tigris in Newell shows weights ranging from 2.08-4.55 grams for one-unit pieces, 5.17-9.24 grams for two-unit pieces, and 8.94-16.72 grams for four-unit pieces.

> The very wide range of weights, on average more than 25 per cent about a mean, cannot be due to wear. Thus, when a bronze coin circulated, its value was probably known by its approximate size and not by its weight.

Here's the much more strictly controlled silver:

> Under Antiochos IV, a reduction of tetradrachm weight is noted between the first and second Antioch series from mainly in the region of 16.90-17.19 grams to mainly in the region of 16.50-16.79 grams, a reduction of about 2 per cent, which can only have been due to an administrative decision. This came at a time when competing coinages had also experienced weight reductions

> There was, of course, not only a question of weight, since, in order to achieve a greater profit, one might also reduce the silver content. Until Antiochos IV, purity of 94-95 per cent silver was still maintained in tetradrachms, but a debasement commenced slowly with Alexander Balas (91 per cent purity) and reached quite low levels of silver by the first century BC (65 per cent purity).

Why did the subjects of the Seleucids tolerate this? They weren't given a choice, and we have a record of their complaints:

> Now one can perhaps understand a passage in the Astronomical Diary of 274 BC. After reference had been made to the collection of 'silver, cloth, goods, and utensils [?]' by the satrap of Babylon to help the king (Antiochos I) pursue the war against the Ptolemies in northern Syria, the Diary continues: 'That year, purchases in Babylon and the [other] cities were made with copper coins of Ionia.'

> The small-silver coinage had apparently been collected by the authorities to help finance the war [because Greek mercenaries required payment in silver], probably in exchange for bronze, and the inhabitants were now compelled to use the distasteful (to them) bronze coinage, distasteful because it was a purely token currency introduced into an area which had possessed a long tradition of exchange based on the value of precious metal

(all quotes from The Seleukid Royal Economy: Finances and Financial Administration of the Seleukid Empire, https://www.amazon.com/dp/0521117763/ )

We may also observe that Chinese coins fill the same economic role as Seleucid bronze coins, being used to transact in small amounts. Large-scale commerce in China was done in unminted silver.