Since the early twentieth century, more than one hundred and fifty silver Sasanian and Arab-Sasanian coins, dating from the fourth through seventh centuries, have been found at Turfan. Sixth and seventh century documents from Turfan mentioning these coins also have been discovered. Analysis of the coins and documents reveals two high points in the presence of silver specie at Turfan. The first was in the late fourth century and the second was from the late sixth through the seventh centuries. This data is somewhat puzzling when compared with finds of Sasanian coins in Central China. For the first peak at Turfan there are no corresponding coins in central China. Also perplexing is that the majority of specie found in central China was minted during the reign of the Sasanian emperor Pêrôz (459-84), but only one of his coins has been discovered at Turfan.
    These spatial and temporal fluctuations in coin distribution can be explained by shifts in trade routes. The late fourth century finds at Turfan correspond to the temporary unification of north China under the Former Qin dynasty (351-94) and its attempts to encourage trade with the west. However, warfare and political instability in north China and East Turkestan brought a swift end to the opportunity for international commerce. The importation of coins from the Near East to China revived in the mid-fifth century due to two factors. One was the reunification of north China under the Wei dynasty. The other was a great quantity of coins that Pêrôz paid as ransom and tribute to the Hephthalite tribal confederation in West Turkestan after he was defeated in battle and captured. However, these coins did not pass through Turfan because conflict between the Hephthalites and the Rouran tribal confederation of Mongolia made the middle silk route through Turfan too dangerous. Instead, trade passed along the southern route via Khotan and Xining in Qinghai before entering the Hexi corridor. The final peak in trade coincided with favorable conditions in China, Turkestan, and Iran. China was unified under the Sui and Tang, mint output in Iran hit a peak under Khusrau II (591-628), and all of Turkestan was united under the Western Turks. At this time coins appear in great numbers at Turfan because the stability engendered by Turkish rule allowed the reopening of the middle silk route.
    The documents found at Turfan show that during the first peak, coins must not have circulated widely in TurfanÐcloth being the most common medium of exchange. However, during the second climax, the coins were commonly used in private transactions involving expensive goods. The fiscal policies of the Gaochang kingdom ruling Turfan encouraged the circulation of these coins because silver specie was used to tax land and trade, and pay government obligations.