Some writers attribute this large number of aspers to a change in the route of the Silk Road during Manuel's reign. The destruction of Baghdad by Hulagu Khan in 1258 revived the trade route running north from Armenia and the upper Euphrates valley to Erzerum and then through the Zigana Pass to Trebizond, instead of to the Mediterranean. However, Anthony Bryer has pointed out the volume of trade carried on this route was minuscule, and taxes and duties extracted from the goods passing through Trebizond (known as the "kommerkion") would at most have been 30% of the Empire's total revenues in a busy year, and more often 6%. Another author has pointed out that the silver mines in the region of Gümüşhane were within the borders of the Empire of Trebizond during the 13th century, and could provide the raw material for these coins; but further investigation has shown that these silver mines were not heavily exploited before the 18th century. Kuršanskis has suggested that these coins represent the tribute extracted by the dominant Mongols of Persia, although this does not explain the care taken in minting all of these coins: most of this tribute would, following its delivery to the Mongol overlords, have been melted down and either used to produce other coins or jewelry.