suspended on columns", as al-Muqaddasi called it. This may have been the congregational mosque of al-Muhtariqa. Port Abbasid Raqqa had an important river port, which played a vital role in trade and communications. Raqqa's location was ideal for a river port on the upper Euphrates – it was ice-free throughout the year, whereas the early 14th century author al-Dimashqi wrote that the Euphrates sometimes froze further north. Archaeologists have not found evidence of this port, but it may have been south of al-Muhtariqa on the bank of the Euphrates because this would have been a convenient location close to the city's main commercial center. Based on Ibn Sa'd's account of the famous hadith scholar al-Waqidi's visit to Raqqa under Harun al-Rashid, it seems that the port of Raqqa was separated from the city proper by a checkpoint and a "poor, simple guesthouse" (khān nuzūl). Most of the boats used on this part of the Euphrates were probably light carriers called harraqs. Economy of Abbasid Raqqa Abbasid Raqqa was an important center of glass and ceramics production. Al-Muqaddasi also mentioned a soapmaking industry at Raqqa, which is connected to the glass industry because both make use of alkali. Minerals used as colorants in glassmaking or glazing pottery may have come from Jabal Bishr to the south, since Yaqut al-Hamawi recorded glassmakers in Aleppo using minerals from Jabal Bishr as a colorant in the early 1200s. According to Julian Henderson, Raqqa is one of the earliest places where an important shift in glassmaking technology occurred at the turn of the 9th century (i.e. around 800). Before, glassmakers had been using mineral alkali as a flux or purifying agent in the glassmaking process. Around 800, though, Raqqawi glassmakers switched to using plant ashes, which were readily available and much cheaper to obtain. On top of that, the plant ashes contained potassium, which lowered the melting temperature for the glass furnaces, reducing production costs even further. Henderson connects this technological change with the famous alchemist Jabir ibn Hayyan, who is known to have had an interest in glassmaking at around the same time, indicating that he may have been a resident of Raqqa during this period. The port of Raqqa was probably the main entrepôt (shipment point) where food and goods from northern Syria and Mesopotamia were shipped to before then being exported to Baghdad and the rest of Iraq. For example, Ibn al-Adim