The discovery, in the first century B.C., that molten glass could be inflated to create objects was soon followed by the realization that vessels could be formed and decorated in a single operation in a mold. Since their introduction by the Romans in the early first century A.D., molds have been used continuously and remain one of the most common tools of the glassmaker. The technique spread from the eastern Mediterranean region, where it originated, to all Islamic glassmaking areas, becoming particularly popular in Iran.
Two types of molds were typically used in Islamic production. Most medieval examples were created in so-called full-size molds, though no examples of these survive. Molten glass at the end of a blowpipe was inserted into a two-part hinged mold in the shape of the vessel and then inflated. The pattern, carved on the interior walls of the mold, was impressed in relief on the glass upon inflation; the mold was opened to release the object, which was then finished with a rim, foot, or handle as necessary. Such a mold was called full-size, as no changes were made to the size or shape of the glass object after it was removed from the mold. Popular patterns include vertical ribbing or fluting, honeycomb and chevron designs, a variety of geometric and vegetal motifs, and sometimes inscriptions.
The dip-mold was also popular. Molten glass was inflated in a cylindrical mold in order to impress the pattern; the glass parison, or bubble, was then removed and further inflated outside the mold and tooled in a variety of forms to create the desired object. The subsequent inflation would make the pattern appear in a lower, less distinct relief than a pattern created using a full-size mold.
By its very nature, molded glass was duplicable to a certain extent, though the shaping and finishing details could produce a unique object. A mold had to be conceived, designed, and cast before a glass vessel was created. Most molds were probably made of bronze, though less durable materials may have been used. Thus, a metalworker was also involved in the process, making the chain of production more complex.
Carboni, Stefano, and Qamar Adamjee. “Glass with Mold-Blown Decoration from Islamic Lands.” In Heilbrunn Timeline of Art History. New York: The Metropolitan Museum of Art, 2000–. http://www.metmuseum.org/toah/hd/mold/hd_mold.htm (October 2002)
Carboni, Stefano. "Glass with Mold-Blown Patterns." In Glass from Islamic Lands: The al-Sabah Collection, pp. 197–259. New York: Thames & Hudson, 2001
Folsach, Kjeld von, and David Whitehouse. "Three Islamic Molds." Journal of Glass Studies 35 (1993), pp. 48–56.