The invention of glassblowing led to an enormous increase in the range of shapes and designs that glassworkers could produce, and the mold-blowing process soon developed as an offshoot of free-blowing. A craftsman created a mold of a durable material, usually baked clay and sometimes wood or metal. The mold comprised at least two parts, so that it could be opened and the finished product inside removed safely. Although the mold could be a simple undecorated square or round form, many were in fact quite intricately shaped and decorated. The designs were usually carved into the mold in negative, so that on the glass they appeared in relief.
Next, the glassblower—who may not have been the same person as the mold maker—would blow a gob of hot glass into the mold and inflate it to adopt the shape and pattern carved therein. He would then remove the vessel from the mold and continue to work the glass while still hot and malleable, forming the rim and adding handles when necessary. Meanwhile, the mold could be reassembled for reuse. A variation on this process, called “pattern molding,” used “dip molds.” In this process, the gob of hot glass was first partly inflated into the mold to adopt its carved pattern, and then removed from the mold and free-blown into its final shape. Pattern-molded vessels developed in the eastern Mediterranean, and are usually dated to the fourth century A.D.
While a mold could be used multiple times, it had a finite life span and could be utilized only until the decoration deteriorated or it broke and was discarded. The glassmaker could obtain a new mold in two ways: either a completely new mold would be made or a copy of the first mold would be taken from one of the existing glass vessels. Therefore, multiple copies and variations of mold series were produced, as mold makers would often create second-, third-, and even fourth-generation duplicates as the need arose, and these can be traced in surviving examples. Because clay and glass both shrink upon firing and annealing, vessels made in a later-generation mold tend to be smaller in size than their prototypes. Slight modifications in design caused by recasting or recarving can also be discerned, indicating the reuse and copying of molds.
Roman mold-blown glass vessels are particularly attractive because of the elaborate shapes and designs that could be created, and several examples are illustrated here. The makers catered to a wide variety of tastes and some of their products, such as the popular sports cups , may even be regarded as souvenir pieces. However, mold-blowing also allowed for the mass production of plain, utilitarian wares. These storage jars were of uniform size, shape, and volume, greatly benefiting merchants and consumers of foodstuffs and other goods routinely marketed in glass containers.