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A key invention. Until the Hellenistic period, few ceramics were produced by molds, and those few were usually molded only in front and handformed in back. The Hellenistic invention of keys, however, made the bivalve mold—a mold with two halves—practical. The keys, protrusions (male keys) from one half-mold that fit into indentations (female keys) on the other half-mold, insured a close, secure, correctly matched fit between the two halves. A half-mold for the base of an oil lamp (left) displays three male keys, at top, lower left and lower right.

The invention of the bivalve mold with keys revolutionized the oil-lamp industry. Before this invention, lamps were individually crafted, a slow and costly process; but afterward only one lamp, the archetype, needed to be crafted, and then many negative molds of plaster or ceramic could be made. The two halves had matching keys that, when fit together, made a perfect match. The molds were filled with clay and closed. When the clay had dried, the mold was opened, and the clay lamp was removed from the mold and fired in order to harden it into the finished ceramic lamp, such as the lamp at right, dated to the second century C.E. This easy production of any number of molds from one handmade archetype lamp meant that high-quality lamps could be mass-produced and that division of labor could be applied to the process.