An optic mold is an open, cone-shaped mold with a pattern on the inside. When hot glass (on the end of a mandrel or punty) is pressed into the optic mold the pattern is transferred to the glass. After removing glass from from the mold the glass is often twisted to create a spiral design.
Optic molds are available in many shapes and sizes. Small versions that are just a couple inches tall are intended for glass artists working with a torch. Bigger versions suitable for the glass blowers working with large gathers of glass are also available. Common patterns include stars that have from four to more than thirty points.