jet

In hand typecasting, a jet is the small protrusion of metal left on the body of a type after casting. It forms where molten metal enters the mold cavity through a channel called the sprue, and remains attached to the type after the metal cools and solidifies. The jet must be removed before the type can be used for composition.

The jet is broken off by hand or with a snapping tool once the type has cooled completely. Because the metal is brittle when cold, the jet snaps cleanly at its base rather than tearing or distorting the body of the type. The point where the jet was attached leaves a small burr or raised edge, which may be smoothed with a file or rubbing block to prevent snagging on other types during handling.

The term jet derives from the verb to jet, meaning to project or stick out. It describes exactly what the sprue remnant does: it projects outward from the body of the type. Other trades use jet for similar extraneous metal projections on castings, though in general foundry work these are more often called sprues or gates. In type founding, the distinction is precise: the sprue is the entire channel; the jet is the broken-off stub remaining on the finished type.

This term is now largely historical. Modern type is rarely cast by hand, and digital fonts have made metal type obsolete in all but a few specialist printshops. Typecasters and letterpress printers still use the word, but younger workers in industrial printing are unlikely to encounter it outside of technical literature or restoration work on legacy equipment.