For glassblowers the jacks are a major shaping tool, but their main purpose is to put a neckline or constriction in the glass so it can be removed from the punti-rod or blowpipe.

The "snow-person" or caterpillar demo is a common exercise to practice cutting jack-lines into glass. We will be using the marver—a steel table used to cool and shape molten glass—the jacks and tweezers.

Focus is on using the gather heat, consistent turning, speed, and timing. There is no use of a reheating furnace for this exercise, as the goal isn't to make a "good" snow-person, but to move through the process quickly using just the gather heat and break the glass off into the annealer at the appropriate moment. We can learn when this moment is by paying attention to the color of the glass and it's movement (glass that's still moving is probably too soft to break easily). At the right moment we can chill the neckline that we want to break—using the cold parts of the jack-blades—and the piece will easily break free from the punti rod.