Split 4A - Split Optic Mold for Glassblowing. The Split 4A has 8 ribs with semi-circular indentations running vertically. It is 4" tall with a 2" opening at the top that tapers to a 1-1/2" opening at the bottom. This is a great mold for putting rounded ribs and bumps on a piece. It comes equipped with base plate and handle or kick lever. The mold is made of cast aluminum.