What is Overlay?
An overlay is the coating of a molten glass surface with another layer of glass, this glass is usually of a different color.
What does a glassblower do when he makes an overlay? When the glassblower makes an overlay, he gathers glass of a different color over an existing one and produces a glass piece with two layers acting as one. The artist uses a piece of colored rod, called "rapping cane". The rapping cane is used to cover the clear borosilicate glass under it. While turning the blowpipe slowly, the rod is softened and wiped on a hot surface. The glassblower does this by starting at the top. The cane is kept very hot which makes a good seal to the surface that is being rotated. To make a solid color, the glassblower repeats his previous actions. The glass piece is then reheated and formed into the desired shape.