Taken from the Web:  Cased glass, frequently known as lining, plating or flashing, was not new in the nineteenth century, but that is the time period in which it became very popular. This type of glass is still in demand and continues to be one of the techniques that many glassblowers use today.

Cased glass is identified as a glass piece that has two or more layers of different colors. The inner or outer layers of this glass can also be clear.  In some instances, the outer layer of cased glass can be cut away in sections to bring out the color(s) of the previous casings beneath.