Cased glass is a piece of glass that has two or more layers in different colors.
Of course, it’s not always that simple, either. (What’s life without some controversy?) Some collectors consider casing to be only when there is more than a single extra layer of glass. They call pieces with just one extra layer “plating.” There’s also a difference in technique. While casing is done on the blow pipe, plating is actually done after the piece is made.
For the purposes of this blog and eBay, though, let’s just stick with the initial definition that’s says casing is layered glass that happens to be in different colors.
Now, here’s some cool examples:
I usually try to look for the part along the edge where it looks like one layer of glass is “encased” in another. That’s a pretty good indication that what you have is cased glass.
Hope this helps!