(1) The application of a very thin layer of glass of one color over a layer of contrasting color. This is achieved by dipping a gather of hot glass into a crucible containing hot glass of the second color. The upper layer may be too thin to be worked in relief. “Flashing” is sometimes used (erroneously) as a synonym for casing. (2) The act of reheating a parison by inserting it into a glory hole and then withdrawing it rapidly.