Flashed glass is where two or more layers of colour are created in one sheet of glass. This is made by gathering one blob of molten glass on a blow pipe and dipping it into a crucible of a different coloured molten glass.
The glass is then made into a sheet and the process of glass blowing spreads the second colour into a thin layer over the first colour. This means that you end up with a piece of glass with two colours overlaid and the upper layer can be etched or abraded to reveal the colour underneath.
The relative thickness of each layer and even the shape of the initial gather will affect the final colour intensity and whether the colour is even or varied.
Flashing glass allowed extra artistic techniques to become possible. Methods were developed to remove the coloured layer of glass to reveal the layer below and create more than one colour using one piece of glass without having to lead together separate pieces of glass. In the example shown in the photograph, the red layer has been removed to reveal the clear glass below. Over time different colours have been produced in flash, so you can also use base colours other than clear. Removing the layer of glass was a very skilled process as the thickness of each colour layer would vary between batches and across individual pieces of glass as a result of the handmade process.
Layers of red and blue glass together appear as purple but you can remove the blue to show red, or remove the red to show blue – all on the same piece of glass. You can also remove only some of the layer thickness to create different depths of colour. You can also use silver stain or more than two layers of glass! The methods used to remove the layers of glass varied over time. Originally it would have been painstakingly ground away by hand, but later, Hydrofluoric acid was used to dissolve the glass. This gave greater control over how much was removed and considerably speeded up the process.