A process of floating and stirring a small amount of bitumen* paint on water and then picking it up by carefully placing a sheet of glass against it. When dry, the glass is treated with hydrofluoric acid which etches the glass in areas clear of the bitumen.
Source:Dictionary of Glass: Materials and Techniques by Charles Bray
(* Bitumen is a tar-like mineral pitch produced during the distillation of coal and petroleum.)
