Opalescent glass
Opalescent glass is milky, semi-opaque sheet glass in which colours often swirl and blend. Light passes through it but is scattered, so it glows rather than sparkles, and it hides what is behind it much better than transparent glass.
It was developed in the United States in the 1870s and 1880s, with John La Farge and Louis Comfort Tiffany both involved in its early development. Opalescent glass suited the new pictorial style of American windows, in which shading and layers of colour were produced in the glass itself and not by paint, and it is closely associated with the copper foil method and with lampshades.
Opalescent and transparent glasses can be used together in one panel, but different makers’ glasses should be tested for compatibility if the work is going to be fused.