WHAT IS THE DIFFERENCE BETWEEN OPAL, DICHROIC AND OPALESCENT GLASS?
Opal glass: A translucent white glass made by adding a finely ground powder tin oxide or bone ash to the glass batch to give it a. First made in Roman times and fully developed in Venice, Germany and Bohemia in the 17th/18th Centuries.
Milk Glass: Opaque white glass, usually opacified by tin oxide or arsenic.
Dichroic Glass: Glass which shows different colors depending on whether the light upon it is being reflected or transmitted. This iridescent effect is due to multiple micro-layers of metals or oxides in the glass. In Roman times colloidal silver-gold alloy was used; today uranium or copper oxide are also used. Example: Lycurgus Cup Roman 4th C
Opalescent glass: Is similar to iridescent glass. It is a surface treatment applied on translucent white glass. Frederick Carder (1863- 1963) introduced alabaster glass at Steuben Glass Works in Corning, New York. Carder’s alabaster glass has an iridescent finish made by spraying the object with stannous chloride and then reheating it.
Iridescence: Optical property in which fine colors are produced on a surface by the interference of light that is reflected from both the front and back of a thin film. Things that appear iridescent show rainbow-like luminous colors, and often change color as their positions change. The iridescence on ancient glass is different. It was cause by natural processes and was unknown by the glass makers at that time. see this link to Iridescence on Ancient Glass