This is a positive photographic image made from a negative upon a light-sensitive coating on opal glass. Opal glass, also called a milk glass, was a white translucent glass made by the addition of oxides to basic silica soda glass. Flashed opal glass had the translucent white layer on top of a sheet of clear glass. Sheets of opal glass were also used as a diffuser when rephotographing negatives by transmitted light. The terms opaltype, opalotype, and milk glass positive are synonymous.