Verre églomisé,

(Wikipedia) from the French term meaning glass gilded, is a process in which the back side of glass is gilded with gold or metal leaf.  In one of a number of related processes, the metal is adhered using a gelatin adhesive, which results in a mirror-like, reflective finish in which designs are then engraved. The metal leaf may be applied using oil-based adhesives (goldsize varnish) to achieve a matte finish. The gilding may also be combined with reverse painting on glass. It is to be distinguished from “sandwiched” gold glass where gold leaf is glued to a piece of glass, and often then scraped off to form a design, and a further layer of hot glass is then applied on top and fused to seal the gold inside.