At the end of the 19th century, glassmakers discovered that uranium glass with certain mineral additions could be tempered at high temperatures, inducing varying degrees of micro-crystallization. This produced a range of increasingly opaque glasses from the traditional transparent yellow or yellow-green to an opaque white. During the Depression years, more iron oxide was added to the mixture to match popular preferences for a greener glass.[8] This material, technically a glass-ceramic, acquired the name "vaseline glass" because of its supposedly similar appearance to petroleum jelly.