Fer-ret′to.
(Glass.) A preparation of copper employed in glass-coloring. It is made by placing thin sheets of copper interstratified with powdered brimstone in a crucible which is luted tight and exposed to the heat of a blast-furnace for about two hours; when cool, the copper is found to be calcined so as to be readily crumbled between the fingers; it is then pulverized and sifted for use. A superior article to the foregoing is prepared by using vitriol instead of brimstone, and exposing the crucible to the heat of the glass-furnace for three days. The old vitriol is then replaced by fresh, and the heating operation repeated six times.