Slag glass is defined as colored, opaque glass wih an altenate color swirled throughout giving the appearance of marbling. For this reason it is sometimes called marble or malachite or mosaic glass. Popular colors include butterscotch, green and purple. The name has been used by glass-makers only in recent times and is derived from the belief that the colors were acheived by adding “slag” from iron smelting works to the glass.

Many companies made slag glass including the Imperial Glass Co. of Bellaire, Ohio. (Imperial was a late comer, not beginning slag glass production until the 1960′s and continuing through the 1970′s.) The Ohio landscape readily provided the raw materials of clay, sand, and tempering agents for making ceramic vessels. European immigrant glassmen were drawn to the area for this reason. Slagware – variegated colored glass – was a popular European form at a time when demand for decorative glass was high. It appealed aesthetically and was affordable.

The English glassmakers Sowerby and the Davidson Glass Co. produced slag glass from circa 1850 until the mid-20th century. By using a blast furnace, they powdered their wares with cryolite. This produced “vitro porcelain” with streaky veins of purple or green of an opaque quality.