Image: Northwood Glass Works, Factory about 1898-1919. CMoG 79.4.92
Slag glass" is a collectors' name for opaque pressed glass with colored streaks, usually white and/or cream streaks.
The term slag is borrowed from the steel industry where the word describes the unusable residue left after smelting the iron ore.
When they were produced, these items were called by names such as "Marble glass" or "Brown malachite" or "Brown marble vitro-porcelein".
Between 1650's and 1900 in American Glass Factories at the end of the formal working day glassworkers were allowed to use the leftover batch for their own use. In the course of this practice they would occasionally go from pot to pot, gathering different kinds and colors of glass. Thus various colors came into existence.
