"Slag glass" is a collectors' name for opaque pressed glass with coloured streaks, usually white and/or cream streaks like this brown bugle made by George Davidson's in the North East of England in the 1890's. It is a name which was not used by glass-makers until recent times. The name derives from the belief that these colours were achieved by adding "slag" from iron smelting works to the glass.
When they were produced in the 19th century, these items were called by names such as "Marble glass" or "Brown malachite" or "Brown marble vitro-porcelein".
Slag glass is commonly found in purple, less common in blue and brown and green. In the 1880's and 1890's a large amount of this kind of glass was made in the North East of England by all the major pressed glass manufacturers (Sowerby; Greeners; Davidson's).
In the USA slag glass was made by Imperial Glass, by Westmoreland Glass, by Akro Agate, and several other companies. It is still popular today, and is made by US glassworks such as Boyd Glass, Summit, and Mosser, who each make a range of slag glass items in a wide range of colours including red and orange.