Slag glass, also known as marble glass or malachite, is a type of opaque, streaked pressed glass. Production of slag glass originated in late-19th-century England, where glass manufacturers are thought to have added slag from iron-smelting works to molten glass to create a range of effects—from tortoiseshell to marbling. Among other uses, slag glass was a popular material for lampshades.

One of the first glass foundries to produce slag glass (although it was not called that at the time) was the Gateshead, England, firm of Sowerby, which patented its recipe for purple malachite glass in 1878. Sold into the U.S. market as "blackberries and cream," this popular formula was followed by other malachite colors, including a lemon-yellow called Giallo, a drab green called Pomona, a blue malachite called Sorbini, and Sowerby’s famous Brown, which is very difficult to find today.

Since the process of making slag glass was shrouded in a certain amount of mystery, stories sprang up to try and account for the process behind the effects. For example, it was a good bet that Sowerby’s Blue Nugget color of 1883 was the result of adding cadmium to molten glass, but how to explain Gold Nugget? Stories soon spread that John George Sowerby, son of the company’s founder, was tossing gold sovereigns into batches of amber glass to create this dramatic hue.

In the United States, manufacturers such as Westmoreland and Akro Agate picked up on the techniques developed in England and produced their own versions of slag glass. As in England, one of the most popular applications for their pressed opaque glass was in lampshades. Wide bands of creamy colors allowed the light source in a lamp to fill a room with a soft ivory glow, while the purples and greens and reds pieced together in detailed leaded shades resulted in multi-colored illumination.

Just about anyone who was making lamps during the Art Nouveau period, from Tiffany to Roycroft to Steuben, might have used slag glass in their shades. But whether they did or not in a particular lamp is another matter. The problem is that today, "slag glass" is used rather casually by dealers and collectors alike to refer to almost any type of pressed opaque glass containing colored swirls or streaks. The key to determining if a piece of glass is "true" slag or just a handsome example of pressed glass is to look for glass that is obviously and richly marbled rather than simply colored or streaked.

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Recent News: Slag Glass Lamps

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The last days of the Big K
New Statesman, November 4th

The last deep coal mine in Britain is an arresting sight – a sprawling tangle of towers, conveyor belts, processing sheds, railway lines and “muck heaps”, as its mountains of grey-black slag are known in Yorkshire. .... The caretaker – Kitchen's son...Read more

Cornucopia of decorative arts to be auctioned
Boothbay Register, October 25th

The auction will feature four table lamps by Tiffany Studios, including one with an Acorn pattern leaded glass shade and model 531 bronze base, one with a geometric leaded green slag glass shade with pearly inserts and patinated model 181 bronze base, ...Read more

Lamp with lighted base is more desirable
Chicago Daily Herald, October 13th

Q. The lamp seen in this photo was purchased by my grandmother at an estate sale. It has two light bulbs in the shade, and the base lights up as well. I cannot find any marks to identify the maker. Other than a few cracks in the metal on the shade, it...Read more

Mysterious tungsten mill site in Tucson used in WWII
Arizona Daily Star, October 4th

Tungsten is a metallic element that has a variety of uses ranging from a hardener of steel for armor and the nose cones of rockets to a variety of electrical applications such as lamp filaments and television tubes. It has the highest melting point of...Read more

Some collectibles better at weathering economic storm
Mid Columbia Tri City Herald (blog), August 23rd

Q. No one has been able to identify the maker of our metal-base, slag glass shade table lamp. We've shown it to many people with some expertise in the field and even took it to the Antiques Roadshow this summer. Can you identify the manufacturer and...Read more

Treasure: Art deco lamp shining example lighting of era
The Detroit News, July 30th

Thomczek did too, telling her that she had a fine example of what is known as an early 20th century slag-glass table lamp. According to a fascinating article with a variety of illustrations on, ( more

Honeywell agrees to settle Jersey City chromium lawsuit for $10 million, June 9th

Residents in the southwest section of Jersey City who filed a class-action lawsuit against Honeywell over chromium contamination on their properties would receive roughly $2,000 each in a $10 million settlement. Honeywell will put $10 million into a...Read more

Glass lamp, pickle jar, Victrola and Sitzendorf figurine: Attic Finds
The Plain Dealer -, November 6th

ANSWER: Pittsburgh Lamp, Brass & Glass Co. made your lamp around 1915. The hexagonal, intricately modeled base was used on a variety of styles made by the firm. 1825 is the model number. The marbleized, caramel-colored glass is known as "slag" ...Read more