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

Source: Google News

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

New NAIMA Guide Recommends Installation Methods for Insulating Chilled Water ...
SYS-CON Media (press release), July 20th

NAIMA is the association for North American manufacturers of fiber glass, rock wool, and slag wool insulation products. Its role is to promote energy efficiency and environmental preservation through the use of fiber glass, rock wool, and slag wool...Read more

Outstanding Two-Day On-site Estate Auction!
Maine Antique Digest, July 7th

duck decoys; good duck decoy lamp; brass cannon; Kentucky curly maple percussion gun; powder horns; early drum with eagle decoration - Peabody, Salem label; double student lamp; pair of ball made shade lamps; Victorian slag glass lamp; other lamps; ...Read more

Signed 1929 cat drawing by Leonard T. Foujita will be sold at auction June ...
ArtfixDaily, May 27th

Lamps and lighting will feature an antique oil banquet lamp with etched hurricane shade and gold base, kerosene lamps, stained glass and slag glass lamps, a stained glass hanging fixture, and a floor lamp with painted shade. Antique clocks from the ...Read more

'3 Scientists Walk Into A Bar' sounds like a joke, but it's really a new show ...
Mother Nature Network, May 27th

Other topics include why clouds seem to defy the laws of gravity, why storms spin, what causes the northern lights, the difference between a meteor, a comet and an asteroid, the most earthquake-resistant building materials, what Marfa lights are, where...Read more

Scientists Convert Packing Peanuts Into Better Battery Parts
Huffington Post, March 23rd

"These initial laboratory-based coin cell batteries are already powering light emitting diodes (LEDs) and wristwatches," Pol told The Huffington Post in an email. "After scaling up the carbon-production process, we will be able to make larger-size...Read more

Advice From Dallas' Best Interior Designers
D Magazine, March 3rd

So we posed five questions to our winning designers that would shed a little more light on their tastes and preferences, how they work within a range of price points (which, we should point out, do not necessarily reflect realistic dollar amounts they...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