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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Hundreds of items auctioned off at former restaurant
Johnston Sun Rise, April 9th

dining room, and included some serious and high-powered antique dealers who bid on and bought items that ranged from kitchen accessories and stained glass wooden doors to butter churns, hand-painted items by artist Jean Dewey and even slag lamps...Read more

Itronics Provides 2015 Operations Update and Outlook
Azomining, April 2nd

The Company has discovered that one of its fertilizers is able to leach silver from the silver bearing glass slag produced by its refining operations. .... Energy Saving Technology: About half of the lighting retrofit project started in 2014 was...Read more

Furnishings and Decorative Arts, Asian, Jewelry, Coins and Fashion in Rago's ...
ArtfixDaily, March 26th

Lighting: an English cut glass chandelier, a continental gilt-bronze chandelier, a Charles et Fils table lamp, a pair of gilt-bronze sconces; lamps by Handel, Slag and Bailey Banks & Biddle. Louis XVI and Federal gilt mirrors. Clocks: Louis XV style...Read more

What's it Worth: pottery lamp base, cameo collection, March 14th

I would like to know if it is worth anything before I give it away. — C.H.. ANSWER: This pottery lamp base probably once held a cloth or leaded-glass shade. Unfortunately, the original sockets have been replaced with inexpensive modern sockets, which...Read more

Auction watch: Upcoming sales range from art to toys to sports
Tribune-Review, March 8th

Jerry Smith will be staging Smith's Antique & Auction Service's biggest-ever coin auction and a sports memorabilia event that's a grand slam in the making. The two-day sale opens March 21 with a coin auction with estimates reaching as high as $6,000...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

Arts and Crafts to Modern Masters
Maine Antique Digest, December 30th

It is copper, patinated metal, and leaded slag glass, and has four sockets. A fine pot from Arequipa ... If this two-day sale of early 20th-century decorative art and modern ceramics and glass on Saturday and post-1950s design on Sunday does not bring...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