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

Auction focuses on furniture
Clinton Herald, January 24th

The same Wisconsin buyer also bought the slag glass table lamp, which had several bidders trying to get this item and sold it for $550. The oak framed hall mirror walked away with a $105 top bid. The glass coffee table managed to bring $25 and 6...Read more

Busby hat not the same as a bearskin
Waterloo Record, January 23rd

The lamp is 56 cm in height (22 inches) with a six-panel slag glass shade measuring 42 cm tip to tip (16.5 inches). The original (two) light sockets with pull chains contain the markings of Bryant. It is in good shape with no cracks or dents and works...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

The Wilton Fall Antiques Market
Maine Antique Digest, December 16th

On a quintessential New England fall day in October, 88 dealers offered furniture, portraits, landscapes, folk art, silver, and other collectibles to a Sunday crowd of buyers and lookers. The field house at the Wilton High School, Wilton, Connecticut...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

Auction Watch: Heinz, other collectibles will be on the block
Tribune-Review, November 2nd

A large selection of glass bottles includes locally made vessels from McKees Rocks and other nearby towns. Of special note are milk bottles and other items made exclusively for the Pennsylvania Railroad that featured the rail giant's famous keystone logo...Read more

The Strange Blowpipe 19th Century Miners Used to Analyze Ore
Wired, August 19th

Then you'd combine the ore with some tiny lead pellets in a scorification dish (those are the little clay dishes inside the front part of the case), and heat the whole thing with the alcohol lamp (the silver object in the middle of the case with two...Read more

'Antiques on the Diamond' returns to Ligonier June 14
Tribune-Review, June 5th

Duane Hall, owner of Gerty's Attic Antiques in Ligonier, stands near the front window of his shop, featuring a slag glass and brass lamp, circa 1915-1925, and various glass, china and silver pieces...Read more