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

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

Iron Age religous offering may have been buried to 'ride' into a new season
Daily Mail, October 14th

Researchers also unearthed what are thought to be equestrian tools, including an object that was probably a grooming comb and two curved blades which may have been used in the care of horse hooves. The discovery was made by students from the ...Read more

Celebrity Big Brother recap: Lauren has too much whine while Dee gets Ricci ..., September 7th

It's a classic Celebrity Big Brother line-up this time around, and tonight's highlights show was definitely not lacking in entertainment. The biggest surprise was that none of the drama included Gary Busey. Instead, it was Lauren Goodger who took...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

Maine Antique Digest, August 8th

Betty lamps, Whale oil, Fluids inc. tin & bull's eye & drape patt., Cut glass apothecary jar full of shells made into lamp, Copper milk can lamp, 3 lg. retro. metal bank lights 34” high, Finger lamps inc. blue cut overlay etc., Student's desk oil lamp...Read more

Has the Midlands Meteorite been found in Coventry? Local claims to have ...
Daily Mail, July 15th

'There have been some meteorites that have gone over us recently and I just want to find out exactly what it is as it has baffled me. 'I've found some other stones before that are really interesting to look at but they are just slag, which is common...Read more

Boston Blockfront Chest Leads Willis Henry Sale
Maine Antique Digest, June 7th

Other sale highlights included glass from the collection of the eminent Shaker dealer Ed Clerk (1924-2013). Willis Henry has been selling Clerk's effects for several sales. A Tiffany Studios desk lamp, 14" high, in counterbalance style with a green...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