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

The day Scunthorpe was bombed: 100th anniversary of Zeppelin raid which killed ...
Scunthorpe Telegraph, January 31st

No street lamps were lit and the lights in most of the houses were subdued. "The exception was at a slag bank, where a bright fire was burning in a devil to prevent the engine going over the end of the bank when tipping slag. There is no doubt that...Read more

Burma: Mining companies putting lives at risk as jade rush continues
The Independent, January 30th

It is an event that has become all too familiar in the jade-mining community of Hpakant – the collapse of a heap of slag that engulfed scavengers scrabbling for discarded fragments of the precious stone. At least six died in the latest tragedy...Read more

Before government became reviled
The Missoulian, December 31st

In 1840, when William Henry Harrison, scion of wealthy Virginia planters, ran for president as the hardscrabble "log cabin and hard cider candidate," the resulting paraphernalia included glass log cabins containing whiskey from Pittsburgh's E.C. Booz...Read more

Craft brewers handtool new South Portland brewery
Bangor Daily News, December 12th

A slag hammer nicknamed "John Henry" hangs on a chimney of reclaimed bricks at Fore River Brewing in South Portland on Tuesday. The hammer was found .... With beer tourists flocking to Maine in droves and outfits such as The Maine Brew Bus making it...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