Slag glass, also known as marble glass or malachite glass, 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

Industrial look strips old furniture of paint
HeraldNet, April 27th

Today the industrial look is “in,” and shows have old workbenches, industrial tools attached to wooden tables, lamps made of old gears and gym lockers stripped of their original paint. All have been given a modern look by exposing the original silvery...Read more

Kovel's antiques: Creating the 'industrial look'
Observer-Reporter, April 23rd

Today the industrial look is “in,” and shows have old workbenches, industrial tools attached to wooden tables, lamps made of old gears and gym lockers stripped of their original paint. All have been given a modern look by exposing the original silvery...Read more

Kovels: Old furniture can be transformed into unique pieces
Winston-Salem Journal, April 21st

Today the industrial look is “in” and shows have old workbenches, industrial tools attached to wooden tables, lamps made of old gears, and gym lockers stripped of their original paint. All have been given a modern look by exposing the original silvery...Read more

Ramp it Up! Aims to Celebrate Food, Shops and Architecture of Cleveland Heights
Cleveland Scene Weekly, April 6th

The 5,000-square-foot second-floor space, originally John D. Rockefeller Jr.'s real estate offices, boasts 20-foot ceilings, oak-beam rafters, iron chandeliers, leaded-glass windows, and slag-glass lamps – not to mention the stonework, archways and...Read more

Collectors covet Oscar Bach's distinctive work
HeraldNet, March 17th

The original heart-shaped dishes had a grapevine or a hearts-and-flowers design pressed into the glass. The signature of the ... Bradley & Hubbard, lamp, slag, hexagonal shade, moored boat, buildings, trees, turquoise, burgundy, 13 x 20 inches, $530...Read more

Treasure: Tiffany-style lamp shines with intrigue
The Detroit News, February 18th

Unusually tall, the lamp includes five light sockets and a pierced shade with detailed leaves and vines. “A lot of their lamps seem to have piercing,” the appraiser said of the company. “Some had mica shades, others had slag glass and some jewel-like...Read more

Hopes dim for 100 miners missing after landslide near Myanmar jade mine
South China Morning Post (subscription), November 23rd

Rescue workers had recovered 104 bodies when the search was suspended on Sunday night, state newspaper the Global New Light of Myanmar reported on Monday. It is unclear what caused a mountain of mining debris to give way early on Saturday in ...Read more

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 collectorsweekly.com, (collectorsweekly.com/lamps/slag...Read more