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

Greensburg Daily News, September 9th

The use of various coloring agents turned milk glass into blue, pink, yellow, green and slag glass colors. Several quality glass ... “Milk glass” of this type will be a solid white and when held to the light will not show “fire” (a slight rainbow of...Read more

Some collectibles better at weathering economic storm
Mid Columbia Tri City Herald (blog), August 22nd

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

The Last Ride of King George
Texas Monthly (subscription), August 6th

The reception area is the front porch, glassed in with the Saltillo-tile floor left in place, and the way into the office proper is through the old front door, still flanked by exterior house lamps and the doorbell. .... Country people don't publicly...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, ( more

Honeywell agrees to settle Jersey City chromium lawsuit for $10 million, June 9th

Residents in the southwest section of Jersey City who filed a class-action lawsuit against Honeywell over chromium contamination on their properties would receive roughly $2,000 each in a $10 million settlement. Honeywell will put $10 million into a...Read more

The Louis Daniel Brodsky Collection of Art Nouveau
Maine Antique Digest, April 27th

Brodsky's buys were from major names in the genre. Purchased primarily in Europe, New York, and Chicago, the collection encompassed furniture, ceramics, silver, glass, posters, prints, art glass, and some jewelry. Some came from his home territory in...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

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