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

ArtfixDaily, February 24th

Enthusiasm in these categories can be represented by a French cameo glass and bronze table lamp that drew $23,750 against a pre-sale estimate of $1,500/2,500, an Art Nouveau slag glass and gilt bronze chandelier that drew $27,500 against a pre-sale ...Read more

Oscars 2015 live: winners and highlights of the 87th Academy Awards ceremony
Sydney Morning Herald, February 22nd

But that would be a little glass-half-empty, wouldn't it? Alexandre ..... Butter Lamp (La Lampe Au Beurre De Yak) .... Jack Black gets out of his seat to slag off the self-congratulatory Neil Patrick Harris/Anna Kendrick tribute to the fabulousness of...Read more

Index Numbers of Wholesale Price in India (Base: 2004-05=100), Review for the ...
Invest in India, February 21st

The index for 'Non-Metallic Mineral Products' group rose by 0.6 percent to 172.9 (provisional) from 171.9 (provisional) for the previous month due to higher price of grey cement (2%) and asbestos corrugated sheet, glass bottles & bottleware and lime (1...Read more

India whole sale inflation dips to -0.39% in January
Hellenic Shipping News Worldwide, February 16th

from 171.9 (provisional) for the previous month due to higher price of grey cement (2%) and asbestos corrugated sheet, glass bottles & bottleware and lime (1% each). However, the price of bricks & tiles (4%), marbles (3%) and slag cement (1%) declined...Read more

Jan WPI inflation at -0.39% yoy
Indiainfoline, February 15th

from 171.9 (provisional) for the previous month due to higher price of grey cement (2%) and asbestos corrugated sheet, glass bottles & bottleware and lime (1% each). However, the price of bricks & tiles (4%), marbles (3%) and slag cement (1%) declined...Read more

PUBLIC SALES: Feb. 15, 2015
LancasterOnline, February 13th

$1,100; step-back display cabinet, $1,018; Willard-style tall case clock, $990; Hepplewhite bow-front chest, $880; harvest drop-leaf table, $715; corner cupboard, $715; high-wheeled baby coach, $660; slag glass table lamp, $605; Shotwell gas pump...Read more

Untitled Short Story, by Jessica Sired
The Mancunion, February 2nd

NEW LAMP?” she shrieks, their argument finally reaching a crescendo. A door slams. The sound of glass breaking rings through the quiet, suburban street and finally John appears, red-faced and dishevelled at the front door. ... He only drives me to...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