What is Slag Glass? Price Guide and History

Though it is known by several names, slag glass is a variety of pressed glass that is universally celebrated by collectors for its characteristically subtle variations in color and opacity. Adding to the beauty of slag glass – also known as malachite glass, marble glass, and mosaic glass – is its rich history that reflects the ingenuity of turn-of-the-century glassmakers. This article strolls through this history and highlights some of its most popular forms for those who want to surround themselves with exceptional slag glass pieces. 

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What is Slag Glass? A Discarded History

Slag glass gets its name from slag, or the byproduct of steel production. When iron ore is smelted in that process, the remaining residue is a glass-like, often glossy material that takes on different colors depending on the minerals and elements present in the iron. The visual marbling of this slag is reminiscent of slag glass, however, the presence of actual slag in slag glass pieces depends on how it was created.

Early creators of slag glass were rumored to incorporate pulverized fragments of iron slag chunks into their molten glass, before pressing it to accomplish the rich variety of colors seen in these glass pieces. The Corning Museum of Glass also notes a secondary process, in which the “slag” of slag glass was developed from lingering scraps of glass leftover in the pots of glassmaking factories at the end of each work day. When melted together, these colors took on a marbling similar to that seen in iron slag. 

Regardless of which technique was used by various makers, slag glass reflected a brilliant repurposing of materials otherwise resigned to the refuse bin, resulting in some of the most dynamic and eclectic colors known within the glassmaking industry. It is perhaps thanks to this novelty that the introduction of slag glass in the later years of the nineteenth century resulted in excitement from glassmakers and glass collectors alike.

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Early English Slag Glass Makers

The Sowerby firm in England filed one of the first patents for slag glass during the later years of the 1870s for their striking purple malachite glass. Sowerby’s slag glasswares, along with the similar pressed glass designs of contemporaneous English makers Davidson and Greener & Company, are still some of the most coveted today, particularly those that feature their unique brown malachite glass. Of course, brown was not the extent of their color palette: Sowerby especially experimented with a wide array of colors, including their vibrant giallo (yellow), sorbini (bright blue), and pomona (a muted green) shades.

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Leading American Slag Glass Makers

With the export of these beautiful creations to the United States market, nineteenth-century American producers also entered into slag glass production. These included Atterbury & Company, who was one of the United States’ earliest producers of slag glass. They created beautiful pieces like this striking kerosene slag glass lamp base, from 1860 to around 1900. There was also Challinor, Taylor & Company, who excelled in smaller slag glass home goods, like these multi-color salt and pepper shakers, from the closing years of the nineteenth century. So celebrated are their designs that they rank among the collections of the world’s most prestigious museums, like this goblet housed at New York’s Metropolitan Museum of Art.

This tradition carried over into the twentieth century, with prominent makers including Imperial, Boyd, and Westmoreland, who experimented with an array of delightful slag glass novelties. For example, Westmoreland conjured this Three Kittens Plate or these multicolored and iridescent “Hen on the Nest” covered dishes, while Boyd developed a specialty for the novelty figurine, like this airplane. Perhaps the most unique addition to the slag glass market during this period was Ohio-based maker Akro Agate, who had originally earned acclaim for making marbles. They transitioned in the 1930s into pressed slag glass designs, like these paired vases, to expand their reach (although, they preferred to call their pieces “multi-colors” rather than “slag glass”).

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Art Nouveau and Arts & Crafts Slag Glass

Perhaps the most celebrated examples of twentieth-century American slag glass are those of the Art Nouveau and Art & Crafts era, such as Tiffany Studios, Steuben, and Roycroft, all of whom sensed that the natural variation in the marble textures would complement the organic sensibilities of their designs. These pieces, which ranged from spectacular lamps to smaller decorative pieces, like jewelry boxes and picture frames, showcased the artful way in which these makers could use the subtle variations of slag glass to great effect. 

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How to Identify Slag Glass  

With this extensive popularity of slag glass among these producers, collectors can find numerous examples of antique and vintage slag glass pieces on the market today. This predominance can make identifying true slag glass challenging, a point reinforced by the fact that there are so many associated colors. One clear indicator, though, of true slag glass is the richness of its tonal variations: real slag glass will not simply bear streaks of color but instead will offer more subtle variations in opacity and tone like one would expect from marble or alabaster. Meanwhile, imitation slag glass, like this early twentieth-century ashtray, reveals an unusually uniform color.

What is Slag Glass Worth?

With these maker names in mind and these tips to guide your research, you should be well on your way to a successful search for a splendid new slag glass piece to your collection. The final question to contemplate, though, is the value of these pieces. Like so many fields of collecting, slag glass can be acquired at a wide variety of price points.

Condition is a big factor, as is age – newer vintage pieces tend to sell for less than earlier antique examples. However, perhaps the biggest indicator of price is the maker of the work: signed Tiffany Lamps, like this spectacular Dragonfly Lamp, can sell in the tens of thousands, while the diminutive and delicate collectibles like this Boyd train set sell for mere dollars. The benefit of this incredible price range is that the slag glass market is accessible for virtually any budget and can satisfy those seeking bold statement pieces as well as smaller accent decorations. So, whether you are ready to make a big investment or simply want to spend a few dollars, memorable examples of slag glass can be yours.

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