Slag Glass

Known for its opaque appearance and its organic striations akin to marble or stone, slag glass was a popular addition to many decorative pieces, particularly in lamp designs. Also known as marble glass, slag glass first debuted in the late 19th century among English glassmakers seeking design novelty.

Earning its name from the rumor that it was created through the addition of iron-smelting slag, slag glass first appeared in the late 1870s. Early producers such as Sowerby out of England brought to market fantastic examples of opaque patterned glass whose motifs ranged from marble to tortoiseshell. The popularity of the style soon spread to the United States, with American producers like Westmoreland getting into the slag glass market.

Slag glass was prized for its versatility, but it was perhaps best loved for use in lamp designs. The inherent opacity of the glass combined with its coloristic effects and patterns proved a perfect conductor of warm, glowing lamplight. Art Nouveau glass designers including Tiffany Studios and Steuben turned to slag glass for many of their lamp designs in part for the organic nature of the glass patterning but also because of this warm diffusion of light similar to antique stained glass. Today, slag glass continues to find followers, its rich colors and patterns as vibrant today as they were at the turn of the century.

Quick Facts

• Sowerby patented its purple slag glass in 1878, one of 437 total designs the company patented between 1874 and 1894
• Many offshoot companies attempted lower cost pressed-glass versions of slag glass, but these can be detected through a close look at the richness of the marbling. True slag glass will have a range of colors among its striations akin to true marble or alabaster, while impostor pieces will appear simply as streaks across the glass pane
• A Tiffany Studios Dragonfly lamp that features a shade filled with slag glass elements recently set a new sales record at a Sotheby's New York auction in late 2015. A relic from the collection of Andrew Carnegie, the lamp sold for $2,110,000

Learn more from the Invaluable Guide to Slag Glass.