Vitreous marble

Pigmented structural glass, also known generically as structural glass and as vitreous marble, and marketed under the names Carrara glass, Sani Onyx, and Vitrolite, among others, is a high-strength, colored glass. Developed in the United States in 1900, it was widely used around the world in the first half of the 20th century in Art Deco and Streamline Moderne buildings. It also found use as a material for signs, tables, and areas requiring a hygienic surface. Over time, the trademarked name "vitrolite" became a generic term for the glass.

Pigmented structural glass was developed in 1900 in the United States by the Marrietta Manufacturing Company of Indianapolis, Indiana. The product was made by combining borax,cryolite, kaolinite, manganese, silica,feldspar, and fluorspar. The fluorides made the glass opaque.

These materials were fused into glass at a temperature of 3,000 °F (1,650 °C) and then annealed. The annealing process took much longer than it did for plate glass, often lasting three to five days. This left the glass very strong, with a compressive strength about 40 percent greater than marble. If the product was to be affixed to another surface (such as the exterior of a building), one side of the slab was grooved before the glass hardened.

The exposed side(s) of the material was flame polished, which left the product highly reflective and brilliant. Later manufacturing techniques used fine sand to polish the surface, followed by felt blocks and iron(III) oxide powder.

Originally, only beige, black, and white colors were available. But by the 1930s, new manufacturing methods could make pigmented structure glass translucent, and more than 30 colors were available. In time, even agate- and marble-like color patterns were available. Black structural glass was sometimes silvered, to give it a reflective finish.

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