What Those Beautiful Glass Prisms in the Sidewalk Are Really For

Have you ever walked along a city street and noticed grids of colored glass on the sidewalk? Though the patterns are lovely and may seem decorative, they actually served a purpose — or at least they did at one time. The glass pieces are vault lights, sometimes called pavement lights in the U.K. They were inserted into the sidewalk to allow light into the basement areas below ground.

The first vault light was patented in 1834 by Edward Rockwell, reports Glassian, a site devoted to glass collections and glass history. It was a round iron plate surrounding a large glass lens.

In 1845, Thaddeus Hyatt proffered his own patent application complaining that Rockwell's lights were easy to fracture. He instead proposed an iron plate containing small glass pieces, protected by protruding iron knobs. Those are the lights you're most likely to still see today.

The top of the vault lights are flat with the sidewalk so that people can walk right over them, but the bottom often has a different shape.

Some of them have a prism design so the bottom can spread as much light as possible through a broad area, explains GBA Architectural Products. "In some cases, multiple prisms set at different angles would be incorporated to spread the light evenly throughout an even larger room."

These sidewalk prisms were first used on the decks of ships.

"It's long been the traditional way of lighting the interior of ships," Diane Cooper, a museum technician at the San Francisco Maritime National Historic Park, told KQED News. "While kerosene lamps were sometimes used, the smoke could make interior spaces uncomfortable. And candles could become a fire hazard on wooden ships."

The lights became popular in U.S. cities like New York, San Francisco, Chicago, Philadelphia and Seattle. Internationally, the lights came to be found everywhere from London to Dublin, Amsterdam to Toronto. The idea eventually spread to even smaller cities.

They were a way to illuminate spaces where natural light wasn't available and a way to avoid using gas, oil and candles.

Vault lights can be various colors, but they are often found in shades of purple.

When the lights were originally put in place, many of the glass pieces were clear. But during older glass manufacturing, chemists would mix in manganese dioxide during the process. That would stabilize the glass and take away the greenish tint it got from other elements.

Over the years, as manganese is exposed to ultraviolet rays, it turns purple or even pinkish, KQED reports. The colored glass today is either very old or has been dyed to look like old glass.

The use of vault lights declined by the 1930s when electricity became more common and inexpensive. As the glass pieces cracked in places, they became hazards to pedestrians as well as the subterranean spaces down below as they let in moisture. Cities began to cover or remove them.

However, some preservation groups are working to restore the lights for their historic and aesthetic value. Some cities, like Seattle, offer tours that show where the vault lights are located and have done studies on their history and value.

Says GBA, "Since many vault lights panels have lasted for more than a century, these cityscape artifacts have become prized historical treasures."