They are changing the architectural preservation rules to permit replacement with concrete or diamond plate steel.
Two hundred years ago, ships designed to carry coal had glass bricks in their decks so that crews could check out the holds without blowing up the boat. This evolved into Prism Glass, which were cast glass lenses designed to spread light through the room below.
They are a wonderful historic feature that is still visible in much of New York City, often surrounded by cast iron. Rebecca Paul explains in 6Sqft:
In 1845, Thaddeus Hyatt, an abolitionist and inventor, patented a system of setting round pieces of glass into cast iron sidewalks. His “Hyatt Patent Lights,” as they were often called, were technically lenses, since their underside had a prism attached to bend the light and focus it to a specific underground area. Hyatt eventually moved to London and brought his lights with him, opening a factory there and having them produced in cities throughout England. The lights brought him great wealth, and he went on to also patent several designs for reinforced concrete floors.
Paul notes that many have disappeared:
The use of vault lights decreased when electricity arrived and they became expensive for property owners to maintain. And with years of neglect, some of the metal frames began to corrode, and the tiny glass windows were deemed hazardous.
And now, the few that remain might be lost, as the City of New York does a major overhaul of its heritage regulations. The Historic District Council complains:
Vault lights are a defining feature of former manufacturing districts like SoHo and Tribeca, providing evidence that these districts were once industrial powerhouses, as opposed to the domain of wealthy property owners, shoppers and tourists that we see today. This rule change states that the staff will approve the removal of up to two panels of exposed vault lights that are deteriorated beyond repair if no other vault lights exist on the same side of the block. They may be replaced with diamond plate steel or concrete/granite to match the adjacent sidewalk.
The shame of it all is that vault lights and other types of prism glass are exactly the kinds of products that we should be using more of, because they bend and direct natural light, reducing the need for electric light. Not only is that light free, but it is becoming more evident that a bit natural light is important for our health, for maintaining our circadian rhythms. We should be learning from this stuff, not ripping it out.
If these regulation changes pass, it is likely that New Yorkers will be walking on diamond plate steel, because of course it is cheaper than fixing cast iron and glass vault lights. And anything involving heritage preservation these days is seen as an expensive frill and a tool of NIMBYS. Why should this be any different?