2-27. Purple Prisms


While wandering the streets of San Antonio recently, I was delighted to find so many purple prisms embedded in the sidewalks, especially in the area near Alamo Plaza.

Most people probably think these are old decorative tiles as they walk over them without noticing, but they’re actually sidewalk lights or vault lights for allowing sunlight into the basements of the buildings in the area. They were often shaped like prisms so that light would spread out once it reached the basement. They are based on deck prisms, which were used on ships, especially in areas where flames would be dangerous.

I was fortunate enough to find one that had been broken out. And doing what anyone would have done, I poked my phone in the hole and took a picture. What was down there you ask? I’ll tell you: old rags and paint cans. What else did you expect to find in a basement?

There’s one odd thing though… why are they purple? After all, purple isn’t going to let as much light through as clear gas, so why would they bother using what was probably more expensive glass just for the sake of decoration?

In fact, they didn’t. The original installation, which probably happened in the early 1900s, used clear glass. And the glass wasn’t replaced: it was photochemically altered. Before 1915, American glass companies used manganese in their glass formula in order to clarify and stabilize the glass. What they didn’t know was that UV light caused the manganese to oxidize and turned purple. The process took years, and now such glass is known as “sun glass” and is sought after by collectors.

So if you see a piece of purple glass in the sidewalk, take a moment to realize that you’re looking at something over 100 years old, placed by someone who thought they were putting a piece of plain glass in place. The mundane can become beautiful given time.


The secret underground beneath the prisms...

The secret underground beneath the prisms…

Purple Prisms in the sidewalk of San Antonio

Purple Prisms in the sidewalk of San Antonio

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