Quote Originally Posted by Ralphxyz
"Are those bricks in the wheel?"
Quote by Frank S
"They might even be hollow glass bricks"

Very possible. Shallow cavity and smooth texture however, says they could be solid glass bricks. Most people haven't seen them, commonly installed in areas wanting natural light, but elevated security.
This post does not indicate I've had direct contact with either side of such material, only that by way of interest in common hollow glass brick.

https://www.gbaproducts.com/security...ention-windows