What material is under this paint?

[u/paullyprissypants]

Home built in the early 40s in San Francisco. The bottom side looks like a mortar bed. Remodels in the 60s and 90s but I’m pretty sure it’s original. The material sounds like stone or concrete when I knock on it. I don’t think it’s plaster.

I don’t know much about how they made this stuff back then.

[u/rjl12334567]

It’s probably wood

[u/paullyprissypants → u/rjl12334567]

No way it’s wood based on how it sounds when I knock on it. But did they make concrete molds for stuff like this in the 40s?