I just bought a house in West Philadelphia that was built around 1925. I immediately started scraping the texture off the walls in the foyer (fortunately it had never been painted and comes off easily) with the plan to wallpaper the walls once I got the texture off. I found the wall underneath to be made of something unfamiliar to me. It is speckled with black dots with a few tiny holes here and there and is quite smooth and even to an extent that makes me think it can't be any kind of plaster. It can be scratched easily and sounds like drywall when i knock on it (somewhat hollow and low). It seems most similar to drywall that just doesn't have paper on it-- it's clear that there is no paper in what I'm scraping off. Can anyone tell me what it is?
In this photo you can see the texture on the top region of the picture and the wall in question on the bottom.
And secondly, what would I need to do to prepare it to be wallpapered?
Thanks so much in advance.