tldr; Why do magnets stick to my ceiling?
I have an old house, built in 1905 in the Boston area. I want to install a light fixture in a room that does not currently have one. I tried using my stud finder to find the joists, but it proved futile.
I switched to trying to use some neodymium magnets to try to find nails in the ceiling. Right off the bat, the magnet stuck. Thinking I was lucky, I grabbed a second magnet and started hunting for another nail. That magnet also immediately stuck.
I started hunting around and found that the magnets stuck to my ceiling virtually everywhere that I put them.
The ceiling has a textured plaster finish on it. There are no vents running through the ceiling that I am aware of.