What is the reflective material used in antique hand mirrors?

I have an art deco hand mirror from the 1920s or 1930s that belonged to an ancestor. I was storing it in a drawer and I noticed that a couple of things in the drawer had rusted and then I picked up the mirror and it's black behind the gl where it used to be reflective. The mirror has a celluloid handle, back and frame. The edges of the frame are flaking. I ume there was some sort of liquid between the gl and back and it has leaked out through the sides of the frame. I hope it didn't contain anything dangerous like mercury.