To doorknob or not to Doorknob

[Old man digger]

I found this today while surface hunting. I believe it to be a Doorknob made from porcelain.
When I found it had this blue looking material inside the hole. I was a little disappointed because I thought it was plastic.
When I tried to remove the plastic, I found out it was some blue/green corrosion. When I picked at it, I realized it was a type
of soft metal and not plastic. Maybe even lead. Need help in deciding what is and its age.

 

[CanadianBottles]

Doorknob seems right to me. Not sure if this sort of thing is possible to easily date. I think they were commonly in use around the turn of the 20th century but no clue how far back before that they go. I've found similar ones before but I don't remember ever seeing one in use in a house, so presumably they're pretty old.

 

[Step Back In Time]

If it's a green colour then it might be copper.