Here's an interesting fact. Hand carved wooden frames are hard to find, if you get your hands on one, keep it, you've got a gem.
Like all modernization, picture frames also underwent a way to mass produce ornate frames that were popular in 17th, 18th & 19th century. Composition materials are cast in mass, then added to a wooden base. If grandma's frame ends up in an attic or a non-control enviroment, guess what? The glues and other materials become unstable and it does not take much for a corner to 'pop' off.