A couple of my old bristle brushes have loose ferrules. I am a bit hesitant to tighten the metal with pliers in fear of crushing the wood and making them even looser. Has anyone had any success tightening loose ferrules?
If the hairs go out of the ferrule
One idea which I plan to test (without destroying a good brush just for testing purposes) is to inject some kind of glue at the base of the bristles / hairs.
My plan is to use a medical syringe, fill it with some glue (even oil paint should do, I suppose), and fill the base of the hairs (inside the metal "clamp") with that glue. Upon hardening, everything should stay in place together.
Additionally, to prevent losing the hairs as a block, I would stick some small nails through the metal, so in the end these nails would be stuck with the glue.
If the ferrule goes out of the handle
Just screw a screw through the ferrule into the handle. Maybe even two screws, from opposite sides. If you worry that the handle will break due to the pressure of the screws, drill some "guiding" hole first. The diameter of the hole should match the diameter of the "core" of the screw.
Alternatively, use some thin nails.
Or just glue it - see notes below.
Always keep the brush with the bristles upwards (or at least diagonally upwards) - to avoid the glue sticking them together.
Use a glue which does not become very rigid / breakable after curing.
If the glue is too "liquid", temporarily wrap the ferrule / handle area with adhesive tape. It will hold the glue until it cures. Remove the tape after the process finished.