Yesterday I bought a small Ultrasonic cleaner with 35 watts power and 16 fluid oz. capacity. It is normally used for cleaning jewelry and other fine metals and I believe railway enthusiasts use it to clean etched brass frets and metal train parts. I recall that I read somewhere that it can also be used to shake up paint bottles, which seemed to me a good enough reason to pick one up.
I filled the cleaner with tap water and began to place handfuls of 17ml Vallejo paint bottles into it. Most of my Vallejo bottles are several years old and have not been stirred/shaken very often since I purchased them. Even the metal pellets I put into some of them had become impossible to shake loose due to settled pigments.
I put each batch of bottles through one to two 3-min cycles (auto-timed default for the model I have). I then took the bottles out and towel dried them. The result was better than I expected. After the ultrasonic treatment, a quick shake was all it took to re-integrate the pigments with the medium/carrying fluids.
So I am quite happy with this purchase and can't wait to see what else I may be able to use this machine for. Perhaps I will even use it to clean something some day...