Where Does Volcano Lightning Come From?

The weird appearance of lightning during volcanic eruptions has been documented for some time now, but until recently, scientists weren't sure what caused the lightning bolts to appear.
They were pretty sure it had something to do with the particles of ash from the eruption getting an electric charge as they spewed out of the volcano, but nobody could figure out how the charge got there. And getting a closer look wasn't exactly an option, since directly observing a volcanic eruption isn't the safest thing to do.