Courtesy of the Church of the Transfiguration in Orleans, MA, we have a marvelous picture of the inside of several super shiny bells. Now why would there be those grooves inside the bell? To tune them, of course! Remember the five lowest partials in a bell?
The bell founders tune those partials by shaving off metal from the inside of bells. The precise location where the metal is taken off will determine which partial is affected and which direction it will go (whether tuned up or down). And the tuning process is a true art form. Tuning one partial invariably affects the tuning of another, so it’s a very gradual, careful process of trying to achieve the closest approximation of the desired overtone structure. Here’s a video of some of that tuning process at the John Taylor & Co. Bell Foundry in Loughborough, England.