Beerstone. No matter how much you much how you squeeze it, you'll never get beer from it. Beerstone, the colloquial term for calcium oxalate and water salts, is the ugly brown coating that grabs hold of the inside of your shiny brewpot. Not only is beerstone unattractive; it affects the performance of your kettle. Remember a clean brewery is a good brewery.
Now the Phantom has seen many brewers laying into their kettles hard with a scrub pad to remove that stone. My fellow brewers to borrow from Dupont, "Better ways for better living through chemistry." Unlike organic by-products of brewing, proteins, hops, etc.; beerstone doesn't clean easily with alkaline chemicals (soap, detergent, caustic, PBW). After a light scrub to remove organic residue, the cleaning agent of choice is an acid wash.
Myriad choices exist for the brewer with different effects. Yet the basic process remains the same: an acid salt or liquid is mixed into hot water. The solution sits in contact with the stone for 10 minutes (soak directly or apply via a sponge). A soft sponge can then be run around the kettle and the beerstone will peel right off, exposing the bright shiny stainless steel underneath. Rinse with hot water To protect the kettle, you need to passivate the steel. With solutions of citric acid or vinegar, the cheap acid washes, you need to let the kettle sit dry exposed to the air for a week before use. Professional acid washes (including the Acid 5 solution at Daume's) include acids that will passivate the steel quickly.
Rescue those kettles from the grip of the notorious brown! Give them acid and bring them to the bright and shiny enlightened path.