i boiled mine but i think the boil was to "rough as some would put it and the shells either faded or turned a really sic pattern! but for most they just faded. so i reacon a "gentle" boil for 10-15 mins like instructed above.
Boiling water is still 100C (212F) regardless of whether it's a gentle boil or a rolling boil.... granted the pot is hotter with the rolling boil and some heat might transmit straight from the pot to the shells.
I didn't think to boil my shells, I'm not to the point of adding fish yet though. Yesterday I cleaned mine out... dumped them all in the 29gal tank and filled it with water, blowing the water into the shells. It filled up before I got all the shells cleaned out (yes, I have alot of shells), so I pulled out my as-yet-unopened sump pump and used that to finish cleaning out all the shells
. I may do another rinse cycle, or just pile in some extra filter pad on top of the wet/dry (when I get it built) and run it for awhile, churning the shells periodically.
-Rick (the armchair aquarist, who is cleaning several hundred shells instead of several shells)