"Ice Box" rivets work nicely on the lighter skins, there is noticeably less force needed to drive them, reducing skin pucker. Some of the nicer RVs were skinned using them. It's for cosmetics, so there is no need to use them on internal structures.
Basically, you are re-heat treating the rivets. Once quenched, it takes some time for them to age harden, and you can take advantage of the fact they are soft for a while. They will age harden back to full strength after a few days, they also work harden when you set them, so they don't lose strength in the process. The important thing is to have a good oven that is accurate and to quickly water quench them. You can store them in the freezer for about a week, after which they are back to normal hardness. I would throw away the ones still in the freezer after a few days and start a fresh batch.
From Sport Aviation:
Heat treating can be accomplished in a small dental oven. The critical temperature not to exceed is 935 degrees, as the rivets will be ruined above this temperature. For small batches--say 100-200 rivets--use a 15 minute 'soaking' period at 850 degrees, followed by an immediate quenching in cold water. Intergranular corrosion, the nemesis of warbird restorers, can be caused by letting too much time elapse before quenching, so be sure to get the rivets from the oven to the water in less than 3 seconds. I like to check a few rivets for overheating just to be on the safe side. This can be done by smashing a few flush head rivets with a hammer on the anvil of your vise. If the rivets got too hot, they'll crack around the thin adge of the head. If this happens, reject the whole batch. Otherwise they're ready for immediate use, and you'll be surprised at how easily they can be driven. It's actually fun! If you don't use all the rivets in a few hours, age hardening can be retarded for up to 2 weeks by storing the rivets in the freezer. The rivets will return to full strength either in the process of driving (work hardening) or by letting them sit for about 6 months (age hardening). Sport Aviation, December 1987, Pg. 54.