What happens is that the sun, whilst supplying our lovely planet with energy for us to live (yay, sun!)
also gives off these lovely rays in the energy domain of ultraviolet light, which most of you probably know as the reason we get sunburns when we are outside too long in the summer (boo, sun. ). Another detrimental effect of this UV radiation is the fading of LEGO pieces. Why/how does it fade them? Well, Earth is extra special in that its atmosphere has wonderfully healthy amounts of oxygen (O2) gas which we need to breathe and live (yay, oxygen!). Not only do we like oxygen, but so does ABS plastic, from which LEGO is made. The plastic has a compound in it that possesses bromine which, for those of you who do not know your periodic tables, is a halogen in the second to last column of the periodic table and is, thus, highly reactive when on its own. Fortunately, it is nestled in the ABS compound, but this doesn't quite satiate its need for buddies to bond with it (because it's greedy that way), so it decides to find more buddies to bond with in our own air – the very oxygen we breathe!
What does UV radiation have to do with this? Well, it turns out that bonding takes energy, and the bromine within the ABS does not have the energy by its lonesome to absorb a buddy oxygen from the air (since oxygen gas is fairly stable and thus requires more energy to separate). So, the UV radiation of the sun is just the kick it needs to bond with oxygen, thus producing this:
Ugly, huh? But, someone discovered that our friend hydrogen peroxide (with a catalyst found in oxi-clean detergents) is able to reverse this process with the help of – guess what – UV radiation. That's right, the same thing that triggers the fading is also what allows it to reverse! Weird, huh? I decided to try this process on some of my faded white and gray pieces from some of my older sets (circa 1998-2000, mostly Adventurers theme) and this was the lovely result:
The difference is like night and day. For those of you wondering if it affected the printing on the skulls of the skeletons or the minifig bodies, the answer is no, it did not. Truly remarkable and a relief that my old pieces can shine as the pearly whites they were meant to be.