My kids came to LEGO age so my parents dropped off a few boxes of my LEGO yellowed bricks. Memories!
Many of those old pieces look, well, old. Discolored because of years of sunlight exposure, the white bricks look yellowish. They really stand out in a set with newer pieces so I DuckDuckGo-ed (yeah, that doesn't sound right) me a solution: hydrogen peroxide!
It is possible to reverse the effect of the UV light on the pieces and remove the yellowish color. We'll need some hydrogen peroxide, a bowl, and the help of the sun to reverse the damage.
Hydrogen peroxide you say? Yes, I had no idea what that was neither and hope I didn’t set off any FBI flags looking to get my hands on a bottle. Turns out it’s very common and easy to find. Hydrogen peroxide, or H2O2, is normally used as a sanitizer to disinfect cuts and you can find it in most supermarkets’ hygiene departments (for Belgians: Delhaize & Kruidvat, probably others as well).
Throw the bricks in a transparent bowl and add the hydrogen peroxide (I added water as well since the bottle didn’t mention the concentration and I wanted to be on the safe side). Place the bowl in a sunny spot and wait a few hours. You know it’s working when you see bubbles forming on the bricks.
It’s important to fully submerge the bricks or a line will form on the part right where it sticks out of the liquid. The line is etched in the plastic and hard to remove so it’s easier to weight down the pieces and make sure they don’t stick out.
Thanks to The Retr0bright Project for the tip!