Bleaching white Lego

 Old white Lego parts can darken to a yellow tan colour. Transparent and blue parts can also discolour to display rather brownish hues, as can grey parts to a much lesser extent. I've read somewhere that it's a flame-retardant bromide component in the original plastic mix that causes this, but other than that your guess is as good as mine. Why red and yellow parts are not affected I haven't found explained either.

Blue and transparent parts cannot be helped, but thankfully the white parts can be bleached back to their original white splendour, usually entirely so. Hydrogen peroxide is what's required, available from the chemist's and other sources. I got a 12% solution online. Its most known use is to bleach dark hair to what has often actually been called "peroxide blond" hair. Blue Lego will just get more and more faded over time and any printing will go in days. There's no point in putting in transparent parts. I have yet to try grey parts but apparently peroxide works on those as well.

Apparently the peroxide requires daylight to work, so transparent containers are a must. I use an empty instant-coffee jar sat on a window sill in the kitchen. Not shown here, it's now sitting on an empty tin can to get it up to the height of the window frame. I've cut a section of colourless PET water bottle that nicely holds plates upright. The bubbles seen in the picture are a reaction to surface dirt I think, they disappear after a while. 

So far, the peroxide in the jar has been in use a good year. I dunk in a load, leave it a week or sometimes longer, and then fish out the bricks with a throwaway pair of wooden chopsticks. I've topped up the level a few times with some water, where I'd say the solution will be more like 8 or 9% now. Now that the central heating in the kitchen is on since the start of Autumn, I think it's evaporating a bit as well. And maybe it's finally losing its potency, for the last batches seem to take longer.

Ive found that a jar half-full with parts works best, and stirring once a day probably doesn't hurt to get even exposure to light. Lego itself doen't float, but air trapped inside a brick does provide buoyancy. Tapping with a chopstick releases most if not all and a few layers of bricks on the bottom of the jar will still work out though with the odd stir. The first months of the year there was less light, but a week could show a marked difference as seen above. The summer months were very good of course.

Most vintage Lego will come out nice and white. Some of the old plates may still keep a buttery yellow hue. And newer Lego parts can come out whiter than white indeed. (The small grey patch is a leftover gunpowder mark, a souvenir from when we blew up Lego structures with little firecrackers many decades ago. The peroxide didn't much affect it).