Bricks are getting dusty on the shelves. They get dirty from building and just playing with them around. And used bricks we buy... sometimes it is better to not to know what kind of dirt they got. My advice is to clean every single used brick you buy. Even if they look clean.
What is the best way to clean LEGO bricks? I'll show you the way I do it. I think it the safest for bricks, very precise and easy to do.
1. If the bricks are not separated yet now it is best time to do it. Make sure to put aside all elements with stickers, prints and glass parts.
2. Put rest of the bricks into a bucket with warm water (max 40°C / 104°F). Pour lot of dish-washing liquid. Some ordinary, traditional type. In Poland we got "Ludwik". Not sure what's the brands in other countries. Keep bricks in a bucket for like 1 to 3 days. Depending on how dirty they are. Stir it from time to time. After so long time the most dirt will get soft and fall off by itself.
Washing LEGO in dish-washing liquid has an additional advantage. If you have kids you know how they like to put everything to mouth, or (oh no!) bite bricks. In this case it is better when kid will accidentally eat some remains of dish-washing liquid then some washing powder or toilet cleaning liquid...
3. In next step put bricks in a plastic strainer. If they were in decent condition do it like oldschool gold digger - just pour whole bucket through the strainer. Why strainer you ask? To be sure that brick won't fall into water draining hole. If the bricks were very dirty inspect them 1 by 1 and put in the strainer.
4. The LEGO bricks which are still dirty I clean with soft, children toothbrush. It does not scratch bricks and handles with most of the dirt. If the bricks are still soiled or marked with pen, crayons etc. toothpaste is the way to go. Just put it on the toothbrush and brush bricks gently. It really works well!
5. For the really nasty dirt like the plasticine inside the brick use wooden toothpicks. It won't scratch LEGO bricks and it will help with really bad dirt.
6. When all the bricks are already clean wash out them by running water in the strainer. It will eliminate any dirt and dish-washing liquid remains.
7. And the last step is drying bricks. Put them on a towel and from time to time mix them. More often you will mix the faster they will dry and you will minimize chances to have damp patches.
And that's it. After this cleaning method your LEGO brick will shine like new!
Few additional information about cleaning the LEGO bricks:
1. Bricks with stickers - in the water they can fall off so don't put them with the rest of the bricks. You can clean them gently with a toothbrush or a soft cloth.
2. Printed bricks and glass parts - wash them in the separate container. I think glass is much easier to scratch then ordinary bricks and prints can worn off from toothbrush. So just gently wash them with soft cloth.
3. Many people wash LEGO bricks in the washing machine or dishwasher. You can do that but I think this is less safe for the bricks. They can easily get scratched and matt so I would not recommend these methods. However, if you decide to do that remember:
• 40°C / 104°F water temperature is max for the bricks. Higher then that can get them deformed and matt.
• Do not use any strong / aggressive cleaning agents because the bricks may become deformed and matt.
• Turn off spinning to minimize the chance of getting scratches on the bricks.
• Put all of the bricks in the special underwear bag and tie it well to avoid spilling all the bricks in the machine.