I purchased a large bulk load of bricks for my 7 year old. Most have washed up quite well, but some of the bricks have various substances stuck in their underside.
The types of foreign substances vary, eg. mud, playdough etc.
What are good ways of getting the muck out of the bottom of bricks?
best way i found so far is simply using a plastic-toothpick (wooden ones should also do the job). just get the muck out as good as possible and then wash the bricks again the remove the rest.
if the muck is too hard for a toothpick, use a metal needle or a very thin screwdriver - but only do this if you can't avoid because you might be scratching/damaging the blocks (wich shouldn't be so problematic to the inner- and underside, but anyway you should avoid it).
I suggest to let them soak in water for a long period of time (2 days) before trying to clean them. The water will not affect the Lego plastic but some other elements (especial dirt and food) will soak it up. This makes it a lot easier to simply rinse out or scrub off the dirt.
The plastic toothpick (like @oezi suggested) is great for very small corners. For the tougher crud, I suggest wood tools (like sculpting tools) since they won't scratch the soft plastic but are hard enough that they don't just bend.
You could also get yourself a can of compressed air - it works on very small areas on my motorbikes so I'm sure it'll do an equally good job on some lego bricks. Here's some on Amazon...
Soaking in water for a long period of time (a few hours is usually enough) and then using a pipe cleaner to sweep out the residual muck has proven to be very effective. The pipe cleaner is gentler than some of the other things that I've previously used (toothpicks, small screwdrivers, etc.). A Q-tip with a very small hide should also work well.
For larger quantities of really tough muck that's stuck in the tubes on the bottom of bricks, I've had some success with scewing in a cross-headed scew and then pulling it out.
I have to wash my Legos (mostly Duplo, but also some regular ones) regularly, as my creations are actually used as toys for my sugar gliders (small exotic animals), who can't be "potty trained" and often use their toys as toilets. I soak the blocks for a few hours or possibly overnight in a 50/50 solution of hot water (hottest I can get from the tap) and white vinegar, adding the blocks after the water and vinegar are mixed since temps above 104F can damage Lego pieces.
The warm vinegar/water solution takes care of most of the cleaning and disinfecting on its own - remaining debris can often be removed just by rinsing the piece in a strong stream of warm tap water. For more stubborn muck, I've had good luck using the small (nipple) end of a baby bottle brush and for crevices where that doesn't fit, I use pipe cleaners (in both cases, after the pieces have had a nice, long soak).
