Yes, dirt, mud, hardware stores...but avoid petroleum products. Some will advise you to walk on fresh tar, oil spots, grease, etc. Do you really want to walk in that kind of stuff?
Be surprised how a clean looking floor is worse then anything else. Floor like this in this Google image looks clean but not really. Defiantly more dirt then the street unless it has not rained in a real long time. Feet will get dirtier walking around the mall or a big retail store then they would out in the parking lot unless its new pavement I guess.
I agree with TheGreatShoeScam. Shopping arcades and stores which look clean are really not! Five minutes of walking around some of my locals shops and my soles are black. As the floors are usually smooth and flat you can "scuff" your feet as you walk to maximise dirt collection on your soles. I don't recommend this method outdoors or on rough surfaces. Good luck and have fun finding different ways of getting them dirty!
I agree. Now the warmer weather is with us walking around town centres and shopping centres makes for filthy soles. Warm slightly sweaty feet attracts the dirt lovely.
Go camping or something, the ash from campfires fill make them black quickly. Barefooters are a minority so very few people have actually gone barefoot camping but I sometimes feel like camping feels wrong if it isn't barefoot just as much as it is without a tent. The idea of camping is to get close to nature, so doing it without being barefoot the whole time feels weird. A bit artificial.
