On the metal filings and such. I ride a bunch of rail trails. The cinders have a lot of rail dust that is nothing but metal dust from the wheels and the rails. It is very fine and it gets everywhere on my bike.
I took the bike down to the bare frame yesterday, hand cleaned the chain and relubed with what I use. While I sometimes like to soak a chain, I didn't have enough Simple Green to do it properly.
When I use Simple Green, I heat it up slightly to about 100-110 Degrees F when I dilute it by adding hot water. I use about a 1:4 ratio which is Simple Green's recommendation and what we used at the shop in the parts cleaning tank. I use a stainless 1 quart mix vessel that I have and it is amazing what I get off of a chain using this method.