Art I really do not know. What is your guess?I have no idea just were that stuff came from. After I scrubbed that bore with a brush and then hit it with that first solvent saturated patch, I was stunned. I could not believe that came out of my rifle. I am guessing it is all the carbon that is somehow trapped in the pores of the metal, but you would think that with the water bath it would have been gone. In fact when I was giving it the water bath I even commented on a what a good water flow I had going through the barrel. And the clean Rusty Duck patch did not surprise me being clean. But that Montana patch sure did.