Ok, My Wasr-10, which has maybe 1500 rds total through it in it's life, maybe more, can't remember

It's been cleaned twice since I've had it(when I got it, and after about 500rds) I have shot only Wolf, and GT through it, so it's all NC ammo....
I noticed the Carbon on my piston, has turned from that light flat black color, which wipes off easy, has turned more to like a tar consistancy, not sticky like that though

I haven't fired it for maybe 6 months maybe, so I wondering if the carbon is changing forms or something, never noticed it before......
I know the solution is to clean the damn thing, which I am doing as we speak, but I was wondering what would make the carbon do this????
(yes I'm bored, and no I'm not drinking

)
What 'HM' said. +1
When the carbon fouling deposits on my AKs' pistons gets a little "heavy", my routine for removing it as always been:
1. "Pre-soak" the piston "head" in some transmission fluid (whatever you've on hand will do just fine) over night; two nights if the deposits are really severe
2. Then, after the "pre soak" is complete, use the phosphor bronze "toothbrush" to
very gently scrub the carbon deposits away (this requires very little effort)
....works everytime.
TK
I use CLP and a "medium" toothbrush from the drugstore to clean all my guns. Works great.