It's just heroin, slightly purer than the gear on the foil you started with because a lot of the crap has been left behind as you run it, but it's exactly the same stuff in exactly the same form. You're not burning heroin when you chase it remember, you're vapourising it, so it's not a tar residue or an oxide or anything like that. Solid turns to liquid when heated and then vapour when it boils, some of that condensing back out when it hits a cold surface like the tooter you're pulling it through and sticking to it. Can't give you a citation but not sure one's needed? It's that straight-forward.
Great post Sepher!

Back before the drought when good gear was plentiful i used to save up loads of tooters, & then if there was a night that i was broke or if i couldn't score i'd make up a nice big tray out of these tooters! Those trays always seemed stronger, sometimes i wondered if i was imagining it, but it seems they were indeed stronger!!!

God i miss strong gear, i'm smoking a tray of Dublins finest crap right now as i type! 8(