I am the one who wrote its Wiki (before some clown rewote it without adding anything worthwhile) so forgive me if I repeat some of it. If you want to understand Tar examine NZ Homebake, a lack of reflux apparatus renders it very impure and NOT #4. Number 4 is a salt. Tar is an acetate.
As for adulteration, I wish that I could tell you that it is free of Fentanyl but these days it has just as much. Showing how little Mexican cooks understand, they toss Fentanyl in on both ends of the equation, completely asinine. Tossing it in with opium before acetylisation, then acestylising it sans reflux, they toss more Fentanyl in.
In days gone by it would be adulterated with plain old lactose after first liquifying the Tar end, then, if they wanted Mexican Mud, or as it was called in NYC, "Chocofan" [sic], pronounced "Choco-fine," they would toss in rice grains for each Mx Oz (25 gm) and once dried naturally, they wouldvremove the rice, grind it, add more rice and add lactose, place in a very gentle steam bath to rehydrate and noone could tell seeing as how Tar's purity ranges so widely.