Many months ago SWIM had some pure off white freebase bufotenine and did a D-limonene boil on it to get rid of any traces of xylene in it (it was originally precipitated from xylene). The white bufotenine turned brown and tasted of oranges.

After several months passed, SWIM got out his bufotenine and it was all black and hard.

SWIM has seen this happen before with impure bufotenine where it takes on a black hard shiny appearance, almost looking like coal after a few months.

Because of the high pKa of 9.67 freebase bufotenine should theoretically form bufotenine carbonate after a while, just like freebase mescaline ( pKa of 9.56 ) forms mescaline carbonate after a while.

I don't think this can happen with DMT ( pKa = 8.68 ) because it's a weaker base than bufotenine.

The black material that forms is insoluble in acetone and insoluble in water. Is this bufotenine carbonate? Does anyone know?

It could also be the limonene degrading. its sensitive to oxidation and things like that.

That's very likely.

SWIM has not seen pure bufotenine turn black ever.

I think I agree here, it's probably some leftovers from the D-limonene, the fact that it tasted oranges and turned brown are reasonable evidence! Anyway, if you want it pure again, why not try to precipitate it out of xylene again? I find it odd that you mixed in the D-limonene from the beginning, there can't possibly be any traces of Xylene left in the extract when it's dry. Xylene is a really volatile solvent!

There actually was. You couldn't smell xylene on the crystals, but when SWIM took it orally, after about 30 minutes he burped xylene flavored burps. It’s was horrible! Somehow the xylene got trapped in the crystal matrix. Next time he’ll try re-crystallizing in MEK:heptane to get rid of the xylene.

Xylene is not that volatile. Its boiling point is higher then water. 130-140 C.

I always recommend that after evaporating xylene redissolve material in a more volatile solvent like hexane or ethanol and reevaporate to get rid of traces of xylene. I think people not doing stuff like this is why people get harsh smoke that tastes like plastic. Even if the material smells completely normal xylene molecules get trapped literally within the crystals of spice.