I reduced a bunch of fairly weak material with Zn and HCl, and following basification and freeze precipitation I placed a bunch of wet freebase on aluminium foil for solvent evaporation. When I came back maybe 30-40mins later I noticed both the spice and foil gone dark gray to black at the point of contact.

What sort of chemical reaction is this and how would you get rid of it?

Apply 2 - 3 drops of tears per gram (depending on how much spice you might need more tears)

Place said spice in dustbin

Meanwhile extract some more spice, and allow to dry on clean glass surface

Allow garbage to be collected

You now have clean spice and no self harm done

lol, thats would be a wasteful solution to my problem.

I redissolved it in warm solvent and passed the solution through a 0.2 µm fluoropolymer filter and harvested crystal clear solution.

Sorry if thats not the relevant board but seems I'm still green and not allowed access to seasoned poster chemistry boards.

Could anyone shed some light on the possible chemical reactions there? Why would that happen??

oxidation, and I wouldn't do that to begin with. never use an acid or base with aluminum, unless your goal is a chemical reaction. use stainless steel or glass.

Yeah definitely glass from now on. I wonder if there has been some zinc hydroxide picked up with the naphtha and subsequently precipitated that gave this peculiar reaction. After I did Zn+HCl reduction adding NaOH resulted in dense white precipitate which I speculate to be DMT freebase and zinc hydroxide. The naphtha appeared to pick up some insoluble flakes (zinc hydroxide maybe?). Anyway I'll filter that shit.

I'm really curious about the mechanism of this reaction. I decided to dissolve zinc in HCl, and react the chloride salt with NaOH (thus getting the zinc hyrdoxide) and then use that to see if it will react with the aluminium.