I converted some DMT-freebase to fumarate and back as a cleaning procedure. After recrystallization of this re-converted freebase in warm heptane, i found some white residues on the bottom of the glas, which just wouldn´t dissolve (i added more heptane than enough). Is it possible, that those insoluble residues are maybe fumaric-acid or sodium carbonat?

Well, the residues were soluble in distilled water and the pH was about 10. So i assume the leftovers were sodium carbonat. Anyway, did someone else noticed the same thing?

How did you convert to freebase? Can you describe your exact steps?

Definitely sounds like sodium carbonate excess... But this can be eliminated depending on how you do the conversion.

I followed the Type 1 procedure from the Fumarates to Freebase Conversion TEK, which was kindly posted by you, endlessness, as i assume. I definitely added some excess sodium carbonat water solution (maybe 2-3 ml) to the DMT-fumarate-water, since i did it the first time and was unsure about how "no more clouding" looks like... I´m sure i could have done it more slowly.

However, I don´t mind recrystallizing in naptha/heptane. The possibilty of cleaning the spice, while it is a fumarate-salt, is intriguing to me though Will try that soon.