So the bottom line is:

This bark I've been using is at least 5 years old.

It used to yield very pure white freebase dmt at first, then it got lost in a freezer for 4-5 years. After that it only yielded yellow oily sticky honey-like goo.

But adding elemental zinc powder to the bark while boiling in vinegar gives me pure white crystals now but reduces yield from maybe 1.5% down to 1.2-1.3%