It's been my experience that recovering any usable product from feedstock that is burnt to the state of being black and solid(or soon therafter will be)is not possible. I found that there were changes in expected properties when trying to salvage some that suggest to me that indeed there is changes on a molecular level that aren't necessarilly beneficial(read_ 'fucks it right up'.)Recently I did a few small tests with some very black and crispy solids and found it would not dissolve in H20, or a basic solution, thought it was noted that fizzing occurred when it contacted the base. It was moderatly soluble in acetone but needed encouragement by grinding the solids to finer particles.
A test small chunk of black was tossed into a hypophosphorous/I2 reduction and remained a small black chunk throughout to be collected unchanged at rxn end.......so as I said it's my opinion that burning it is counterproductive and probably not something one should become good at
Now that's only for black type burnt, anything leading up to this ie; tan, brown, golden sahara or whatever you want to call it, has travelled through the reduction without incident the couple of times I've tried it.
Ps. Of course when I did all this I fried the feedstock intentionally for the purposes of research, only a real dummy would accidentally do it, just wanted to clarify that!