Just an FYI, sweet scented wood chips burn better in thin layers on coals. Their purpose is to release their aromatic Offering
of smoke as they slowly smolder to a blackened lump of ash. They never flame unless you set a match to them like kindling
. Some charred matter always remains even using powdered plant material. Scrape off old ash before adding the next thin layer of material for more smoke.
A Kitchen witch observation as comparison - on a backyard BBQ the hot coals are used for a slow cooking process, not for searing, but we can still burn food to the bottom of a pan. It just takes longer. Burning doesn't need flame, just heat. I know from burning just about anything I cook
. The coal discs work the same.
Girl Scouts 101 Re: flame - to make a flame from a coal it needs loose kindling on top & fanned air. Re: burning string - a loosely twisted strand burns better than a tightly braided one. You can lightly UN-twist string to loosen the fibers to aid burning.
In magical parlance, burning any spell-item usually carries the intent of transforming the item from "this-to-that" ; transform in effigy from the physical plane to the ethereal plane where it will then conform to our will. It matters only that the item is transformed, ie destroyed beyond re-use; being reduced to ash is obviously visually & emotionally satisfying so that we let go of our ties to the item &'what it stood for more quickly, & that can have some impact (by association) on the spell's swift completion. Whether the kindling we'd used is fully consumed usually isn't part of a spell (unless you specifically made it a condition). Last - practically speaking, ash is easier to dispose of or scatter than charred chunks.