Top 山 - pictograph of three peaks. Historically 炭 was written 灰 under 厂 (cliff). In the modern shape 山 sits where coal was dug - charcoal and coal both come out of mountains. Indexed under Kangxi #86 火, taken from the fire at the base of 灰 below.
Bottom 灰 - itself 厂 (cliff) over 火 (fire), meaning ash. Carries the indexing fire radical (火 at the base of 灰) that makes 炭 a fire-family word. 山 above + 灰 below gives 'ash from the mountain' - the original picture of charcoal-burning. Pair with 煤 (coal), 烬 (cinders).
Filed under radical 火 (huǒ, #86) by convention. 火 is not a separate component in 炭, so no strokes are highlighted.
