Refers to charcoal as a material, fuel, or drawing medium. Common in cooking, heating, and art contexts.
バーベキューに炭を使う。
We use charcoal for the barbecue.
彼は炭でスケッチを描いた。
He drew a sketch with charcoal.
charred remains
Rare use for something that has been burned to charcoal-like remains, such as after a fire.
火事の後、家は炭のようになっていた。
After the fire, the house was like charred remains.
My grandfather made his living by making charcoal in the mountains.
We lit the charcoal before the barbecue.
Fish grilled over charcoal has a wonderful aroma.
The pillars of the house were burned to charred remains in the fire.
灰 is ash, the powdery residue left after something burns completely, whereas 炭 (in its 'charred remains' sense) still keeps a solid, blackened form.
partly burned charcoal; charcoal that has been extinguished · servant boy at a brothel or teahouse (archaic)
cut-up charcoal; charcoal pieces