unto me - The seraph had been in the temple, Isaiah outside of it.
live coal - literally, “a hot stone,” used, as in some countries in our days, to roast meat with, for example, the meat of the sacrifices. Fire was a symbol of purification, as it takes the dross out of metals (Mal 3:2, Mal 3:3).
the altar - of burnt offering, in the court of the priests before the temple. The fire on it was at first kindled by God (Lev 9:24), and was kept continually burning.