A seraph (singular of seraphim) took a burning coal or glowing stone and touched Isaiah’s mouth, symbolic of cleansing, a “baptism by fire”—the “refiner’s fire.” From ancient times metals have been cleansed from impurities by submitting them to extreme heat.
In this case, the cleansing was literal, but the process was symbolic or figurative. When God commissioned Jeremiah, He touched his lips (Jeremiah 1:9), symbolic of cleansing the inner man. The lips are an apt metaphor. The Savior later said that it was that which came out of the mouth, uttered by the lips, which defiled a person (Matthew 15:11). The heart and the lips are linked: “for of the abundance of the heart his mouth speaketh” (Luke 6:45).
Repentance does involve pain. If we have not suffered, we have not really repented. Isaiah’s sin was “purged”; the Hebrew term t’khuppar means “atoned for.” As he was purged, he was qualified and strengthened for the work.