Sentence:  Reaching for an old clay pot , relic of pioneer days , he tore the envelope in pieces , dropping them into it , touching the little pyre to flame , watching it curl , the red sealing wax <WSD>melting</WSD> and bubbling in the feathery ash . 
Meaning of the word marked with <WSD> tags:  reduce or cause to be reduced from a solid to a liquid state, usually by heating