vulcanize

verb

1. (transitive)To treat rubber with heat and (usually) sulfur to harden it and make it more durable.

   “Local workers hike up the side of the mountain and down into the crater at the top to harvest its sulfur—a byproduct of the gas that escapes from the volcano’s vents and collects near the shores of an acidic lake at the crater’s center. The chemical is used in industry worldwide, from making matchsticks to vulcanizing rubber, but Ijen’s sulfur goes mostly to local factories, which use it to bleach sugar.”

2. (intransitive)To undergo such treatment.

   “These vulcanized hearts, they only become more resistant when you ask for mercy.”

   “Natural rubber […] vulcanizes readily at the double bonds to give a strong cross-linked structure.”