In fact, that fear seemed justified as Spike, whatever he had been doing, let out a small bit of fire that caused a loud bang as the remaining coal dust caught light all at once, blasting open the half-closed door to the furnace. This scared the hell out of everyone, and Handy all but brained himself as he shot upright and clonked his crown off of a rather impolite valve. After all the cursing and berating had been finished with, not least of which came from Spike, who was also under stress and was beginning to give back as good as he got, it turned out Spike had found some fissures in the cast iron of the furnace and spent quite a bit of time modulating his fire breath to effectively weld the seams back together.