Fusible

Lead is a soft, dense, malleable and fusible metal.

Although a fusible plug is provided in the firebox crown of every locomotive, so that by melting if the inner firebox crown becomes uncovered, and thereby releasing steam at full pressure into the firebox, the fire may be extinguished and the crew given an unmistakable warning of the danger, there is no absolute guarantee that the plug will melt.

Try any fusibles you have on hand, making sure that they aren't too stiff for the project you have in mind.