Just so that we don't ignore "conventional wisdom", I present a verbatim quote from the text book "Modern Locomotive Construction" by J.G.A Meyer, published by John Wiley and Sons in the year 1892 (yup, 110 years ago). From page 435, Section 430, "Fusible Plugs"
"Fusible plugs are sometimes used as a safeguard against the collapse of the furnace crown sheet from overheating through a shortness of water. Sections of the different forms of these plugs are shown in Figs 663, 664 and 665. They consist of a brass shell containing an alloy of tin, lead, and bismuth. The plug is screwed into the crown sheet at a distance of 18 to 24 inches from the tube sheet. The water above the crown sheet keeps the alloy at a comparatively low temperature, and prevents it from being melted. When the water in the boiler is so low as to uncover the plug, the alloy is supposed to fuse, allowing the steam to escape, retard combustion, and in the meantime relieve the boiler of its pressure. Whether this action of the plug can always be relied upon is very questionable; we believe its efficiency is sometimes over-rated. A long exposure to the heat in the furnace may cause an alteration in the nature of the alloy and render it valueless. Again, incrustation [sic] on the plug may become strong enough to withstand the pressure of the steam and prevent its escape after the alloy is melted. It is therefore good practice, when these plugs must be used, to renew them at short intervals, say every two or three months." "Two or three months", of course, means 60 to 90 days of continuous steaming in the parlance of the age, not the few hours of steaming that a typical model live steamer might see in two or three months. Remember that in 1892, typical boiler pressures were 150 psi, with a saturated steam temperature of 358 degrees F. Post-War locos had had boiler pressures of 250 or more psi with a saturated steam temperature of at least 400 degrees (which the crown sheet would see, even if the steam to the cylinders received superheated steam). Our Ga1 babies with 60 psi boiler pressures see "only" 292 degrees F. The point is that one has to choose one's fusible alloy carefully. For post-war locos, one might use pure tin, which eliminated the problem of the low melting point alloy constituents leaching out of the plug and had a melting point of 450 degrees F, perhaps not an inappropriate difference between the maximum steam temperature and the melting point of the fusible plug.
