Thermal indicator for wells

A thermal indicator for circulation with fluid in a well and sized to pass through the fluid ports of a well drilling bit for indicating if a chosen temperature range has been encountered in the well, comprising a shallow pan-like retainer body of durable sheet material having a bottom surface portion adapted to bear a substantially permanent identifying marking and having an open top, a meltable solid metal alloy indicator component having a narrowly defined single melting temperature range chosen from the temperature spectrum 76.degree. C. to 351.degree. C. normally encountered in a well, said indicator component at least partially covering the bottom portion prior to melting of said component and being exposed for contact with the fluid circulated in the well, a flange-like formation on said retainer body adjacent the open top thereof, said meltable solid metal alloy indicator component extending across the bottom surface portion and beneath said flange-like formation at opposite sides of said retainer body, and said flange-like formation mechanically fixedly securing opposite sides of said component to said retainer body prior to the melting of said component, whereby when the narrowly defined melting temperature range of said indicator component is encountered in the well said component melts from beneath said flange-like formation and separates from said retainer body exposing said bottom portion and is lost in the fluid in the well.