Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

I believe that is simply a solder joint that has come loose, likely due to your Bix burner's flame not being far enough into the fire box, thus putting flame directly on your sight glass assembly.

It should be a fairly straight forward resolder job, but it is important that you fairly closely match the grade of solder that was originally used, otherwise you may find establishing a complete bond difficult. It does look as though it will be an awkward job due to the lack of a clear line for the torch flame to reach all around the frame mount, unless you do a major disassembly to provide better access.

Best of luck with your repair and keep us informed of your progress!