I can say this from the perspective of having been told by the owner to 'give it the works'.

What you see here is a completely overhauled and restored example of the genre.

It's not something you see much of on modern horns (which feature drawn tone holes, whereby the wall of the tone hole is physically pulled out of the body material), but if your crook falls off its tenon sleeve then it's quite likely that the cause will be oxidisation of the solder - causing it to crystallise and lose adhesion.

In the case of the Martin we have the brass of the tone hole and body, and the lead alloy of the soft solder.

The electrolyte is nothing more than the water that forms in the bore as condensation or gets blown into the horn as saliva.

Granted, the design is perhaps quite simple - and in a modern context I'd throw criticism at such things as the single mount point for the bell to body stay, the static thumb hook and the fixed bell key guards (it's quite hard to set pads on these bell keys when the guards obstruct your getting a pad setting plate inbetween the pad and the tone hole).

On a more positive note the pillars are all well fitted, the bottom bow joint (where the straight portion of the body meets the curve of the bottom bow) features a substantial soldered joint and you get a sense that everything is twice as strong as it needs to be - indeed, the bottom bow plate sports a ridge that looks as though it could easily double as an icebreaker should the player ever find themselves stranded at the North Pole.

Such a process tends to be less 'caustic', and follows the principle of not taking away something that can't be put back - so that the process can be repeated in later years, if necessary.

There's many a fine vintage horn out there that's reached the end of its life prematurely by dint of having been buffed to within an inch of its life every time it's been overhauled.It's also the case that the weak acids in saliva will help to break down a soldered joint in time - so between the two processes it's no wonder that these tone holes are often problematic.There's a separate article here that goes into more detail, and discusses the remedies.There are pros and cons with this design, of course, and the big drawback is the possible lack of integrity of the joint where the tone hole meets the body.It's very common to find Martin horns with leaks around the tone holes, and this is most likely due to the process of oxidisation of the soft solder that holds the tone holes in place.It also raises the question as to whether silver plated horns with soldered on tone holes are more susceptible to galvanic corrosion than lacquered brass ones.