Kind of an unusual question, but I was thinking about how there's a litany of commercially available guitar polishes, waxes, fretboard cleaners, fretboard oils, etc. When did this attention to cleaning or polishing string instruments come into being?
Before the advent of specifically made stringed instrument polishes, what did musicians use?
Even further before that, how about in antiquity? Was it a common practice to polish and clean string instruments in the 15th to 19th centuries?
Before the advent of specifically made stringed instrument polishes, what did musicians use?
The answer is what they still use today: a clean cloth, and if things are really awful a little bit of a solvent like mineral spirits followed by a clean cloth.
But there's a problem: the manuals don't say anything about it. The earliest printed manuals from Spain for vihuela?[1] Nothing.
Nothing. Geminiani and Leopold Mozart?[2] Not a word. Baillot's text for the Paris conservatory?[3] Nada. The late 19th century banjo manuals[4]? Nope. Even today's basic manuals for guitar such as those from Mel Bay don't talk about this. Taylor guitars has a brief FAQ, saying exactly the same thing as I said.
I spent some years in early music, and among all the discussions of instruments and details of their making, stringing, and playing, there was never any mention of cleaning. We just all whipped out a clean cloth, wiped off the fingerprints and resin, and on we went.
As for when the various commercial products designed to separate amateur musicians from their money emerged, I apologize: I don't have access to an archive of magazines aimed at amateur musicians through the 20th century. The advertisements in there would be the easiest way to document the rise of such things.
[1]: Chapter 7 of Music Education in the Middle Ages and Renaissance, edited by Russell E. Murray et al. says, "The printed vihuela books, although aimed at the beginner as well as the accomplished player, pay little more than lip service to mechanical matters." And it's true. Luis de Milan's Libro de música de vihuela de mano intitulado El maestro has barely anything on the actual playing, much less cleaning. On playing the vihuela from Juan Bermudo's treatise says nothing either.
[3]: Pierre Baillot, L'art du violon. Kreutzer and Rodé wrote sets of etudes that we still use...but they aren't concerned with the day to day care and maintenance at all.
[2]: Leopold Mozart's Versuch einer gründlichen Violinschule (1755) doesn't talk about care at all. Nor does Geminiani's The Art of Playing on the Violin (1751).
[4]: Briggs' banjo instructor, 1855 has nothing. Nor does the 1851 The Complete Preceptor for the Banjo. There's a literature review that doesn't suggest anything more.