The new battlefield in the loudness war?
A brief introduction to soundproofing
What is production? Part 2: Arrangement
Recording acoustic guitar in stereo - should you use spaced or coincident mics?
A brief introduction to acoustic treatment
How to become a better singer
Avid and Apple conspire to heist 9 decibels of level
What is production? Part 4: Mixing
Does inverting the phase of one channel of a stereo signal always sound bad?
What would happen if a spider got into your microphone?
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Parts 1 and 2 of this article are essential reading otherwise this part won't make as much sense as it should. All three parts consider mainly the steel-strung acoustic guitar.
Some guitarists will say that it is better to play on new strings, others prefer the sound of strings that are played in. For a recording, it is important to choose strings that sound great for the song. If your song needs new strings, then that's easy - just take out a new set and put them on. If it needs strings where the zinginess has toned down, then hopefully there will be a used set of strings available that is at the right point in its life cycle from too zingy to too dull.
If guitar strings are so variable, then maybe a better option is to try and gain control over them to at least some extent. We can consider what actually happens as a string is used over time...
Firstly, it is very possible that the actual metal of the string changes. As you probably know from your school science, metal is subject to fatigue on repeated flexing, ultimately to the point of breaking. To my non-metallurgist thinking, it is quite conceivable that the structure of the metal of the strings changes as they are repeatedly vibrated (played). There could also be an issue with the structure of the winding due to repeated vibration, but this of course would only affect the strings that are wound.
Secondly, unprotected metals oxidize in the atmosphere. Left long enough, guitar strings will rust. Actual visible rust probably wouldn't ever be a good thing, but a very thin layer of corrosion that you couldn't see might well affect the sound. I'm not so sure however that this is all that much of an issue as I have used gold-plated strings in the past, and they still became dull over time and use.
A third factor is the deformation of the string where it presses against the frets. The string starts off being very similar to a mathematician's concept of a one-dimensional object (search online for "mathematics of vibrating strings" if you want to see how much thought has gone into this). As it is played however it develops flat spots. This makes the shape of the string mathematically more complex, and you could expect the sound it produces to change too.
From the above it would seem that controlled experiments in vibrating, corroding and deforming your strings would help take you forward on the path the the sound you want, and to be able to achieve it when desired. However, there is a more significant factor in a string's life cycle that you can reproduce easily whenever you want. And that is...
As a string ages it picks up contamination, which is mostly a combination of sweat and dead skin cells that work their way into the winding of the lower three (or four) strings. If you can reproduce this contamination, then you can condition a string so that it sounds exactly as you want. You might consider going for a run to work up some sweat, then rub it into your strings. No, that's disgusting. But there are various fluids you have around your home that will work to a very useful degree. A very small amount of cooking oil rubbed into a string will take away surplus zinginess immediately. Bear in mind that you can very easily overdo it and end up with a string that is horribly dull. Only a tiny amount of oil is needed, and probably not over the entire length of the string (think of where contamination normally happens).
At this point, I shall leave the choice of intentional contaminant to you. I've found one substance that works reasonably well, but there might be others that work even better. Be advised that although you might consider PTFE bicycle chain lubricant a possibility, it is highly flammable, as other likely candidate substances might also be. There are also so-called 'string conditioners', which might be worth a try. They are sold as string cleaners with the intention of extending strings' useful life. However some users complain that they make the strings sound dull, so used in tiny amounts they could be useful to de-zing new strings that are fresh out of the packet.
Since there is an optimum point in the life cycle of guitar strings where they don't sound too zingy but are still nice and bright, it would be desirable to gain control over this process. Finding a way to condition new strings to your liking could be a valuable way to do this.