I thought I would throw a few more words at this thread, just to address one point that seems to have been overlooked - that is, a lacquered horn where some (or a lot) of the lacquer has worn off or become scratched.
As has been mentioned, nearly all brass-colored instruments are lacquered. The lacquer forms an air-tight coating that keeps the brass from tarnishing. If the lacquer were always intact, the brass would never tarnish and it would be shiny forever so that a soft cloth and maybe some window cleaner would be enough to keep it looking clean. Again, as has been mentioned, hot water can soften or loosen the lacquer and damage it so a lacquered horn should only be washed (it's monthly bath) in lukewarm water with mild soap (like hand soap).
In the real world, however, the lacquer becomes worn, scratched, or cracked over time through normal use (and abuse) so that in some areas, the protection is gone and the tarnishing or even oxidation (shows as reddish-colored areas called 'red rot') can occur. Now the problem is, that if you attempt to use the aluminum foil bath to clean the tarnished areas, the hot water, and the electrolyte, in the bath will attack the remaining lacquer. Depending on the type of lacquer that was used, it may even turn a very dark color (I speak from experience) and come off. Now, this may be a benefit if your goal is to remove the remaining lacquer and make the horn all raw brass. However, the later types of lacquer (baked on epoxy) will not come off this way so stripper will still be required if you want it all off.
The point is, that if you have a horn with only certain areas missing the lacquer and you want to keep the rest lacquered, there is another approach. To understand this approach, keep in mind the following facts about polishing metal (which is what you are trying to do to the metal where the lacquer is missing):
(1) Tarnish is metal which has reacted with oxygen or sulphur. It is the metal's way of trying to protect itself by forming a barrier against further oxidation. So, when you REMOVE tarnish, you are removing metal. NOTE: If you use anything on tarnish which then turns black (the 'yellow cloth' for example) - that is your metal coming off the horn!
(2) Because tarnish is metal, it can be turned back into untarnished metal by reversing the chemical process which caused the tarnish. That is what the aluminum foil bath does - it turns the tarnish back to the original metal rather than removing it.
(3) Silver plating is VERY thin. It does not take long to completely remove it by repeated applications of polish which turn the cloth black - that is your silver! So, you should NOT polish silver to REMOVE the tarnish. I have not tried TARN-X so it is possible that it duplicates the process of the aluminum foil and restores the metal (if the cloth turns black - it is removing your silver plating).
(4) Brass is NOT thin. So, polishing the brass areas where the lacquer has worn off will not, in your lifetime, wear the brass away significantly. However, some brass alloys will vary the composition with the depth of the metal so it may be possible that the brass will change color if you continue to polish it vigorously and often - that is something you might want to watch for.
(5) While the brass is not thin and will not wear away quickly, it can be restored with the aluminum foil bath the same as sliver. If you have a raw brass horn where you polish the entire thing, it is possible that fine engraving can be worn away so in this case, using the aluminum foil bath is the preferable method.
(6) Back to the horn with lacquer partially worn away, you can use a very gentle polish (not BRASSO) and lightly rub it over the areas where the lacquer is worn. If the tarnish is very light, or if you are trying to remove smudges from handling, a high-quality automotive-type CLEANER/WAX (Mequiars makes one) may do the trick. This will also leave a protective wax coating on the brass to retard tarnish in those areas. If the tarnish is too heavy for the cleaner/wax, try a high quality, non-abrasive polish (Mothers Billet is one candidate) followed by the wax treatment. This will avoid further removing the lacquer surrounding the areas that you are cleaning. If, however, the oxidation is so bad that actual pitting of the metal has started, it may be necessary to use an abrasive (like Brasso) to clean up the pitted areas and then polish with the non-abrasive and the wax to restore the mirror finish to the brass.
(7) For those wondering about the aluminum foil, it is necessary that the metal of the horn be in contact with the aluminum foil. Wrapping the horn in foil simply speeds the process because there is more contact area involved. It is not necessary for every square inch to be covered and it does not need to be tightly wrapped. Any sort of contact will work. Also, the hotter the water, the faster it works. Boiling water will not melt solder joints. For those that cannot find Sodium Carbonate, regular laundry detergent or even pre-soak aids can be used. It is just an electrolyte to aid the chemical process (actually, you are creating a mini-battery in the water which is what helps the conversion of tarnish back into the original metal - silver or brass, whichever it is). It does not hurt to leave the horn in for a long time. Once the tarnish has been returned to the original metal, the reaction stops.
(8) I have tried this on horns which had nickel plated areas and it did not seem to affect the plating that I could detect. It should also be safe for any brass, copper, or silver horn, regardless of age. However, if you have some real exotic horn made from titanium or beryllium or the like, you may want to do further research. I'm not sure what it does to other metals.
I hope this clears up some of the obscure issues that some were wondering about.