I always assumed the liquid coming out of the spit valve of my trumpet was spit. But today D18 Jr’s music teacher told him it ain’t spit, it’s condensation.
So is it spit? Or is it condensation? And if it’s a combination, which makes up the greater share?
Good question.
As a former brass instrument player (trumpet, French Horn, baritone, tuba), I always thought the “spit” that came out of those valves (or in the case of the FH, in the numerous pipes without valves) was more watery than spit should be.
I’m sure SVW (spit valve water) has a good amount of saliva in it. But as evidenced from winter breath, there’s a great deal more water contained in regular breath.
Probably some gas physics dictate that air moving under pressure through metal pipes naturally condenses the water vapor out of it.
In cold weather (like at late autumn football games), I don’t recall the air coming from the bells of our instruments having condensing water vapor clouds like our uninstrumented breath did. So I’d say that somehow the instrument is removing the water vapor from the air passing through it.
Condensation. Lord knows I don’t spit that much. Also, you get much more condensation playing in cold weather. There may be some spit that works its way down there, but if so it’s a very small percentage of the total volume released by opening the water valve.
Hint: Buzz your lips while holding your hand in front of your mouth. Not a whole lot, if any, spit hitting it, is there? (I better quit doing this; I’m at work now.)
I have not played trumpet for 40 years, but I’d say it’s 85% spit and 15% condensation. The amount of condensation could vary depending on the temperature of the instrument. Colder instrument, more condensation.
I am a woodwind player, but I’d say it’s more condensation than spit for two reasons:
As has been stated, instrumentalists (unless you’re talking elementary school students) have learned how to control their saliva. This holds true for brass and woodwinds (I know my clarinet doesn’t drip incessantly).
The nature of brass instruments, i.e., metal being a better heat conductor than wood or plastic, lends them to more condensation accumulation. The metal conducts heat away from breath, causing more condenation to precipitate out of the cooled air. For a wood instrument such as my clarinet, the air stays warm, and less condensation can form on the innards of the instrument. Also, brass instruments tend to be longer than woodwinds (add up all the coils), offering more space for the air to cool.
IMHO, anyway.
This link has a better explanation. Pretty much what Scott said in point #2. Pretty good, for a woodwind player. 
Thanks guys! Glad to see I’m not the only brasshole on the SDMB!
It may not really be spit, but it’ll still really bother your guitarist when he’s walking around in bare feet.
To support what’s already been said, when it’s really hot out, there’s little or no liquid to drain. However, when it’s really cold, I’ve sometimes been able to get fog to come out of the bell of my trombone instead of having it condense in the horn. I’m probably just full of hot air, though. Other than that, normally the colder it is, the more spit/water (whichever you prefer to call it).
Our vocalist would drool too, but I don’t think that was the same thing.