They are a mixture of raw brass & silver plate up until now. My new lacquered Willson arrives soon. I prefer the look of lacquer, we'll have to see how it sounds.
You might want to search on this subject, as it has been discussed ad infinitum recently. You'll find a number of folks that have already chimed in on this.
I play on silver because I like the look and because I think a silver finish stays good looking longer (admittedly an unscientific sampling of a dozen horns, but they have all been decades old).
It is impossible to make things foolproof because fools are so ingenious.
I play on a silver euph. I don't think it sounds any different than a lacquered horn (some say there is a difference, but I don't think the human ear can hear it). A silver horn is easier to keep looking new.
Miraphone 5050 - Warburton BJ/RF mpc YEP-641S(recently sold), DE mpc (102 rim; I-cup; I-9 shank) Symphonic Band of the Palm Beaches: "Always play with a good tone, never louder than lovely, never softer than supported." - author unknown.
I really like the look of both the satin silver and the lacquered silver, but both my euphs are lacquered brass. I don't notice any playing differences; I think the difference between instruments is so great that the finish is undetectable.
MA, whose horn is also brass