In Handel’s “Messiah” can a solo for a bass be sung by an alto or vice-versa?
Answer:
Now on the subject of Handel and Messiah solo roles. It is true that Handel himself used the most capable available solo singers for his various performances and, yes, an alto was sometimes chosen to sing the “Refiner’s Fire” aria. However, a man (baritone) ALWAYS is designated to sing “The Trumpet Shall Sound”.
Whereas SOME bass solos are given to an alto, I’ve never known the reverse to be so. Therefore, for example, a bass is not assigned to sing “O Thou that Tellest Good Tidings to Zion” or “He Shall Feed His Flock” within the performance of Messiah unless he is a countertenor (a man singing in the alto range). [There does seem to be some long-standing preferences (or prejudices?) along these lines, doesn't there?!] I think the bottom-line issue is, “what is appropriate?” and “what has Handel himself allowed and designated?” You will find that the Novello scores print the multiple versions of the various arias allowing the best available soloist to be given the appropriate key.
Having told you what is allowable by “general practice” within the performance of the oratorio, I should also add that within the scope of a varied recital or worship service, even more choices exist and the idea of a man singing “He shall Feed His Flock Like a Shepherd” might be considered a possibility - depending on the circumstances. For example, I can imagine the scenario where the resources of a tiny church may not allow for the possibility of an alto soloist, but have a capable baritone. But now, I think I may be going beyond your question and making the waters on this subject murky.