Deb, if you know that Vivaldi or Joanambrosio Dalza composed it or that Vicenzo Galilei (Galileo’s papa) is believed to have composed it, then it’s not folk.
“Plaisir d’Amour” is often sung by folk singers (including me), but it’s not a folk song (composed by Giovanni Martini aka Martini il Tedesco in a classical context).
Spirituals are folk songs even when sung by opera singers. They rarely sound good when sung by opera singers; opera singers have too much voice. (That’s my opinion; don’t get mad at me.)