The sculpture is a metaphor for the gateway to purgatory in Dante's The Divine Comedy. The spirit of the angel is represented by a bodiless spiral with three hollow-eyed faces in one head; the open mouth is speaking. The spiral movement symbolises the workings of the intricate lock on the gates. The suspended keys and gate-hinges for the angel's wings reflect Dante?s humour: Quantunque l'una d'este chiavi falla, che non si volga dritta per la toppa, diss'elli a noi, Non s'apre questa calla (Purg. IX, 121?123).
- Susan Dorothea White