In Greece it was a solar symbol and the sacred bird of Apollo. It also has the role of the messenger of Gods and accomplishes the prophetic functions.
In Alchemy the raven is associated with the phase of putrefaction and matière au noir. They call it the Head of the raven. It is necessary to whiten it when washing seven times in the waters of Jordan. These are the imbibitions (the displacement of one fluid by another immiscible fluid), sublimations (the transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase), cohobations (the process of repeated distillation of the same matter, with the liquid drawn from it) or digestions of the matter. That’s why the raven is often represented on the Hermetic texts.
In recent times ravens gained a negative reputation because they were present in the battle fields eating the cadavers. Thus it’s symbolism started to be related to the fair of bad luck.