As I learned it back in introductory Greek, there's significant debate in the classics community about whether Classical Greek Ζ was pronounced
/zz/, or something else.
What evidence is there to favor one over another?
Latin Language Stack Exchange is a question and answer site for linguists, teachers, and students wanting to discuss the finer points of the Latin language. It only takes a minute to sign up.Sign up to join this community
W. Sidney Allen's famous Vox Graeca, which is well worth the time of anyone with a more-than-casual interest in Greek pronunciation, has three-and-a-half pages' worth of things to say about the subject, which I will attempt to summarise.
It's uncontroversial that from quite early in the Archaic period through the late 4th century BCE, ζ represented [zd] in every major dialect. Evidence for this:
However, before it was [zd], it was likely [dz], because:
The metathesis of [dz] into [zd] isn't rare and would be particularly natural in Greek, which didn't have any other affricates and also lacked an independent /z/ phoneme, but did already have [z] as a regular allophone of /s/ before voiced stops, as e.g. in Λέσβος [lézbos].
Later in the 4th century, ζ apparently began to represent [z], as evidenced by its being used to represent Persian z and occasional confusions between ζ and σ in inscriptions. When Aristotle said that some people would analyse ζ as σ + δ but others consider it a separate sound that does not comprise already recognised elements, he may have been referring to the start of this process. It's usually held [z] would more naturally develop out of [dz] than out of [zd], so it's been suggested this development is not a normal phonetic development but a dialectical replacement from the Koine (just as σσ replaced ττ). After short vowels at least the original quantitative pattern is likely to have been preserved by gemination, i.e. [zz]; this is also indicated by its representation as ss in the early Latin borrowing massa < Greek μάζα.