Face mask or face guard in late roman period

[Robert Vermaat]

But, as far as I recall, he was not describing their armour, so there was no occasion to mention the facemasks.

That's the chicken and the egg, because now you seem to be saying that face masks were present but Ammianus 'just so happened' not to be describing armour that day.

It is my understanding that the Roman clibanarii were based on the Persian model and the Persian's wore facemasks. The sources, though few in number, seem fairly explicit; you can't reject them just because you don't like them.

That's too simple. a) we don't know what Roman clibinarii were based on, but I think that yes, we can assume that Sassanid clibinarii were a large influence. b) Persian cavalry did have facemasks but here, too, we don't see many of those in art. Mail face coverings are more common.

The sources, though few in number, seem fairly explicit; you can't reject them just because you don't like them.

Is that what you think I'm doing? Because to me it seemd that I was discussing those sources and looking for a context for my interpretaion, as you do. ;-) Never did I reject them. Ammianus discusses cavalry with facemasks, that's a given, and even discusses them (once) within a Roman context. The Column of Arcadius, too, shows facemasks. neither however shows them unequivocally. There's room for discussion.