On Sea leather would not rust, right, it would rot if you would not grease it very often. No real advantage. Weight? Hardened leather thorax armour is not light and not comfortable. It needs a lot of caretaking. It's not like a leather jacket from soft leather. (Or the ECW Buff coat) I have an old long leather coat that I do not wear. It's heavy. And that's the light soft leather.
Leather armour is good if you use scales. Like the Samurai or Horse fighting nations armour. Flexible armour for horsemen. A massive, inflexible, leather thorax has no advantage.
There are much more leather findings from Roman times than the public thinks. Not from fens or deserts only, Vindonissa and other sites produces an astonishing number of findings. But they are thought boring for wide public publication. A sword, helmet or skull photo sells the book, some leather fragments.... Who cares to read the JRS (Journal of Roman studies) or dry reports by excavotors? Only ivory tower nerds like me.
Imperial Roman naval forces 31BC- 500AD is written by whom? D'Amato. He is the modern author that repeats again and again the leather armour story. That he ommits in his 290 page book the name Lindenschmidt shows that he is a, I don't dare to write a fitting word.
He is a specialist for Byzanine studies. In Roman military regards he is an amateur with no high reputation among the ivory tower specialists. He wrote a lot and Osprey gave him a certain public reputation. In academic circles...... .
He states on p. 160 in his 2009 book that the archers are clad in long leather corselets. Bad news for the warlord sculptors, the foundry ones and the ones from XYZ . Not to forget the late Robinson and most of my fellows. And not to forget the gamers that field the Eastern archers.
His simple (and only) argument is: Smooth surface on sculpture is leather. And that the sculptors show exactly what was worn. Painted deatails, no.
You find leather armour mostly in societies that have limited metal industrial power or that field gigantic armies. Resources and costs, that's why it was used. The moment the Roman state started to equip the miles the equipment changed to 1st rate metal armour.


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