Trying to identify the ink binder...

[Bernd]

If we assume the VM came from an amateurish and not exactly wealthy background, it's not far fetched that the creator(s) chose DIY approaches over commercial ingredients like Gum Arabic. Cherry or plum gum would fit into this picture, as well as the overall unusual ink composition.

I've never worked with FTIR so I don't know how age and reaction with other ink components modifies the spectrum compared to a pure and fresh binding agent. But this should probably be taken into account.