Pherc1667 was quite small and just so happened to have readable ink, so it was easier than I expect most others to be.

Most of the evidence so far points towards carbon based ink. I am not sure if any of the scrolls we have scanned show strong evidence of iron gall based ink. I know that there are different types and preparation methods for different carbon based inks, but I do not know if it is possible to determine which kind(s) were used solely from inspecting the xrays.

I am, though, not a papyrologist, so historical ink making, preparation, and usage are not my field.