There's a lot of nuances to old ink.
For example, Confederate stamp cover dealer and forger John Fox was pretty good at his craft. He even engraved his own postmarks on metal. But even with all his knowledge as a stamp dealer, he didn't realize that 1860's postal ink was made from a mixture of lamp black (carbon soot) and linseed oil. Instead of mixing his own ink, he used some type of modern ink without oil, and as a result, the ink didn't spread evenly across the metal postmark, leaving a spotted, "mottled" look to the ink. That gave him away. Super small detail, right?
See report here. It's on the last page.