This is aimed at Chrissy and those others knowledgeable about inks, really. I remain woefully ignorant.

My latest vintage ink arrived in a large bottle, half full. It's Stephens Fixed Blue, as the title states. Not knowing what it is, other than something likely to be indelible. I loaded a cartridge pen and wrote a line or two. I was surprised - even disappointed - to see that it was purple, rather than blue. I did something else and returned to the page after a few moments and it had become blue, and much more intense than it had at first appeared. Someone suggested that as it "develops" in this way it may be an iron gall ink. What do you think?