Why my shop smells like pickles
Maelgwyn, my go-to leather guy, turned my attention to a dying method from "The Plictho of Gioanventura Rosetti" which was first published in 1548. It simply uses vinegar and iron filings. He and his lady have used it to good effect on leather and since I was strapping stuff for Martel anyway I decided to give it a shot. The vinegar was only $2.35 for a gallon and a few minutes poking around my shop with a magnet will win you a more iron filings than you'd care to ask for. Here's the initial leather and the vinegar: 
The buckles are for Martel's leg harness.
Iron filings...
And the concoction mixed together with the leather jammed inside like a specimen in the Mutter museum:
Now what I didn't anticipate is that this reaction creates a good amount of gas, and it's both fairly rapid and somewhat forceful. By the next morning it had made the plastic container bulge, and that sturdy looking green threaded cap leak out almost half of my dye concoction, which then leaked through the slats of my work bench. If I'm complaining about my router being rusty and smelling like pickles later, remind me that this is why, and to thank Maelgwyn. (Honestly it was a fun experiment anyway, and no one else had seen this phenomenon in their attempts.)
So the final product looks like this:
There are some parts that are really black, but most of the leather is a pleasant and pretty uniform charcoal gray despite my haphazard and broken attempt. A bonus is that it dyed all the way through the leather and on both sides. So it was plausibly period, cheap, and did a good job. What more can you ask for?
The buckles are for Martel's leg harness.Iron filings...
And the concoction mixed together with the leather jammed inside like a specimen in the Mutter museum:
Now what I didn't anticipate is that this reaction creates a good amount of gas, and it's both fairly rapid and somewhat forceful. By the next morning it had made the plastic container bulge, and that sturdy looking green threaded cap leak out almost half of my dye concoction, which then leaked through the slats of my work bench. If I'm complaining about my router being rusty and smelling like pickles later, remind me that this is why, and to thank Maelgwyn. (Honestly it was a fun experiment anyway, and no one else had seen this phenomenon in their attempts.)
So the final product looks like this:
There are some parts that are really black, but most of the leather is a pleasant and pretty uniform charcoal gray despite my haphazard and broken attempt. A bonus is that it dyed all the way through the leather and on both sides. So it was plausibly period, cheap, and did a good job. What more can you ask for?
Comments
You might also want to try it without the vinegar, merely iron filings left to rust, free iron ions will then be able to react with the tannin in the leather.
Certainly in the 15thc 'the black liquid from under grindstones' was used, ie iron water.
Also a solution of copperas, ferrous sulphate, either naturally occurring or made by pouring sulphuric acid over iron, copperas was the agent most commonly used for making ink and 'shoe black'.
You might want to gen up on the conservation issues related to iron tannin related dyes.
Many years' back I made a leather pencase and dyed it in the above way and eventually the surface layer simply degraded. Also the Burgundian Hours of the Dead, parchment dyed black in the same way and the leaves can only be held together by sheets of glass.