As mentioned in my previous post, given the lack of success with my previous blue recipe, I've moved on to another one to try. This one is a bit different in that it uses an acid (strong vinegar) instead of water.
Once I figured out the measurements, the recipe basically uses 1 oz of indigo and 450 ml of vinegar for each skin. I've cut down my amounts to 1/4 of a batch because I don't need that much for a test, and I'm starting to run low of the indigo after so many failed attempts.
The recipe says you can do it two ways, either just mix the ingredients together, or heat them (which supposedly gives you a brighter colour).
Here are my notes from the test run:
For the first try, I made the mix without heating it and let it sit for approximately 2 hours. The recipe doesn't say to let it sit, but it seems to me it makes sense to let it "brew" for a bit.
After letting it sit, I painted it on with a brush. I applied two coats, letting it dry in between. I then manipulated the leather to help it soften and work the dye into the grain, and applied a third coat.
The pigment seems to still be rubbing off the leather, although not as bad as with the other recipe. It does still wipe off with a bit of moisture, so I wouldn't consider it safe to use. The colour is barely distinguishable as blue, rather than black or grey (at least in indoor light).
For the second try, I followed the same steps as round one, but I let the dye sit for a full 24 hours to see if the vinegar would dissolve more of the sediment.
The pigment settled a lot more so the tinted liquid was much more transparent. In fact, it was barely dying the leather at all until I mixed it back up again. Colour is pretty similar to the first try, certainly not much blue-er under indoor light. I'll let it dry overnight and see if it rubs off like the last try.
Next rounds I'll try boiling the dye a bit to see if it would affect anything, since the book does say it will generate a brighter colour.
I may also try a bit of a change by going outside of the recipe to test my theory that the modern tanning process is different than in period, and so we need to do some extra steps to prep the leather in order to get period dyes to work.
I'm thinking of pre-treating the leather with a mordant like alum to see if it might help the dye stick..