I have made about 60 to 70 natural leather dyes, and here is some detailed information on what I have learned.
1. The best mordant for dyeing leather with a vegetable dye is potassium alum. Don't use too much, 5 grams dissolved in 100 ml of water is adequate. Apply only one or two coats of this mordant solution, and then allow to dry before dyeing. (The following vegetable dyes do not require a mordant: indigo, lac, walnut, woad.)
2. The following vegetable substrates are proven leather dyes, but generally they are not wash-fast or sun-fast:
Brazilwood (pernambuco) (red and purple)
Buckthorn berries (green, lavender, purple)
Lac insects (red)
Poke berries (red and purple)
Turmeric root (yellow)
2. You can either soak the leather in the dye, or paint the dye onto the leather.
Indigo - very powerful, so dilute and brush onto leather sparingly
Walnut - soak the leather for a day or two
Every other dye - soak the leather for 1 to 3 days, or brush 5 to 10 coats of dye onto leather. Allow each coat to dry before applying another coat of dye.
3. To prepare a vegetable dye usually requires the following process: (a) crush the vegetable substrate and soak for one to seven days in either water, vinegar, or potassium carbonate solution (depending on the color that you desire); (b) boil and simmer for an hour, or two, or three, and then soak for a few days to fully extract the colorant; (c) strain the dye to remove the vegetable dregs; (d) brush the liquid dye onto the leather.
4. In general, you want to concentrate the final vegetable dye by simmering. A good goal is a final dye volume of 1 milliliter for every gram of dry vegetable substrate with which you started. In other words, if you start with 400 grams of dry brazilwood shavings to make a dye, then add as much water as you need to boil the shavings, but simmer the final dye bath until it is down to about 400 ml.
5. Adding crushed gum Arabic powder to the dye thickens it and gives the dye a nice texture.
6. There are a few natural non-vegetable dyes that are excellent. Iron-tannin-acid reactions produce wonderful grey and black dyes that are wash-fast, sun-fast, and rub-fast. A mordant is not required.
7. You can make an iron-tannin-acid dye by dissolving steel wool in household vinegar for 1 to 2 months (with the cap off). aka "vinagroon." Soak the leather in the vinagroon overnight, then rinse well.
8. To get grey, you'll need to boil 1 part ferrous sulfate with 1 part copper sulfate in a vinegar and tannin dye bath. Boil and simmer for a few hours. Allow to cool and settle, then decant the liquid and discard the dregs on the bottom. Apply only one light coat of liquid dye. The color takes about 3 hours to fully develop, so be patient before applying any more dye.
9. Making and using natural leather dyes is a bit of an art, so don't be too disappointed with your first results. Practice, experiment, practice, experiment.