I learned about this technique a couple years ago from a sandal-making video, where it was mentioned in passing. I didn't try it until last week when I was making a bag for Laura. Back when I made our sandals, I used standard leather dye. They looked great, but the bottoms of my feet were grayish-blue for weeks. Black dye is notorious for staining and running. Since I was sewing a black strap to a white canvas bag, I really didn't want the dye to run. So I figured it was time to try this new technique.
Ok, ready for the big tip?! To make the dye, stick some steel-wool in white vinegar.
Let it sit for a while. That's it.
I used mine after it sat for a couple hours. It continues to grow more dubious-looking as time goes by, but I promise, it's just fine. Here's my batch, it's about a week old.
Paint the mixture on the leather with a foam brush.
In a moment, the leather changes to gray-black.
For a darker color, add more coats. To me, the process is just as magical as watching indigo dye oxidize on fabric.
Why is this happening? According to this website, the tannins in the leather react with the ferric acetate that the vinegar and steel wool creates. This causes a chemical reaction that permanently changes the leather- there's no dye to rub off, stain or run. Also, this will only work with vegetable tanned leather, not the soft chrome tanned stuff.
Some websites suggest brushing on a baking soda and water solution to neutralize the vinegar, but I skipped that step, instead coating the leather in a few coats of neatsfoot oil.
I'm working on a bag for myself now, and this process is so fun, you can be sure it will have black straps!