When you think the ferric acetate solution is ready, put on some nitrile gloves and pour a small amount into a ziplock bag. Submerge a piece of the discarded leather from the prior step into the solution. The leather should react almost immediately, turning black within a few seconds.
If it does not, either the solution hasn’t fully “fermented” or the leather has been treated. If you think the solution needs more time, wait a few more days. If you think the leather isn’t vegetable tanned, you will need to find another suitable piece. In my experience, most leather goods that are a light tan color will work, especially the cheaper ones because they most likely have not been sealed, oiled, etc.
Please note, because this solution is cheap & easy to make and used infrequently, I carefully discard the leftover down the sink with some baking soda when finished. If for some reason you want keep some solution around for another future use, I’d recommend straining it from the vinegar bottle into another suitable storage container utilizing a filter to remove all the small bits of metal. Make sure the container is clearly marked with a warning label and kept away those pesky and always curious corporeal lifeforms.
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