Oak galls are a natural source of tannin that can be used to colour natural materials. Galls form when a tiny gall wasp lays its egg in the leaves or twigs of an oak tree. Iron-gall dye first saw use in the 12th century and continued to be a source of dye and permanent ink for over 700 years, according to the Maryland State Archives. Black iron-gall dye can be made and used at home to dye leather or natural materials for crafts.
Place oak galls on top of the large, flat stone. Break them into small pieces with a hammer. The pieces should be small enough to grind in the mortar and pestle.
Place the pieces of oak gall into the mortar. Use the pestle to grind them into a powder. The tannin in the oak-gall powder will react with a mixture of iron and vinegar to produce black dye.
Pour the oak-gall powder into a bucket. Add 1,892ml. distilled water. This is the tannin mixture for the first stage of the dying process.
Place the leather thongs, shells and beads into the bucket containing the oak galls and water. Leave them to soak for 48 hours to let the gall mixture penetrate the material thoroughly.
Pour the vinegar into the second bucket. Add the nails and steel wool. This mixture will react with the tannin to turn the items black and act as a mordant to fix the colour in the items you are dying.
After 48 hours have passed, put on the rubber gloves and take the leather, shells and beads out of the tannin mixture. Let them drain for a couple of minutes. Place them into the bucket containing the vinegar and steel wool, and soak for another 48 hours.
Remove the items you are dying from the vinegar mixture, and allow them to dry. They should be dyed a deep, permanent black.
If you can't find oak galls to make your black dye, crushed acorns will work as a tannin source.
You should always wear rubber gloves when dying with oak galls, because the dye is caustic and can stain your hands black. Documents written using oak-gall inks are caustic enough to burn the paper.