Updated: Oct 27, 2019
By watching the Native-Americans, the conquistadors discovered that avocado seeds produced a milky liquid that turned red when exposed to air. They then used this as a natural ink, and some documents written with it are preserved to this day. Many of these works reside in the archives of Popayán in Columbia.
The ink was also used to mark cotton and as a textile dye. In fact, in the dyeing process you will often hear people speak about "mordants". A mordant is a plant or metal based fixative that is used to extend the colour and wash fastness of natural dyes.
One of the magical things about avocado stones is that they contain tannin that acts as a mordant that binds wonderfully to cotton fibres. The depth of the shares you can achieve depend on how many pits you use and how long you leave the solution to steep.
If you want to try using this natural dye, it really is quite easy: Wash your avocado stones removing any fleshy bits that may have stuck to the pit. Add the whole stone to your pot filled with water; the more stones, the darker the colour. (Make sure your pot is big enough to hold the fabric and water as otherwise the textile won't be able to move freely, which is important to ensure an even dye.) Bring to a low boil, then reduce the heat to a simmer. Simmer until the avocado pits begin to turn the water to pink and then a deep maroon, which should take anywhere between 20-40 minutes for the colour to change. You can either place your material in the mix whilst it is simmering or later, once it's cooled. If you do the latter, probably best to leave it overnight. Basically, the longer you leave the fabric in the liquid, the brighter the colours.
Do note: the “avocado ink” sold today is typically named for its green color and has nothing to do with the red ink of the avocado seed.