Those who want to do onion skins for 100% cotton fabric, here is a simple method:
1. Collect the skins.
2. Place into a large glass jar/container (mayonaisse) with water to cover them; cover.
3. Let it sit 24-48 hours (but not longer or it ferments - Pewwwww!!!) The resulting colored liquid is called the "liquor".
4. In a large kettle/pot boil several quarts of water.
5. Strain the "liquor" into the water. Stir with a wooden stick.
6. Wet the fabric with clear water; and fold gently so it will fit the large kettle/pot
7. Place the fabric into the boiling water and gently stir with a wooden stick.
8. Turn off the heat and stir occasionally to make sure the fabric is thoroughly saturated with the color-dye.
9. When cooled, carefully remove the fabric and hang it to dry or lay it out flat "out of the sun"!
10. When dry, use as needed.
There is no mordant, in this recipe, to change or brighten the natural dye. Refer to a book or the internet for choices.
This simple method can be done with a number of different natural materials for the dye.
You can also try 100% natural wool yarn, for spinning/hand crafts, etc. or 100% silk if you can afford it. Those are also natural fibers that will take the liquor (dye) well.
Good luck.
cpfrog (retired art educator)