Few plants can be used to dye hair conveniently and safely. Henna, indigo and cassia powders are green powders, similar in appearance, because they are all dried powdered leaves, and all contain dye molecules that can safely and effectively dye hair. The green chlorophyll in the leaves hides the dye molecules. Cassia contains a yellow dye molecule. Henna contains a red orange dye molecule. A dark blue dye molecule can be derived from indigo. These dye molecules will stain keratin, the structural protein of hair.
The shape of a hair keratin molecule has sites for dye molecule binding, and these sites have specific shapes and conditions for binding. Henna has lawsone, a napthoquinone molecule, which binds very efficiently with keratin. Cassia has chrysophanic acid, an anthraquinone, which does not bind as efficiently as henna. Fermented indigo has a molecule, indigene, which can be broken down to an indoxyl that will bind to keratin, but this molecule is unstable.
One way or another, the pigment in the powdered cassia, henna, or indigo leaves has to migrate from the leaf to the hair, and then bind with the keratin in hair, rather like ‘tetriminos’ that you coax into place during a game of Tetris, or like a wet tea bag staining a white tablecloth.
Only a few pigments will bind with hair and dye it permanently. Beet juice won’t dye hair red; it will wash out. Blueberries won’t dye hair blue. Beet juice and blueberries will color ice cream, but their dye molecules aren’t the right shape for ‘keratin tetris.’ Madder can temporarily stain hair rich red, but fades after a few washings. Wool and silk dyers often use heat and mordants to make differently shaped molecules attach to hair. Heat expands the keratin molecules, so it’s easier to jam in a molecule that doesn’t quite fit. Mordants ‘rough up’ a molecule so it’s easier to jam in a dye molecule that doesn’t really fit. When these cassia, henna, and indigo molecules bind with keratin, they dye hair.
Read more: https://www.mehandi.com/v/vspfiles/downloadables/chapt5_plants-that-dye-hair.pdf
Links to specific topics in this chapter:
Three Plants Dye Hair: Henna, Indigo, and Cassia
Henna, Lawsonia Inermis
Indigo, Indigofera Tinctoria
Cassia Obovata
What is the difference between Cassia Obovata and Cassia Auriculata?
