For the best coverage and permanent results, it is important to mix henna powder with a mild acid and allow the paste to sit for a period of time. This process is referred to as “dye-release.” Improper dye release can lead to weak stains that fade overtime, and undesired color results.
A Quick Chemistry Lesson
Henna leaves naturally contain lawsone, a red-orange dye. In powder form, henna will not stain keratin. Henna powder must be mixed with a liquid to allow hennocides, the lawsone precursors to be released from the plant material and migrate out into the paste. The precursors are converted into aglycones, through the replacement of the glycosyl group with a hydrogen atom. These hydrogen atoms function like the nub parts of puzzle pieces, so the dye can bind effectively to other molecules. Aglycones are intermediary molecules which will bind to other molecules to convert into a stable form. If the bond occurs with keratin, the result is a permanent stain on the outer layers of the hair.
Low pH (acidic) mixes are rich in hydrogens, which keep aglycones relatively stable until it is time to use the dye. This hydrogen-rich environment provided by the acid liquid allows for a longer and fuller dye-release as well as a more stable bond to keratin aglycones to bond to the keratin of the hair through a .