Warning! Science content! If you can’t cope with a chemistry lesson today skip the following paragraph and move right on to the pictures.
I’ve been learning a lot about the chemistry of henna in the last few years. One of the most confusing and notable things about henna is the apparently contradictory information on pH. Henna powder needs a mildly acidic solution (pH of about 2-4) in order for the dye to be “released” from the plant material. What that means is that the dye molecule (lawsone) is attached to a sugar molecule in the leaf. A process called hydrolysis breaks the sugar and lawsone apart so that its free to do its job. The next thing that happens is that the lawsone attaches its self to cellulose (in the case of paper and cotton) or keratin (in the case of skin, hair, and silk.) This happens by a reaction called Michael Addition which prefers alkylin (above a pH of 7) environments. So, lawsone requires a low pH to be made available for use, but a higher pH in oder to be used! What can we do? Here’s what I did:
I soaked one craft stick in ammonia (A) and the other was left alone as a control (C.) The pictures below show the result immediately after removing paste, several days later, and after exposure to heat several days after removing the paste.
We can see clearly that the ammonia soaked stick is darker, especially at first. We can also see that with the addition of heat and time, the stick that was not soaked in ammonia nearly “caught up” with the other. This brings me to the conclusion that the dye will attach to the wood even in a neutral envirnonment it just takes more time. You may also notice that the color is warmer in the stick that was not soaked. In the end, you may prefer not to use ammonia on wood if you like the warmer color. If your in a big hurry, on the other hand, you may want to give it a try!