The Origins of Henna
Before I delve into mechanics of henna, I think it’s important to know a little bit about its rich history. The henna bush is indigenous to North Africa, the Middle East, south Asia, and northern Australia. As far back as the times of ancient Egypt, people have used henna to dye hair, skin and fingernails. They would harvest the henna leaves at the end of the summer season just before the rainy season began because the leaves had the highest amount of dye content during that time period. The leaves would be dried, milled, and sifted; and then later sold as a fine powder for dying.
What is Henna Exactly?
The active ingredient that enables henna to color hair is lawsone- which is also known as 2-Hydroxy-1,4 Napthaquinone (Figure 1). Lawsone is a weak organic acid that has a typical concentration of 1.3-1.5% in henna leaves that are harvested at the height of the summer season, as mentioned above. In order for lawsone to be effective, it must be preserved.
Figure 1. Lawsone
How is Lawsone Preserved During the Manufacturing Process?
Lawsone is unique because of its structure. It’s an extremely unstable negatively charged structure due to the presence of 2 carboxyl groups (C=O) and one acid group (OH). It will ionize and then react (decompose) quickly, which makes it ineffective for hair coloring; therefore it needs to be preserved. In today’s manufacturing practices, lawsone is preserved with citric acid during the milling process. However, during ancient times, lemon juice or tea was added to the henna powder to create an acidic paste of permanent hair colorant. The over abundance of protons from the citric acid make it less likely that the acid group or carboxyl groups on the lawsone molecule will ionize and decompose. Remember, it’s the release of lawsone in the henna leaves that gives henna its pigment.
Tip: Make sure the Henna product you purchase has citric acid on the INCI ingredients listing. If the lawsone has decomposed, it will not color the hair.
The Color of Henna
Lawsone’s color is red to orange only. So when you see henna in colors other than reddish orange, other pigments have been added. The important question to ask is, “What are those pigments?” Are they natural pigments such as indigo, turmeric, catechu, amal, vashma or black walnut shells? If so, you should have a basically safe colorant for your hair. However, if you see colors listed like the ones below, the odds are that the manufacturer has added a synthetic dye, which studies suggest are toxic.
Toxicity of Henna “Knock Offs”
In some cases hair colorant manufacturers will market their henna based products as all natural or organic but, in reality they are using metallic salts (lead acetate/copper acetate/silver nitrate), paraphenylediaamine, or p-aminophenol with henna to achieve mahogany, golden brown and black hair colors. There is scientific data, which links the use of metallic salts in hair dye to heavy metal poisoning. It is equally important to note that studies have linked paraphenylediaamine and p-aminophenol (PPD) to severe dermatitis/allergic reactions as well as hair damage and/or hair loss.
If you currently have a henna colorant in your hair and are uncertain about the presence of metallic salts in your “natural” henna product, you can do this simple test. Mix 30 grams or 1 oz of hydrogen peroxide (20%) with 1 cc or 20 drops of ammonia (28%). Add a few strands of your henna treated hair to the mixture and look for the following results:
- If the hair color changes immediately, the henna colorant most likely contains lead acetate.
- If the hair boils and gives off a terrible smell, the henna colorant most likely contains copper acetate
- If you see a greenish precipitation and no change in the hair color, the henna colorant most likely contains silver nitrate.
Now that you fully understand henna in terms of its origin, what it is, how it’s manufactured and where it gets its pigment from, let’s take a look what you should do once you’ve purchased your henna product and are ready to use it. Not to beat a dead horse, but I must state it again, you have to preserve your henna product before you can use it.
Acidic Hydrolysis during Paste Making
Once you are ready to use the henna, you will need to make a paste out of the powder. Everyone talks about using lemon juice and black tea as a good medium for making a paste. I recommend making your medium out of dried sour limes. Dried sour limes have the highest amount of citric acid as compared to lemons, oranges or tea. Thus sour limes are more effective at preserving (also known as acidic hydrolysis) the lawsone. Once you make the paste, make sure the pH is around 4.5 to 5.5. Cover the paste with saran wrap and push out as much air as possible. Remember the whole point of adding citric acid to the lawsone is to preserve it. Also, you don’t want the lawsone to oxidize in the presence of air, so force out as much air as possible and wait 6-12 hrs. The waiting period is important to allow the acid to hydrolyze with the lawsone.
Tip: Use a vacuum storage pouch to store your henna paste. It is more effective than saran wrap and ensures that all the air has been removed. It will reduce the possibility of oxidation.
How Does Lawsone Color Hair?
The whole point of adding citric acid to the henna powder manufacturing process and to the henna paste making process is to preserve lawsone for one specific reaction. You want to have as much lawsone available (1.5%) to react with the keratin protein in your hair (Figure 2).
Figure 2. Typical Keratin Structure
The actual reaction mechanism is as follows:
The carboxyl group from lawsone, which is negatively charged, reacts with the amide group from keratin’s peptide bond, which is positive. This reaction attaches the lawsone molecule to the keratin protein in your hair, and thus, gives your hair a permanent color. See Figure 3 and Figure 4.
Figure 3. Lawsone Keratin Protein Reaction
C9H502 – C =O + H2N – Keratin C9H502′ – C = N – Keratin + H20
(Lawsone) (Free NH2 in Keratin) (Schiff’s base)
Figure 4: Typical Peptide Bond
As with every hair colorant, their are pros and cons to using henna, but I think the positives far outweigh the negatives.
Pros and Cons of Using Henna (Lawsone)
- Permanent color system due to the nature of lawsone’s ability to bond with keratin
- Safe product for cancer survivors, pregnant women, and people allergic to synthetic dyes (PPD)
- Natural product as long as it is NOT mixed with metallic salts
- Less damaging to hair because unlike synthetic dyes, you do not have to use ammonia (high pH) to open the cuticle.
- Permanent color system as opposed to semi permanent
- Vegetable dyes in general have limited colors
- Extra effort in converting powder to paste
- Poor solubility of lawsone as compared to synthetic dyes. It will take longer due to the size of lawsone and other vegetable as compared to the synthetic dyes
- Lawsone’s color is different on different hair colors
- Hard to determine if products are truly natural (Use of metallic salts are not always documented in the INCI listing)
Henna Tutorial-The Henna Page
(1) B. I. H. AMRO,* K. C. JAMES, and T. D. TURNER, A quantitative study of dyeing with lawsone, J. Soc. Cosmet. Chem., 45, 159-165 (May/June 1994)
(2) Clarence Robbins(Colgate Palmolive Technology Center Alumni), Chemical and Physical Behavior of Human Hair, 4th edition, Chap 6, Dyeing Human Hair Colorant, Vegetable Dyes, pg 339 – 359.