The Chemistry of Henna
Maybe you’re a person who loves the deeper details of how things work, or maybe you’re an artist who cowers from memories of the trauma of high school chemistry class. Either way, understanding the chemistry of henna can help you make better paste, make use of better aftercare, and even help clients or customers troubleshoot a henna design that didn’t darken as it should have. This exploration of the chemistry of henna will be broken into manageable pieces. First we will discuss all the ingredients necessary to good henna paste and what they do, one at a time. Then we will focus on the dye molecule and what is happening to it through the process of dye release, application, and paste demise. Last we will look at how the dye binds to skin, oxidation, and the life span of the henna design. This really is fairly simple material when taken one step at a time.
Part One: Making Paste
Henna paste, at its bare minimum requires powdered henna leaves, and a liquid. To improve the texture and flexibility of paste we also add a form of sugar. To improve the potency of paste and give a darker color we also use a skin safe solvent, in the form of a monoterpene alcohol.
Henna is a bit like wine in that depending on where its grown, the weather, and other factors in processing and harvesting, each new batch will have subtle differences. Sometimes there is more or less dye present. Typically the leaves contain between 1-3% dye. Obviously higher dye content is appreciated among henna artists. Sometimes there will be an extra abundance of long chain sugars created in the plant which is what creates a “stringy” henna paste. Other times a harvest will have a particularly high concentration of calcium oxalate. This is naturally created in the leaves of henna and many other plants. It also happens to be the most common constituent of kidney stones. In our henna these crystals will make paste sound gritty when stirred in a class bowl and can sometimes cause clogging and unexpected texture changes. While these variables are the results of the unpredictability of weather and nature, there are others to watch for that are the result of manufacturing practices. Henna should be finely ground and well sifted. If not the texture of the paste will clog cones or bottles and be overall, very hard to work with. Henna should be reasonably fresh. Sealed in air and moisture tight packaging, away from heat and sunlight, henna will keep for a few years. It would be reasonable to be suspicious of henna older than that. Henna in store bought kits rarely contains harvest or use by dates making them unreliable. Henna stored in bulk bins will go stale and lose its ability to stain much quicker than properly stored henna. Henna intended for use on hair is often poorly sifted, and often has additives such as indigo or other herbs meant to condition hair. These only serve to dilute the dye and diminish color when used for body art. They should be avoided. Most importantly any henna that claims to give a black stain is certainly adulterated with undesirable chemicals, very likely the dangerous p-Phenylenediamine, or PPD. It is extremely important to avoid these products! Once you’ve chosen some high quality henna we are ready to take the next step, adding a liquid.
The liquid used to wet the henna and form a paste must be either neutral or mildly acidic. This will allow for dye release. We will discuss this in greater detail later. For now what we need to know is that the final mix must be at lest a little bit on the acidic side. Paste with a pH of 5.5 will release dye and stay stable for a 1-3 days after that at room temperature. It will keep longer if refrigerated, and months if frozen. The more acidic, the longer it takes for the plant matter to release dye so that its available to bind to skin, but the paste will also stay stable longer. The less acidic the liquid, the less standing time it needs to release the dye, but the paste will also have a shorter shelf life. There are many safe household liquids to choose from, each with their own pros and cons. Many people use lemon juice in their mix for a more stable paste. The concern with lemon juice is that some people are sensitive to citrus and may experience itching and redness from contact with the juice. Some people use water for the sake of simplicity, safety, and speed of preparation. Tap water in many areas may have an unusably high pH, or may contain minerals that are not helpful to the process. Distilled water is best for this purpose. Henna mixed with water may not have as long of a shelf life, especially when exposed to heat. Another option is tea or coffee. It is important to note that caffeine is absorbed through the skin! People not used to caffeine may even be able to notice the small amount that gets into their bloodstream when used in paste. Also, it is inadvisable to add these liquids to paste when still hot, as heat can rush the process and kill paste as quickly as it becomes ready to use. Another liquid that has been gaining popularity is apple juice. Apple juice is mildly acidic, and not a citrus juice giving us the best of both worlds. Remember though, when mixing with apple juice that it does contain quite a bit of sugar already, so adding less to the recipe may be a good idea. We will talk more about sugar in a bit.
When you hear the word “solvent” you probably tend to think of things like turpentine or gasoline, and you would be correct. In fact such caustic substances are often found in commercially sold premixes, and this is one of the biggest reasons to avoid them! A solvent in henna paste helps to dilute the dye, lawsone, in our paste. Lawsone is hydrophobic, which means that it doesn’t mix well with water. Like vegetable oil, for example, they repel each other. We want to use a solvent to help it dissolve better into the paste, disperse evenly, and make the dye molecules more available to bind to skin. Of course, unlike those premixes, we want to do it safely. Skin safe essential oils high in monoterpene alcohols do the job. The most common suitable oils for this job are cajeput, lavender, niaouli, tea tree, and eucalyptus globulus. You can learn much more about the specific chemical constituents we are looking for in essential oils, judging their quality, and how to safely experiment with using new and varied oils in your paste here. To start, using one of or a blend of these five at about 1-2 tablespoons per 100 grams of henna powder. Some people leave the paste to stand for some hours, then add oils, then leave it stand again. This is fine with a paste mixed with an acidic liquid, but with water mixes adding the essential oil right from the start appears to help increase the longevity and stability of the paste. On the other hand there is no clear advantage for letting paste stand before adding the essential oil.
Henna leaves do have some amount of sugar already. The more sugar the stringier the paste. In most climates though, it is necessary to add additional sugar to henna paste to help it stick better to skin, avoid cracking and crumbling, and to help it stay flexible when dry. Henna also absorbs more readily into skin while its moist. Adding sugar helps prevent it from drying too quickly. There are many forms of sugar we can add, and they break down into two categories. The first is monosaccharides and disaccharides, and include fructose, glucose, dextrose. These are the most common sugars to add to henna paste. They can be found in varying combinations in table sugar, sugar substitutes, honey, molasses, and the like. These are the sugars help keep paste from drying too quickly, prevent cracking and lifting, and help keep the dry paste flexible. Personally I find liquid forms of sugar such as honey, to be unpredictable in how they sometimes change the texture of paste significantly with freezing and thawing, but I also know many amazing henna artists who use them to great success. Choosing sugar is a matter of trial and error, and what works best for you personally. The second type of sugar is polysaccharides. These are very long chains of sugars and they can give creamy paste that egg white-like texture in naturally stringy paste. They are, however, pretty hard to use. The easiest way to add them is in the form of pectin or xanthan gum. I find it best to wet these two before adding to paste. Additionally one can boil okra or chia seeds and strain the gooey runoff and add that to paste. These methods are generally seen as way to much effort for the result. If you prefer a stringy paste, you are far better off buying a naturally stringy henna.
Having chosen your sugar, the next step is how to add it and how much to add. Like in baking, sugar should be considered a “wet ingredient” even when you’re using dry forms such as table sugar. This is because sugar draws in moisture out of the air which will thin your paste just like adding more liquid! This also means that depending on the humidity in the environment where you will be using the henna, you may need to add more or less sugar. Dry climates will require far more sugar than humid climates. If you live in an area where the humidity varies by the day or hour, you may be better off using a standard measurement in your recipe. I find that two tablespoons per 100 grams of henna powder is usually just about right if you add it to very thick paste, then let it stand for 10-20 minutes to absorb moisture from the air. You will notice that the paste is wetter than it was. If this does not bring it to your normal working consistency, then you can add more liquid until its right. DO NOT add sugar to paste that is already thin enough to use or the sugar will make it thinner yet, and you can wind up with a sloppy mess. Read more about my method of adding exactly enough sugar for the current weather here.
Once you understand all of these key ingredients and what they do for your paste, you are much more prepared to make changes to your recipe and predict the results. You may find that changing the texture of your paste improves your work greatly! Good solid stain is of course key to successful henna. Now that you know how to mix good, dark staining paste, lets find out how it works.
Part Two: Dying Skin
This is where things start to get a little more complex, but if you bear with it I’m sure you’ll find it worth your while.
The first part of this process is dye release. It takes place in the paste when we mix it. Adding liquid to powdered henna leaf starts to break down the the tiny bits of plant matter to let the dye molecule be free. Specifically the dye is attached to a sugar molecule called a glycoside. There are two different processes that can sever that link, both of which are probably happening in paste during its standing time. One is enzymatic hydrolysis which is a quick process. Enzymes naturally occurring in the plant can begin to break down the glycosides. This is why crushing fresh henna leaves in your fingers can produce a stain immediately. Acidic hydrolysis is slower and more productive in dried plant material. This process CANNOT take place in an base. Henna leaves are mildly acidic on their own, so using water as your liquid still produces an adequately acidic environment. As the dye is released in this way, it will then begin to demise, which is to say it loses its dye potential.
We’ve talked about how acids slow the process, but it may also be helpful to point out that as in any chemical reaction heat will speed it up. This is why many people apply heat to their paste while standing. I am personally of the opinion that except for in an emergency it is better to let the process take its course at room temperature, only because its too easy to make a mistake. If you let it get too hot you will “cook” you paste and it will speed right past dye release and on to demise before you even have a chance to use it, just like is possible when adding a hot liquid to your henna powder.
Meet the Molecule
This is a good time to get a look at the lawsone molecule. This is the heart of the chemistry of henna. The lawsone molecule, once released from the sugar portion of the glycoside, can then be observed in three forms as seen here. The first stage is the reduced form of lawsone, labeled THN. It is clear, and you will notice that it has some hydrogen atoms in its structure that are not in the next form. That one is pale yellow, and is the useful form in which it is able to attach to skin. The third form is the oxidized form and is has the well known red/brown color.
In this third, oxidized form, the lawsone molecule can no longer bind to keratin in skin. If you’ve ever seen that bit of brown goo that separates from your paste and forms a layer on the surface, that is oxidized lawsone. You may also have noticed that if you can separate that from the rest of the paste and apply it to skin, it does precisely nothing! You also may notice that water mixes look a little more brown. That is because more of the dye molecules are advancing in the process and have reached the yellow or red/brown state. Acidic mixes help slow oxidation of the lawsone molecule. That is to say that it preserves those hydrogen atoms at the edges. This both slows dye release, and slows demise of the paste. This slowing down of the process helps us hit a moment that has the most possible available dye that has been released but has not oxidized.
You can observe the lack of oxidation by noting that acidic mixes stay greener much longer. The trick is to get henna paste into contact with skin while in the middle, pale yellow stage which is when it can best bind to keratin in the skin. This happens after some exposure to the paste and we see the results in the yellow/orange stain when the paste is first removed from skin! Interestingly this bond is permanent, even though the stain is not. We’ll learn why in the third section.
Part Three: Oxidation
We have already discussed oxidation in the previous section, but until now we wanted to avoid it as much as possible. Now that the lawsone is bound to the keratin in the skin we want oxidation to happen. Remember oxidation is the process by which that last hydrogen atom pops off and changes the color of the dye making it the deep dark color we love. Exposure to air enables this process, which is the very same process by which an apple browns or a car rusts! So, when the paste is removed, it is exposed to air and begins to darken as the lawsone molecule changes shape one more time, losing one more hydrogen atom, this time becoming the third form pictured, with the red/brown color. You can learn more about oxidation here.
You may be wondering now about the use of after care balms. Don’t they prevent exposure to air? Don’t they often contain oils with antioxidants? Why yes! And for this reason I tend not to recommend using them until the stain has matured except maybe a light application while bathing. Oxidation is more complex than simple exposure to air though. Other things in the environment will encourage it, and an application of antioxidant oils won’t prevent it, but in cases where quick development of stain is key, such as for brides, I’d hesitate to over emphasize the use of balms in case it would slow the color change.
Part Four: Demise of the Stain
So now we have finally, fully covered the creation of a mature henna stain, we can tackle the rest of its life span quite quickly. As I mentioned above, the lawsone binds to keratin permanently. This is why henna stain on finger nails has to grow out rather than fading. On skin, you may remember from your middle school science class that the surface of your skin is actually dead cells, and as they exfoliate, they are renewed by new fresh cells from beneath. Depending on the individual and the thickness of the skin this cycle takes roughly two weeks. The cells with lawsone permanently attached to them are not permanent themselves and eventually exfoliate away. This is why the average life span of a henna design is about two weeks. It will last longer on palms where the skin is thicker and the process takes longer. The opposite is true on thin skin such as on the face. At this point in the process, balms are much more helpful as they can protect the design from soaps and other chemicals that might fade the design, and slow exfoliation as well.
Final Thoughts on the Chemistry of Henna
Many people wonder why the color is darker on hands and feet than arms and legs, and darker on arms and legs then on the face and tours. The reason has again, like with longevity of the stain, has to do with the thickness of the skin. Lawsone is a translucent dye, and skin is somewhat translucent as well. On places where skin is thicker you are seeing more layers of dye giving a deeper color. Other people may also worry that henna wont show on people with darker skin tone. It certainly does! Again, because the dye is translucent, it adds to the pigment in skin. However dark the natural skin tone, henna will make it darker. Often dark skinned people get rich deep shades of mahogany and even near black on palms. Henna looks great on all kinds of people!
I hope this article has been helpful in understanding the chemistry of henna!
Hydrogen Bonding Motif in 2-hydroxy-l ,4-naphtho quinine Dhumal, Todkary, Rane, and Gejji Colour Chemistry , R.M. Christie
Are Tyrosinase and Thiol Groups Present in Skin Epithelium? Burton and Williams,
The Chemistry of Natual Dves , Epp
A quantative study of dyeing with lawsone, Armo, James, and Turner
The Absorption of lawsone and henna by bleached wool felt, James, Spanoudi, and Turner
The Illustrated Encvlopedia of Essential Oils. Julia Lawless
There were other resources I used, such as the Merk Index and some organic chem texts, but only to help me deal with chemistry. Everything I took from them would be considered common knowledge.