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Henna bonds with natural keratin to strengthen the hair, a big-time boon for relaxed hair, which can feel brittle. Henna also makes your hair smooth and shiny, filling in rough edges on the hair follicle. Henna's safe for use on relaxed hair. Like any hair coloring, you've got to apply it properly to get the sleek and silky hair you crave.
Henna as Relaxer
Some curly girls find that henna takes their curl down a couple levels, mimicking a light relaxer or texturizer treatment. Women with naturally wavy hair or loose "S" curls may be more likely to experience these straightening effects. This is by no means universal, but it is something to consider. If you want to relax your hair, use a traditional product that is designed to do so. If you have relaxed hair that is already fairly straight, the henna can't make it more so.
Henna over Relaxer
You can apply henna over-relaxed hair, so long as you choose high-quality henna powder. Author Catherine Cartwright-Jones, who tested over 50 brands of henna for her book "Henna for Hair: How to Henna," terms body-art quality henna the best choice for use on over-relaxed hair. This type of henna also rinses out of braids. If you've got relaxed hair and braided extensions, you can still henna. It also goes over dyed hair.
Wash and towel-dry your hair before you apply henna. Add henna powder to your hair by scooping it out and coating your hair, or by putting the henna in squeeze bottles and squeezing it over strands. Aim for a thick layer of henna in the hair, since the color will take better. Also, coat the entire length of the hair and the scalp. Work across the whole head, section by section to get everything. Wear latex gloves, since henna can stain your skin.
Not all henna is created equal. Beware of so-called "black henna." Henna is naturally red, so something must be added to generate a black color. Many products use PPD or para-phenylenediamine, a chemical that causes skin blistering, hair loss and other problems. If you want black color, indigo or cassia plants can help you get there safely. Some people can experience henna allergies. Symptoms include itching, wheezing or tightness in the chest. Apply a small but of henna to your skin and let it sit for one hour. If you experience any of these symptoms, do not henna your hair.