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You will need about 50-100g for short hair, 100g for medium hair, and 200g for long hair. Don’t worry about being precise; it’s a pretty forgiving process. When buying henna, there are a few things to keep in mind.  Some henna comes already mixed with additives. If you have purchased a henna that describes a particular color, you may not want to experiment with adding to the mix unless you are an experienced henna user. The additions described here are intended to be added to pure henna powder. Henna out of the box should be green to brown, and smell like dried plants or grass clippings.  Do not buy any henna that is purple or black, or that has a chemical smell. If you have severe allergies or sensitive skin, do a patch test before using. Apply a small bit of the henna mixture to your skin, wait a couple hours, and watch to see how your skin reacts. This is not a precise science. There are a huge number of variables, and you may not get exactly what you wanted the first time around. Results will vary, and your hair may dye unevenly. If you are a perfectionist about your hair, this process is probably not for you.  Pure henna can only achieve shades of red. If a product called "henna" purports to dye your hair black, it contains indigo. Some henna mixes can give you a blonde color, but it will always be a reddish blonde. Rather than obscuring your natural hair color, henna blends with it. This is important to remember when mixing a color. Aim for a color you want to combine with your natural one, not the color you want to achieve. Keep in mind that very light colored hair will probably need to be dyed multiple times to become dark. Because grey hair is translucent, it creates a cleaner canvas for henna. This means the mixing effect that happens with non-grey hair will not take place, and your color will be much closer to the color created by the dye. It also means it is easier to dye your hair unevenly, as hair with more dye on it will become noticeably darker. There is a vast array of ingredients you can combine with pure henna powder to create different effects. The list is longer than can be contained in one article, but here are a few to consider.  For a bright strawberry blonde, use lemon juice, vinegar, or red wine. For an intensified red, use brandy. For a less intense, browner red, use coffee or black tea. If you dislike the smell of henna, you can add good smelling things like essential oils, rosewater, or cloves. You do not need to add anything to change the color of pure henna. Water will work fine as well, although you should add a dash of lemon, orange, or grapefruit juice to oxidize the dye. If this is your first time using henna, you may want to see how it combines with your hair on its own, so that in the future you can decide what, if anything, you want to add. This is a pretty straightforward process. Pour the powder into a bowl. Gradually incorporate liquid, and stir.  Use a ceramic, plastic, glass, or stainless steel bowl. There is no way to tell precisely what amount of liquid you will need. Add it a little bit at a time, stirring until the mixture is the gloopy consistency of yogurt.  This will be a messy mixture, and it will stain any surface it gets on. It may be a good idea to wear gloves, and you should immediately wipe the mixture off of anything you accidentally smear it on. Cover it with plastic wrap and wait at least a couple hours, or overnight for best results. You will know it's ready when the henna darkens from green to dark brown. This means the dye has oxidized and is ready to be used.
Buy pure, natural henna. Know what you're getting into. Gather your supplies. Mix the henna. Let the mixture sit.