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If your hair is very long or very thick, you probably run into trouble when it's time to curl or straighten it. Most heat-setting devices are designed for only a little bit of hair at a time and won't have much effect on thick locks. That doesn't mean you can't change your hair's texture, though. You just need a powerful tool and the right technique to get the curls, waves or flowing texture you really want.
Conventional cylinder-shaped curling irons can produce an uneven curl in thick hair, especially if the hair already has some curl. To define curls or waves when you have a lot of hair to work with, use a conical curling iron that tapers toward the tip. You'll get more delicate curls at the ends of your hair instead of loose ones that return to their original texture in a few minutes.
Iron size has a lot to do with the kind of curl you get. Tiny curling irons create a very tight curl, but often can't heat thick hair all the way through. Very large-barreled irons don't work well, either, producing far looser curls in thick hair than in medium or thin locks. Look for a relatively small curling iron with a big enough diameter to wrap your hair around without overlapping it.
Curling your hair at the lowest temperature possible reduces the risk of damage and dryness, but cool temperatures are less effective on a lot of hair. If your hair is very thick, you'll need a hotter iron to curl it effectively. Look for models that can heat to between 380 and 410 degrees Fahrenheit. High-end curling irons offer multiple heat settings, allowing you to experiment and find the one that curls your hair the best.
Thick hair can take a lot longer to curl than thinner hair, especially if you have a lot of length or the individual strands are very coarse. Section off your hair so that you can wrap it around the barrel of the curling iron without overlapping. Hold the hair on the iron for a little longer than the instructions suggest, so that the heat can penetrate all the way into the strands. Stop immediately if you smell burning or scorching, since this means you're hurting your hair. If you must use very high heat or a very long set time to curl your hair, consider spraying it with a heat protection product before you curl.