In 1875, a French inventor named Marcel Grateau invented a system to style hair using
heated rods. That process is still referred to by professionals as marcel waving, even
though the tools have evolved considerably in the time since. Today's curling irons are
electric, easy to use and readily available. The barrels of today's irons vary widely in
size and can range from the pencil thin - for very short hair or tight curls - to the
enormous - for long hair or cascading spirals. No matter what size curl you're after,
there's a curling iron for you.
 
Here are some things you should know to get the best use of your curling iron:
 
Rule Number One: For best results, start with clean, DRY hair. The hair should be
shampooed, conditioned with a moisturizing rinse-through conditioner, and treated with
a leave-in conditioner for protection against damage from thermal styling. The hair
must then be dried completely. If the hair is even a little damp, thermal styling can
result in severe damage, because the hot iron will effectively "cook" the hair, causing
it to swell, opening the cuticle, and forcing the protective moisture out of the cortex.
 
Rule Number Two: Make certain your iron isn't too hot. If you
aren't sure of the temperature of the iron you're going to use,
lightly moisten a piece of tissue paper and clamp it in the
curling iron. There will be steam, but if there is any smoking,
scorching or discoloration of the paper, you iron is TOO HOT.
Remember, it's better to be safe than sorry. Back in school I
watched a classmate use an untested marcel iron on her
manikin and saw the horror on her face as she wound the curl
around the iron, saw it begin to smoke and watched the curl
break off and fall to the floor.
 
Rule Number Three: If you want your curls to last, use a small amount of styling
product. By using a light mist of hairspray and allowing it to dry (or a small amount of
gel or mouse worked into the hair immediately prior to drying) on each slice of hair
being curled, you will get firmer curls and the curls will last longer than without any
product at all.
 
Rule Number Four: Work with manageable amounts of hair. Don't try to curl too much
hair at once. For a curling iron to work properly, you have to get even heat distribution,
and using too much hair at one time makes this difficult if not impossible in some
cases.
 
Rule Number Five: Allow your curls to cool before working with them further. Because
the heat used in thermal styling is what breaks the necessary side bonds allowing the
hair to be reshaped, you have to let the newly formed curls cool so that these side
bonds are reset in their new shape and the curls will hold.
 
Following these rules will assure you of getting the curls you want, and will help you to
avoid damaging your hair accidentally. The greatest benefit of using the curling iron is
that it allows you to target specific areas of the head for curls, or even specific parts of
the hair (for example, curling the ends of a blunt hairstyle so that the hair ends curl
under or flip outward). In addition, there are two types of curl usually associated with
curling iron styling. Here's what they're called and how to achieve them:
 
The Barrel Curl: This is the type of curl you would typically get from a roller set and is
good for adding body and fullness to shorter hairstyles. To achieve the barrel curl, you
divide the hair into sections a little shorter than the length of your curling iron's barrel,
and take slices approximately as thick as your curling iron is wide. Slide the hair slice
between the iron's clamp and barrel, gently slide the iron to the end of the hair and roll
the iron toward the scalp. As you near the scalp, you can use a comb to prevent
touching the head with the hot iron and burning yourself (or the person whose hair you
are curling).
Hold the iron in place long enough for the heat to penetrate fully then unwind the iron
in place to loosen the hair and allow you to slide the iron out of the curl from the side.
If you want to make sure the curl stays tight, you can use a curl clip or bob pin to hold
the curl in place while it cools. Once cooled, you can style the curled hair as you wish.
More about barrel Curls
 
The Spiral Curl: This type of curl is extremely popular in long hair styles, and is also
great for adding that something extra to special occasion styles. The spiral curl is
achieved by using smaller segments of hair and wrapping them at an angle along the
barrel of the iron from the ends to the scalp.
Again, hold the iron in place to allow the heat to penetrate fully, and unwind the iron in
place so that you can release the curl and slide the iron out from the end. Allow the curl
to cool and you can use your fingers or a comb to soften the curl without losing it. read more here