Let Hair Cool Down
A: I’m going to presume that you are referring to the “cooling stage” after the hair is dried under a hooded dryer, or the act of allowing the hair to cool after being treated with a thermal tool - such as a curling iron, flat iron, or hot rollers - before being brushed or combed through.
The Purpose of the Cooling Stage
The reason for a cooling step in these instances is to ensure the hair is set into its new configuration. For that to happen, it must return to its normal temperature.Just as heat softens and reshapes the hair temporarily, cooling allows the hair to stabilize in its new form. This is why professional stylists often leave curls untouched for a few minutes after removing them from the iron or rollers, even if the temptation is to brush them out right away.
The Hair Science
Styling hair involves altering the side bonds within keratin proteins, which dictate the shape hair falls into. Hair contains strong chemical side bonds (disulfide bonds) and weaker physical side bonds (hydrogen bonds and salt bonds). Thermal styling focuses on these physical bonds (specifically hydrogen bonds), which break temporarily when exposed to heat and moisture.When heat is applied, hair passes what scientists call its glass transition temperature. In this state, the inner structure of the hair softens from a rigid state into a pliable, rubbery one. Think of it like warm wax: while warm, it remains flexible and easy to reshape. Once it cools below that transition temperature, it hardens and retains the new form you created.
How Curls Lock Into Place
This is why curling irons can curl hair, and why wet sets work as well. The hair is wrapped around a heated appliance (or wet and wrapped on a roller), held for a few seconds, and then allowed to cool. As it cools, the hydrogen bonds re-form in their new, curved positions.While styling products provide structural support, it is the cooling process itself that locks those bonds back into place.
If you attempt to finish styling - meaning combing through and manipulating the hair - before it cools down, you will pull the curl out, leaving it limp or straight. Likewise, if you are using heat to straighten naturally curly hair, manipulating warm hair encourages it to revert to its natural wave.
Professional Techniques
This principle is why experienced stylists often clip freshly curled sections into place until they cool completely. Allowing each section to cool undisturbed gives the hair time to memorize its new shape, resulting in stronger, longer-lasting hold.The same rule applies to blow-drying with a round brush. Many professional dryers include a cool-shot button specifically because directing cool air onto a finished section sets the hair before it is released from the brush.
Patience during the cooling stage is one of the simplest ways to improve the results of any hairstyle. Waiting just a few extra minutes before brushing out curls, running your fingers through your hair, or applying finishing touches can make the difference between a style that falls flat within an hour and one that maintains its shape all day.
©Hairfinder.com
See also:
Can I style newly-permed hair?
Is hair easier to style washed or unwashed?
What are the physical effects of blow drying on the hair structure?