The most common way to straighten curly hair without the use of a flat iron or other heat source is through the use of chemical relaxers and straighteners. This method should be used only by a trained cosmetologist because the chemicals can cause burns and severely damage hair.
There are multiple straightening techniques, all of which are more suitable for some types of hair than others. Thermal reconditioning, relaxers and keratin treatments are potential options for straightening curly or kinky hair. Keratin treatments last a few months, and then the treatments have to be redone. All methods have to be touched up regularly to maintain a straight style because new hair growth comes in wavy or curly. Hair that has been colored or otherwise chemically enhanced previously may not be strong enough to withstand the rigors of chemical straightening, so any and all past chemical treatments should be discussed with a hairdresser prior to straightening to ensure that the hair isn't permanently damaged.
Maintenance and continued care should also be discussed after straightening. While there are over-the-counter hair straightening kits available, these are often less effective, and could cause damage to the hair and scalp. Some chemical straightening methods are cost prohibitive, so careful consideration should be taken before proceeding.