The type of perm you get depends on your hair and the style you want. Hair that damages easily, or is already damaged, requires a gentler perm, while coarser hair can handle more aggressive treatment. The way your hair is wrapped prior to processing determines how your curls will look in the end.
An acid-balanced perm is also called warm waves. The solution for these treatments contains glyceryl monothioglycolate and has a pH balance of 4.5 to 6.5 or 7.0. The solution penetrates slowly into the hair and requires a source of heat to be successful. While an acid perm produces looser curls than an alkaline perm, it is much more gentle to the hair. An acid perm is the right choice for hair that is fine or very porous due to damage. It won't have much affect on coarser hair.
Alkaline perms, also called cool waves, have a pH balance of 8.2 to 9.6 and use ammonium thioglyucolate as an active ingredient in the solution. These perms work fast, usually within 20 minutes, and require no heat source. An alkaline perm results in tight curls. The solution, however, has a strong, unpleasant scent and may damage fine or already damaged hair. Alkaline perms are best on coarse or perm-resistant hair.
A spiral perm produces a mass of loose curls. Hair is wrapped around long spiral perm rods in a winding manner. After processing, the curls retain a spiraling pattern produced by the winding. Spiral perms need hair that is long enough to wind around the rods. They are ideal for thin or fine hair as the spirals add fullness and the appearance of thickness.
If you want to add volume to your hair, a stack perm may be for you. A stack perm is used on hair that is all one length. The hair is wrapped in curlers of differing sizes prior to processing. The top inches of hair are left uncurled, while the sides and bottom gain volume and movement from the curls.