Cold wave perms contain the chemical ammonium thioglycolate. True acid wave and acid-balanced wave perms contain glyceryl monothioglycolate. Exothermic wave perms contain ammonium thioglycolate as well as an oxidizer like hydrogen peroxide. Ammonia-free waves substitute ammonia for an alkanolamine such as aminomethylpropanol or monoethanolamine.
The cold wave method is used most commonly because it will make a firm, long-lasting curl. However, because of the ammonia, this method has a strong odor and can damage delicate hair. The true acid method is much gentler on hair but takes longer to set and often doesn't produce the firm curl seen in the cold wave method.
Acid-balanced waves are achieved at a faster rate than true acid waves and create a firmer, longer-lasting curl. Like the true acid method, the acid-balanced method is more gentle on hair than the cold wave process. The exothermic process uses exothermic chemical reactions to generate heat to speed up the curling process. While the exothermic process is faster, it can damage delicate hair and often produces a strong odor.
Ammonia-free waves generally process at the same speed as the cold and exothermic methods. However because they use ingredients that do not evaporate as quickly as ammonia, there is little smell associated with them. For people considering getting a perm, it is a good idea to talk to the hair stylist who will be better able to judge which method will work best with your hair type.