Final answer:
Hair that has been treated with a reducing agent is unfit for permanent waving and will not hold a curl.
Step-by-step explanation:
In order for hair to hold a curl, it needs to have intact disulfide bonds. Chemicals called reducing agents, such as ammonium thioglycolate, are used in the process of permanent waving to disrupt and reduce some of these disulfide cross-links, as shown in Figure 21.6.12. This allows the hair strands to be put on rollers or curlers and set in the desired shape.
However, if the hair has been treated with a reducing agent, such as in the process of chemical straightening or relaxing, the disulfide bonds in the hair have already been permanently disrupted and reformed to create a straightened effect. As a result, the hair treated with a reducing agent is unfit for permanent waving and will not hold a curl.