What significant product was invented in 1938, serving as a precursor to the modern perm?

The significant product invented in 1938 that served as a precursor to the modern perm is the cold wave. This innovation introduced a method for creating curls in hair without the application of heat, which marked a shift in perming techniques. The cold wave process allowed for the use of chemical solutions that could alter the hair's structure to create waves and curls at lower temperatures. This was revolutionary at the time and paved the way for the more advanced perming techniques we have today.

Cold waves provided a less damaging alternative to hot perms and became widely popular due to their effectiveness and the versatility they offered. This method utilized ammonium thioglycolate as a reducing agent, which is still a common ingredient in perming products today.

The other options, while related to hair care and styling, do not represent the significant innovation that the cold wave did in the context of perming hair.