Three types of thread have been used to make merit badges. From 1911 until around 1939 all merit badges were made using silk embroidery thread. In 1939 two factors caused embroidery thread to change from silk to cotton. Conflict in the world and embroidery equipment improvements. In 1939 the BSA issued a directive to all suppliers to stop using silk do the conflict in Asia. In 1939 suppliers of badges to the BSA began to use cotton thread. Cotton was used until the around 1990 when embroiders switched to polyester because of superior performance on embroidery machines.
The images below show the same badge made with Cotton and with Polyester thread. Note the cotton thread is thicker in diameter and has a dull appearance. The polyester is thinner in diameter and has a sheen.