I used to use one, so I was curious. It was hard to find good info by googling, but I eventually came across a presentation which claimed that the Selectric car

I used to use one, so I was curious. It was hard to find good info by googling, but I eventually came across a presentation which claimed that the Selectric carbon ribbon "consists of a carbon wax coating on a polyethylene base": https://www.scribd.com/presentation/419638824/typewriters https://www.scribd.com/presentation/419638824/typewriters (slide 21) Searching for those keywords came up with this, which uses the exact same wording: https://www.google.com/books/edition/Scientific_Examination_of_Questioned_Doc/nQlJw2_gE-cC?hl=en&gbpv=1&dq=selectric+ribbon+carbon+wax&pg=PA182&printsec=frontcover https://www.google.com/books/edition/Scientific_Examination_...