Recently I was tutoring the daughter of a Thai friend who is in her final year at school.
The topic in her text-book was 'prefixes and suffixes' and the way they are used to form English words.
One of the suffixes was -en meaning 'made of'.
Very common examples are wooden, golden and leaden.
Oaken and earthen are slightly less common, but still well-known.
Then, right at the end of the list, was hempen, which COD labels archaic!
My degree is in English Language so phoneme is as common to me as a simple medical term would be to a doctor or a nurse.
I'm not surprised that it is unfamiliar to some solvers, however.