Translation:The clothes were drying.
Is there a past passive form? I know about using the "bli" compound form, but is there a single form like the -s ending for present?
The s-passive is only used in infinitive, and the present, future and future past tenses. In the other tenses you have to "bli" or "være". There are exceptions to the rule, but it is seldom you will encounter it. You can construct the passive past for group 2-verbs (those ending with -te or -de) use the -s-form: betaltes, undersøktes, solgtes.
Tørke is related to the adjective Tørr (dry), it's root also gives us Norwegian Tørst and English Thirst from OE þyrst). It's also related to Latin 'Terra' (land). I can't find any cognates to Norwegian but I'd assume that it'd be something like *'thirch'.