What is that black AC sealant material?

67Dart273 said
We used to use a cork impregnated roll/ tape sealer in HVAC/R made FOR refer lines. I don't remember off hand the brand or name.

Here is one type

Cork Tape - VAPCO Company - Innovating HVACR for more than 65 years

That's the stuff I remember my dad using many years ago. He was a refrigeration mechanic.