Today I learned

… yes they used cow gut/intestines 🐄🐄. 7 layers of cow gut where sewn onto a carrier layer of fabric to create an airtight balloon that could hold hydrogen lifting gas for some days 🎈. 50.000 cows where slaughtered for one gas-cell

Advantages over rubber of cow gut:

• rubber-cotton balloons got brittle with repeated uses with hydrogen filling. Cow gut is a flexible material that lasted longer, though expensive.
• rubber balloons can get statically charged. A small spark can flame all the hydrogen at once. Cattle gut does not charge as quickly as rubber.

Source:

• https://en.wikipedia.org/wiki/Goldbeater%27s_skin#Applications
• https://de.wikipedia.org/wiki/Goldschl%C3%A4gerhaut german article mentions some more numbers data.
• https://en.wikipedia.org/wiki/Airship#Envelope