Usually, bellows have at least 3 layers:
- silk - inside
- stiffeners - middle
- leather - outside
As benjiboy wrote, the leather is best to be treated with bees wax based cream.
I use Woly Sport Wax Active http://tinyurl.com/7kttlp7 on the bellows leather of my Zeisses and Voigtländers.., once every other year.