They use to use some type of sealer on the cork, otherwise the cork would just soak up the gas and sink in the bowl eventually after time.
I cannot remember the name of a product they used but If memory serves me right it was an alcohol based product mixed with Shellac that back in the day was the go to thing to seal stuff, Shellacs are still used today most people associate it with to seal fire damaged house interiors or primer coating something like brand new right out of the factory galvanized steal, It's also used as a primer coat for wall's in houses or apartments when the owner smoked cigarettes for ~10~20 years, Its also good for sealing magic marker's on walls.
If I wanted to use shellack to seal a cork float today I would thin the piss out of it (literally thin as piss) and soak it for a month or two and then let it dry for a month or two.
Source:
Me I have been a painter for almost forty years done a wack of sh*t in my trade and made good money doin it.
