Shaun Eli wrote:
Last week I used a tube that had been opened and then sealed with duct
tape. It was originally used a couple of years ago-- and it worked
just fine. I'd used only an inch or less of it, so there wasn't much
air in the tube. Presumably yours, unopened, should not have hardened.

There is no air in a cartridge of caulking whether it's used a little
or a lot. It's sealed. The only air that comes in contact with the
caulk is at the tip of the nozzle.

It's also pushing your luck to use silicone caulk that's that old.
Sometimes it'll gun right, but it won't set up - obviously you'll find
this out after the fact right about the time the air turns blue from
your swearing.

R