How do you stop silicone drying in the tube?

Why the front sets and the rest doesn't

The data sheets say it in one line: sanitary silicone "cures at room temperature under the action of atmospheric moisture". It starts at the face the air touches and works inwards. On the sheets I read, a skin forms in 5 to 20 minutes, and the cure then runs at about 2 to 4 mm a day, measured at 23 C and 50% humidity.

Inside a sealed cartridge there's no air, so nothing happens. Cut the spout and the clock starts at that end only. That's why an old tube so often turns out to be a solid nozzle sitting on silicone that's still soft.

Do the sum on a standard nozzle, which is about 10.5 cm long. At the rate the data sheets quote, an open nozzle sets 2 to 4 mm overnight. Leave it a fortnight and that's 28 to 56 mm, a good part of the nozzle. It doesn't have to reach the cartridge to cost you, because a nozzle full of set silicone has to be dug out or thrown away.

The two ways to close an opened cartridge

The first is to let the nozzle plug itself, which is how sealant makers describe it. Screw the nozzle fully on, squeeze a little silicone out of the tip and leave it to cure. That blob sets into a plug which seals the cartridge behind it. Next time, pull the plug out. If it has set further down, unscrew the nozzle and clear it with a bit of wire or a small screwdriver.

It works and it costs nothing. The catch is that the nozzle has become the plug, so the longer the tube sits, the more of it you dig out. And the tip you cut for one job is the tip you're stuck with on the next.

The second is to take the nozzle off and cap the spout. Under the nozzle, the cartridge has its own threaded spout. Unscrew the used nozzle, wipe the spout, and screw a cap onto that thread. Nothing is left open to the air, so there's no plug to pull and no nozzle to clear. When you need it again, take the cap off and screw on a fresh nozzle, either a spare or one from a set of ready-cut screw-on nozzles.

Either way, GE Sealants' advice is to push the sealant up to the tip first so you're not storing air against it, and to keep the tube upside down. If silicone keeps creeping out after you've put the gun down, that's the gun rather than the tube, and the answer on using a silicone gun deals with it.

How long does silicone last once opened?

No sealant maker I've read puts a number on it. The closest is GE Sealants: a well-sealed tube "could last for months", while one that leaks air "may only last days". The silicone is the same in both. The difference is the closure.

The shelf life on the data sheet is a different figure, and it's easy to misread. It's 12 to 24 months for the silicones I read and up to 36 months for a decorators caulk. Every one of those figures is for an unopened, sealed pack, and it counts from the date the tube was made, not the day you bought it. Once you've cut the spout, it no longer applies.

Heat and damp shorten it. The sheets ask for dry storage out of direct sun, between 5 C and 25 or 30 C depending on the product.

Will caulk dry out after opening?

Yes, and the same closures work, because decorators caulk comes in the same cartridge with the same threaded spout. The difference is where you keep it. Its data sheet says protect it from frost, and at least one sanitary silicone sheet says the same, so a tube left in a cold shed or van over winter is a gamble. Where each one belongs on the wall is a separate question, and the caulk vs silicone guide answers it.

How to tell if an old tube is still good

Run a test bead on a bit of scrap before it goes anywhere near the bath, which is GE Sealants' advice too. If it won't skin over in the time the sheet gives, or the tube is past its date, don't put it round a bath. A bead that fails costs you the strip-out as well as the tube, and getting old silicone off is a job of its own. Once a good bead is down, how long silicone takes to dry tells you when the shower can go back on.

And if the bead you keep patching is going black rather than splitting, the tube isn't the problem. Black mould on silicone covers what's growing in it, and if the ceiling above is going the same way, the bathroom ceiling answer explains why a wet room grows mould at the top first.