Who knew? Our fancy toilet has been leaking for a while, and at long last I decided to do something about it. Which involved disassembling a bunch of toilet-body parts (remember, fancy), turning off the water and extracting the flush mechanism.
At the bottom of which was a silicone gasket covered in little blisters. After suitable searches and an order for a replacement part, I stumbled across a review by someone who had had a similarly disfigured gasket (and blamed it for their leaks).
Down to my spouse’s sewing supplies for a pin, which I used to puncture every blister and squeeze out the water. More gymnastics getting the flush valve back in and reassembling everything (pro tip: cats should not be allowed to help). Then back to the search engines: why should an ostensibly waterproof gasket develop pockets full of water, even after 15 years of submersion?
Turns out that if there are water-soluble compounds inside an insoluble material that is even infinitesimally permeable, osmotic pressure over the years will force in enough water to dilute the soluble material and open up tiny bubbles. Chemistry!
So why are there water-soluble compounds inside my silicone gasket? Some kind of weird manufacturing flaw? Only sort of. The page I landed on about osmotic bubbles called out soluble metal salts in particular. Anyone remember what silicone uses to catalyze curing? Got in in one. Tin, Platinum, Bismuth… So unless the manufacturer is really careful (and thrifty) there will always be a little bit of excess soluble stuff inside the finished product.
There is one alternative explanation that scares me a bit, though: apparently thermal gradients can also drive water into semi-permeable materials. So maybe every time we might have sorta kinda let the water freeze inside the outside-wall supply lines we were creating a gradient that has now done the terrible damage of costing me $8 every 15 years for a new flapper gasket.