On Saturday, January 2, 2016 at 9:54:38 PM UTC-6, wrote:
I noticed that over time most tank toilet flapper valves
accumulate a layer of a very fine black substance on the
side facing down(toward the bowl mating). Once a year
I flush the tank to empty it, while the other hand holds up
the flap. I then quickly grab a rag and wipe the black
film off the flapper and the mating surface(before the tank
fills appreciably) and I'm good for another 365 days.


Question: What IS the black substance?

After changing a large number of these (a facility with 56 flush toilets) my thoughts would be the water (and its chemical composition) are breaking-down the material in the flapper. The red Korky last longer than the black types.