I am installing a new ceramic handbasin, which came with a pop-up drain fitting. This part has three washers and a steel nut. One washer is soft (silicone?) and thin, one is soft and fat (with a kind of U-shaped cross-section), the third is thin and hard (nylon?). The soft washers are whitish, semi-translucent. Clearly the hard washer goes under the nut. It isn't clear how it should go together. I tried it first with the thin soft washer above (between the flange and the basin surface) and the fat one below. It leaked, and my first thought was the flange seal - I'm not happy about the fact that there is metal-sink contact. The fat washer is too fat to go there, since it raises the flange too high and will not allow the sink to drain. Perhaps I need to combine the thin washer with plumber's putty?
Belatedly I have realized that the bottom seal has two aspects. There is the seal of the washer against the basin, and also the seal of washer and nut with the threads. This basin has that internal overflow catch system, i.e. it is hollow, and the drain catches flow that went out the overflow vent hole. In addition, the contact plane of the flange with the basin isn't quite parallel to the lower mating surface, therefore the drain in not exactly perpendicular to the lower surface. If I'd thought about this I would have avoided this sort of basin, because of the complications it presents. So now I see that there is a complicated sealing issue, presumably requiring plenty of sealant of some kind. Is this a job for silicone?