Re: Nasty Film in Sink Drain
It's mostly soap scum, which is a byproduct of a rather complicated chemical process when soap is used in acidic or mineral-laden water. Insoluble compounds are produced, which then bind to the trap as the water sits. The blackness is from mold growing in the soap scum (yuck!). Many bar soaps contain talc, a mineral, so, therefore, they form scum. The higher the talc content (and possibly the lower the glycerin content), the more likely you are to see scum buildup.
Note that shaving cream may also contain soap. I don't know how the composition compares to bar soap.
About.com: How Soap Cleans
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.