What are my kitchen sink's drain pipes made of, and how should I seal connections?
Those are brass drain pipes, which were once quite popular but now are more expensive than plastic. Most brass drain &amp; faucet assembies are chrome plated.

There should be rubber or plastic seals, not Teflon tape to create the seal (although I have seen Teflon cord wrapped around the pipe to make a seal), but this is not as good as it flows out of the nut.