(as) smooth as glass
1. Of water, exceptionally still and tranquil. Without even the slightest of breezes, the entire lake was smooth as glass.
2. Of some solid surface, exceptionally smooth to the touch. He sanded the wood down until it was as smooth as glass.
*smooth as glass
and *smooth as silkCliché smooth and shiny. (often used to describe calm bodies of water. *Also: as ~.) The bay is as smooth as glass, so we should have a pleasant boat trip. This custard is smooth as silk.