Cleaning the outside of a Meer?

Here's the pics. I cleaned the inside with a pipe tool. I left a thin layer of the cake on so I wouldn't scratch the inside of the pipe itself. I am wondering though, how does it hurt the pipe to scrape the inside if it's made of stone? Not questioning the validity, it's just not very intuitive.

Parts of the pipe are showing the yellowing from smoking but the darker spots look like it's more dirty to me. Thanks for the help guys!