obviously with washing powder. use only a little bit, a teaspoon or so. If that's not available, let them soak and then rinse in fairly hot water. This will get rid of everything but the very greasy...
NEVER use household cleaners like Dawn, Woolite or even plain water They will all cause staining over time. Use a plastic or wooden scraper, carefully remove as much lichen and other biological...
Because they are fire-baked at very high temperatures to make them usable at high temperatures in your kitchen. This makes them very hard, and because of that, brittle. They can stand up to a lot of...