Your clay pieces are fired at amazingly high temperatures. This ensures that any organic particles still present on your workpiece are completely burned away after firing. And that can come in handy!
Want to create a specific pattern with a colourful glaze? Then you can apply wax resist to the parts of your piece that should remain free of the colour you are currently applying. The wax then burns away during firing, and the result is a piece with nice clean lines. Easy peasy!