Dampen your sink. Before you apply the baking soda, lightly dampen the basin of the ceramic sink. You can do this by allowing the sink to fill with water and then draining it, or using your hand to splash water around on the sides of the basin. Make sure that no standing water is left in the sink, as this will dissolve the baking soda and render it useless.
Also dampen the sponge that you plan to clean the sink with. You don’t want the sponge soggy, so wring it out well before you start cleaning. Sprinkle baking soda into the sink. You can start by adding about ½ cup (0.25 liter). Use your fingers or a spoon to sprinkle the baking soda in the basin of the sink, until the ceramic has been fully coated. Make sure to apply baking soda to any areas that are soiled or stained.
Scrub the baking soda with a damp sponge. Baking soda functions as a mild abrasive, and will be effective at cleaning mineral buildup, leftover toothpaste, and general dirt and grime off of the ceramic. Rinse the sponge as needed, and make sure to wring it out fully before you return to scrubbing the ceramic. Once you’ve cleaned the sink, rinse it well with water.
While you’re scrubbing, you should notice the baking soda clumping together into small lumps. This is a good sign: it means that the baking soda is not too wet, and that it’s effectively picking up dirt from your sink.
Whiten the sink with lemon slices. To naturally “bleach” your ceramic sink, start by cutting a lemon into several wedges. Take each wedge and rub it around the basin of the sink, making sure to squeeze out plenty of lemon juice on the ceramic. You can use the tough skin of the lemon to scrub at stains or other darkened areas.