Lemon juice is acidic enough to remove the scales on the inside of a Crock-Pot, particularly if the scales are due to hard water. Take two tablespoons of real lemon juice and pour this in the bottom of the crock. Add a little hot water and scrub the surface gently with a non-abrasive scrubbing pad. Rinse the crock out and use a soft cloth to dry it. If some of the scaling is still present, repeat this process until it is removed.
Vinegar is another acidic natural cleaner that can be used to remove scales. If lemon juice has failed and the build-up is severe, vinegar will most likely work. Use 1/4-cup of apple cider vinegar and some hot water. Let this soak for a few minutes and then use a soft scouring pad to clean away the scales. Rinse out the crock and let it dry. The scales should be gone. If not, repeat and let the mixture soak a little longer.
Nonabrasive household cleaners can also be used on the inside of a crock pot. However, they should be rated for use with food appliances. A gentle dish soap will work on slight or newer scales. Use very hot water and a few squirts of this soap. Let it sit and then scrub the scales away. Other non abrasive cleaners include baking soda. Add a little vinegar for a chemical reaction to get rid of the scales.
Crock-Pots may look durable, but the interiors can be easily scratched. Never use a metallic based scrubbing pad on your crock. Soft scrubbers and brushes will work best, even if they require a little more effort. Never use a harsh abrasive cleaner -- this is particularly true if you are putting your crock in the dishwasher. Make sure you are using a phosphate free dishwashing soap that is gentle.