Originally Posted by mike_d

The mixture makes an effective tooth polish/whitener.

The acid in the lemon makes a fizz at first -- don't be alarmed, it's a chemical acid-base reaction. Plain lemon juice can etch enamel

The 50% calcium carbonate added is less abrasive than baking soda and is used in wine-making to neutralize acid. I found it on Amazon: "North Mountain supply."
Just added a few drops of essential oil (cinnamon, peppermint and clove oil) as the mixture was getting a bit thick or dry to use easily. I still use a fluoride based anti-sensitivity toothpaste or tooth oil most of the time to brush with.