Wonder if alka seltzer tablets would work?

You might want to also google cream of tartar. It works to shine metal pots but not sure if it would work on your LeCruet interior.

I've had good luck making a paste of baking soda and water. Keep it moist with a lid or plastic wrap and let it sit overnight. Then a little elbow grease does the trick for me.

You mean LeCreuset? If you have a tough stain my two suggestions are Barkeepers Friend and a Mr. Clean Magic Eraser.