First just press the pestle down on them to break them into smaller bits, then grind them by rocking the pestle back and forth with some pressure. It takes some effort but finally a small handful of shells grinded in several little batches turned into about 1 1/2 to 2 tablespoons of fine crushed shell and powder in about 20 minutes. Not bad.
It would be wise to sanitized any shell used for adding calcium to the soil. Either soaking it in a 10% bleach solution (before crushing) like other garden equipment or baking at a high temperature for several minutes should do it. Personally I'm gonna try baking it.
Any other home recipes for enriching tomato soil anyone know of?