"linda mar" wrote in message
...
how much shell will it need to cause a noticable hardness increase? 1
sq-cm? more??

linda

The effects are directly related to surface area ... the greater the surface
area the more rapid the reaction. The finer the material the greater the
surface area. i.e. an ounce of powdered oyster shell would probably have a
greater (immediate) effect that a pound of un-crushed shell.

Bob