Questions re: soapmaking.

So! I have been charged with learning how to manufacture a reasonable facsimile of Marseilles soap. As best I can figure, it differs from standard fat-and-ashes soap by

1) being made with oil, not fat, and
2) involving an especially (though perhaps not exclusively) French sort of clay, presumably to add solidity to it.

The question, then: does it matter what kind of clay is used? As near I can tell, it’s not an active ingredient, and there’s a lot of red Missouri clay in the area.

And am I leaving out any vitally-important details?