Myriam (and others). The Danish webpage that describes in great detail the formula and application for "limewash" is http://www.raadvad.dk/7_0/7_2_4.htmlI use masonry lime (hydrated fine poweder) available at any building supply store.
And I've had great success with using only one thin coat of whitewash over stone to arrive at an appearance that allows some of the character of the stone to come through, i.e. not a solid white wall as you would get with paint or several coats of whitewash.
Check out the following site for an example: <http://www.geocities.com/cmasonnos/whitehousefarm1.html>