wow! Between Ralph and Lily, this is sewn up-- the ratio is 70-30 sand to clay with sand defined by particle size and cement, ash, lime or bitumen can be added to get the ratio required.  Ralph's link led to a how to page which is great, but it did say that plastering over rammed walls was okay while Lily's site, which leads to the world renowned David Easton's own site, Rammed Earth Works, says that behind plaster, rammed earth will quickly erode because moisture trapped there will condense and separate the plaster and wall, and therefore is not reccomended.  I guess the water-proofing issue will be the next exploration. Thanks!!