[Cob] Alum and Soap over horizontal Lime Plaster surfaces?

[littlehouseantiques at att.net]

Searching online and reading up on waterproofing and ran across 
a statement that indicated that horizontal lime surfaces in 
Mexico often use a water repellent mixture of alum and soap 
to prevent water absorption.  

Are we talking about the same sort of ALUM that everyone has 
sitting on the spice shelf (used for home canning, etc.) ?

Is the soap added to solubilize the alum into a liquid so that 
it can be painted onto the lime plaster?

Anyone have any experience with using Alum and Soap as a 
waterproofer over lime plasters?

Any reason one couldn't use alum and soap directly on COB??

Kathy