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

[Amanda Peck]

Got links?

There are some whitewash formulas that do contain alum, maybe sugar, I don't 
seem to remember soap.

This contains the right ingredients, but might be more useful for a shower 
curtain.
http://midtown.net/dragonwing/col0005.htm

Kathy wrote:

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

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