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

[Quinn]

Wouldn't the soap just... re-suds up and wash away?

"alum - a double sulfate of of ammonium or a univalent metal (such as sodium or potassium) and a trivalent metal (such as aluminum, iron or chromium): it is used as an astringent, as an emetic and in the manufacture of baking powders, dyes and paper; the commonest form is potash alum (potassium aluminum sulfate).

Cake alum, pickle alum, filter alum, papermaker's alum and pearl alum are other common names of aluminum sulfate, which occurs in nature as the mineral alunogenite. The substance irritates the eyes, the skin and the respiratory tract, and is corrosive on ingestion, according to the International Chemical Safety Card from the Centers for Disease Control and Prevention. It can be a dangerous substance when not used properly. Ingestion of 30 grams (1 ounce) has killed adults. Huh. And you find it right next to the allspice.

According to the National Food Safety Database, alum is used as an ingredient in baking powder and is used as a crisping agent in the production of pickles and maraschino cherries. It is used only in a soak solution and is washed off thoroughly before completing the recipe. It is also used to harden gelatin. "  http://www.straightdope.com/mailbag/malum.html

Quinn

<snip>
> 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?
> Kathy