A genuine black olive oil soap used for tadelakt in Morocco is called with many names: Beldi aka Black Moroccan soap aka Savon Noir. The gel-like soap is made from olive oil and olive paste. The olive oil is saponified with potassium hydroxide.
Black olive oil soap is specially made to be used in traditional Moroccan steambaths called hammams. Once the steam has soften the skin and open the poors, the black olive oil soap is applied all over the body. It works as a cleanising mask and scrub
A solution of the very same black olive oil soap is used to treat the tadelakt, which is traditional finishing material in hammams. Traditional application of tadelakt includes polishing with stone, treatment with olive oil soap solution and dedication, in order to get the water-repellent surface.
The fatty acid of olive oil soap chemically react with calcium ions in lime plaster, forming lime soap. In this chemical reaction the sodium ion in olive oil soap is replaced with calcium ions to form insoluble compounds. The result is water-repellent surface, which is still open to the water vapour diffusion.