Why is there white powdery stuff on the walls?
The short answer
The white powder is efflorescence—mineral salts (mainly calcium carbonate and sodium sulfate) drawn to the surface by moisture migrating through earth walls. It's a symptom of the wall's breathable construction, not damage. Sealing the wall with paint makes it worse by trapping moisture inside.
The wall is blooming. White crystals push through the plaster in patches — fine and powdery near the floor, thicker and crustier higher up. You touch it and it crumbles. The wall behind feels damp.
The word is efflorescence. From the Latin: to flower. The wall is flowering salt.
What's happening: Groundwater carries dissolved minerals — mostly calcium sulfate and sodium chloride — upward through porous earth walls by capillary action. The same force that pulls water up a paper towel pulls it up through pisé and lime plaster. When the water reaches the surface and evaporates, the minerals stay behind. The white crust is what the water was carrying. The wall is functioning as a wick.
Medina buildings sit on ground that has been wet for centuries. The high water table, the proximity of other buildings, the shared drainage channels, and the runoff from a thousand rooftops all contribute moisture that enters walls from below. Pisé and traditional lime plaster are designed to be breathable — they absorb moisture and release it, regulating indoor humidity. But when the moisture carries dissolved minerals, those minerals accumulate on the surface.
Modern cement plaster — used in many renovations — makes it worse. Cement is not breathable. It traps moisture behind the surface, building pressure until the plaster cracks or pops off entirely. The traditional lime plaster lets the wall breathe, sacrificing appearance for structural health. The efflorescence is ugly. The alternative is a wall that rots from within.
There is no permanent fix in the medina. You can brush off the crystals, replaster, and waterproof the base — but the moisture will return because the ground is wet and the walls are earth. The salt bloom is not a defect. It's a symptom of buildings that are still alive, still breathing, still doing what they were designed to do four hundred years ago.