Plaque on a whitewashed wall

Whitewash, also called lime paint, is made from slaked lime (calcium hydroxide) or chalk. Plain limewash is slaked lime mixed with water, and traditional additives include animal glue, egg white, salt, soap, milk, flour and alum; earth pigments give shades from cream to yellow and brown. It cures by carbonation: the lime reacts with carbon dioxide in the air and turns into calcium carbonate, as calcite. Unlike modern paint, which sits on the surface, limewash is drawn into the substrate, so the wall has to be damped down first or the finish can turn powdery. It is very thin and translucent when first applied, and becomes opaque as it dries.

The calcite crystals give limewash a distinctive glow. It has been used on cottages, in dairies and dairy barns, where successive coats smooth rough surfaces and flake off debris, and its mild antibacterial properties were valued. It protects fruit trees from sun scald, and glasshouse roofs were whitewashed in early summer to reduce heat and scraped clean in autumn. Salt is often added to resist mould. It has limits: basic limewash may not stop rain-driven water entering a wall, and silicate paints are gaining ground over it in the United Kingdom. The source does not discuss Portugal, so this describes the material rather than any local custom.