Stucco

Decorations on windows such as this one were common in the Neoclassical period, they are made of stucco.

Stucco is a kind of plasterwork used to decorate ceilings and other parts of buildings. It is made of aggregates, binder and water. These days its main constituent is Portland cement, sand, inorganic lime (a hydroxide mineral, not the fruit) and water. It is applied wet and dries hard.

The same substance is often called plaster and sometimes mortar. It has been used for hundreds of years in many cultures round the world.