A white, powdery deposit of salts that appears on brick, concrete, stone or plaster when water carries dissolved salts to the surface and evaporates.
Soluble salts from the material, mortar or ground dissolve in moisture moving through the masonry and crystallise where the water evaporates. It is a moisture symptom rather than a colour change in the material itself.
It is often mistaken for paint failure or mould; it brushes off dry and returns while the wall stays wet.
New brickwork often shows white bloom in its first winter, fading as the wall dries out.
It can make a dark brick or a coloured render look pale and patchy, and painting over it without curing the moisture fails.
Efflorescence is diagnosed, not graded. The deposit is usually soluble sulphates and carbonates, and the questions are where the salts came from and where the water is getting in. Mortar and concrete products are common salt sources, since they can hold several times more soluble material than fired clay brick. A first test is to brush the deposit off dry and wait: bloom on masonry less than a year old commonly fades as construction water dries out, while a deposit that keeps returning points to a continuing leak, rising damp or a failed coping.
A dark brick, a coloured render or dyed paving is chosen for its depth of colour.
A thin white veil shows far more on a dark surface than on a buff or red one, so the scheme looks faded and blotchy in its first winters. Detailing that keeps water out of the wall matters more for dark masonry.
The white deposit is sealed in or painted over to hide it.
Salts keep arriving behind the film and crystallise there, lifting the paint and sometimes the face of the brick. The moisture path has to be cured first.