Vanadium stains are evident by a yellow- green discolouration on the face of a brick resulting from the vanadium salts which are naturally present in most clays used to produce light coloured bricks.

Vanadium stains are not powdery crystalline deposits and cannot simply be brushed off. They are often mistaken for moss or algae which usually grows in damp areas across the whole face of the bricks and mortar. By contrast vanadium stains appear on dry walls, normally as patches on the brick only.
Causes

Vanadium stains are a result of water soluble vanadium salts. These salts are put into solution by excess water migrating through the bricks and brought to the surface as they dry out. During this drying out process the water evaporates leaving vanadium salts on the brick surface. The vanadium salt turns green when it crystallises in an acidic environment, such as after acid cleaning or exposure to rainwater, Which naturally acidic.

This process can occur whenever the bricks are subjected to excessive water from rain either before or (more often) during the bricklaying process. If the stains are not removed prior to cleaning with hydrochloric acid, they may turn a darker colour and be more difficult to remove.

Vanadium salts are neither permanent or harmful and they do not indicate any defects in the bricks. Such stains in exposed areas generally wash off in time, but their removal can be hastened by chemical treatment.