In the process of cleaning buildings we often have grey to black marks: The substance itself is metabolised by a fungi living in the porosity of the masonry substrate for the purpose of filtering light. It is a natural melamine. It is water soluble and further loses cohesion by photolyse – the influence of UV light. Metabolites are not alive and therefore out of scope for a biocide.
On a roof there is enough UV and water to predictably remove the black tile discolouration within a few weeks. The self cleansing process of black metabolites on concrete tiles is substantially complete in approximately 3 months after treatment with the biocide. This appears to be the time it takes the hyphae to metabolise enough carbon from the substrate to form a new opaque film of black melamine.