Bricks are burned at a high temperature, which incinerates all organic
material.
Bricks often have a coarse surface and this enables dirt to accumulate
on the bricks rough surface – this also helps to binding moisture
to the bricks. In this environment the algae can establish themselves
and the affection becomes a fact.
When algae spores gets stuck to the bricks, they grow fast and will
recur constantly –until all of the bricks pores, all algae spores
and all old residue of algae have been totally sanitized.