The organic constituents in clay are incinerated when the bricks are produced
and therefore there are no nutrients for the mould spores to live off
before dirt and damp becomes attached to the surface.
Then the affection starts and it goes fast.
Bricks have a coarse surface that protects the mould spores and they can then make good grips. The mould becomes establishes and the affections grows fast.
The only way to eliminate all growth is to sterilize the growth and to disinfect the whole brick surface.
Bensaltenside have a granted protection time of four years. Or more.