Lichens with a cyanobacterium as the photosynthetic partner tend to be dark grey, brown or black. They colonise sandstone displaying a dark hard cortex made of glued together (agglutinated) fungal filaments. The cortex can be harder than the stone it colonises and resists pressure washing. The ACP biocide kills the cyanobacteria in a matter of minutes, the black cortex begins to lose it’s cohesive strength. The rate of decay of the cortex is in relation to it’s exposure to sunlight.
After a few weeks the weather begins to wash black marks away. When a faster result is required a vigorous brushing will help return the stone to its original level of aesthetic appearance.
In shady places brushing and light jetting are necessary. Once the stone has recovered periodical preventive maintenance is light. Site testing has revealed good and durable results at a dilution rate of 1:40, but an increased rate of decay can be seen at 1:20.
Using other chemicals on stone: Metaclear is primarily designed for the removal of fungal metabolites on render. One of the active ingredients – sodium hypochlorite – will have some cleaning effects on paving stones. However no chemical will dissolve the cortex and some remaining marks are to be expected.
Oxiclear for brown marks: It is imperative to test the compatibility of the acidic gel on alkali stone before the start. If rust marks are present on an alkali stone acids cannot be used. Thioglycolic acid will be the next fall back option for small work. If unpractical a vigorous brushing with a strong TFR will be -imperfect as it is – the last option.