Lichens (L-Codes)

ATH

Those crusty grey, white, orange or black patches welded to old stone are lichens — among the most tenacious organisms a building faces. They dissolve the surface with acids to grip it, so scraping or blasting them off takes the stone with them. The 'L-Codes' classify lichen by species and the stone it sits on, so removal is chemical and gentle, never abrasive.

The L-Codes catalogue lichenised fungi — symbioses of a fungal mycobiont with an algal or cyanobacterial photobiont — as one of the most aggressive biological weathering agents on architectural stone. Unlike pioneer algae, mature lichens are chemical actors: they secrete oxalic and other organic acids that chelate metal cations and dissolve calcareous and ferruginous substrates (biochemical weathering), while their hyphae penetrate the stone and exert physical fracturing. Crustose species such as Verrucaria bond so tightly to limestone that removal requires controlled chelation and lysis rather than abrasion, which would spall the surface. The L-Codes index species by substrate bioreceptivity — Portland and Lincolnshire limestones are an active research focus for exactly this — and by the conservation sensitivity of the host, which governs whether heritage-grade, non-destructive protocols (steam thermolysis, biocidal chelation) are mandated on listed fabric.

biochemical weathering

The L-Codes catalogue lichenised fungi — symbioses of a fungal mycobiont with an algal or cyanobacterial photobiont — as one of the most aggressive biological weathering agents on architectural stone. Unlike pioneer algae, mature lichens are chemical actors: they secrete oxalic and other organic acids that chelate metal cations and dissolve calcareous and ferruginous substrates (biochemical weathering), while their hyphae penetrate the stone and exert physical fracturing. Crustose species such as Verrucaria bond so tightly to limestone that removal requires controlled chelation and lysis rather than abrasion, which would spall the surface. The L-Codes index species by substrate bioreceptivity — Portland and Lincolnshire limestones are an active research focus for exactly this — and by the conservation sensitivity of the host, which governs whether heritage-grade, non-destructive protocols (steam thermolysis, biocidal chelation) are mandated on listed fabric.