Thallus crustose, dark grey, occasionally pale grey or buff-coloured, rarely immersed and inconspicuous, usually forming a roughened, continuous or cracked-areolate surface, the edges not placodioid or lobed. Isidia, soredia and blastidia absent. Photobiont trebouxioid.
Anamorph: conidiomata pycnidia, 100-200 µm diam., brown-red, often densely covering the thallus. Conidia small, bacilliform or ellipsoidal, hyaline, aseptate.
Teleomorph: ascomata apothecia, to 1 mm diam., scattered to contiguous, ± rounded but usually with the rim slightly flexuose or contorted, flat to convex. Thallin margin absent. Proper margin well-defined and glossy when young, brown-red, algae are absent below the hymenium. Disc rust-red to red-brown, rarely dark orange-red, unchanged in colour when wet, sometimes discoloured with dark patches, pruinose. Epithecium with a well-developed crystalline layer that dissolves in KOH, subhymenium pale. Interascal tissue of thick-walled flexuose paraphyses to 3.5 μm diam., branched towards the apices, the tips only slightly broadened. Asci clavate, fairly short-stalked, 8-spored. Ascospores arranged biseriately, 12-14 × 6-8 μm, ellipsoidal, polarilocular, the septum ca 5 μm thick, to half of the length of the ascospore, hyaline, smooth-walled, without a gelatinous sheath or epispore.
Chemistry: thallus K–, epithecium K+ purple.
Assessed by Woods & Coppins (2012) as of Least Concern. The species is common and widespread, at least in fairly unpolluted habitats.
Distinctive for its large, usually brick-red ascomata on a roughened greyish thallus. The ascospores have a particularly broad septum. Occasionally forms extensive thalli with only tiny, rust-red, pimple-like incipient apothecia; this morph is often characteristic of pollution-stressed sites. Closely related to the bark-inhabiting Blastenia (Caloplaca) ferruginea, but distinct in the darker, rough thallus and smaller apothecia.
Much confused with Caloplaca ceracea which has paler red, flat to slightly convex apothecia with a thick algal layer below the hymenium, the disc turning waxy and greenish when wetted. That species was not included in recent molecular phylogenetic revisions, and its affinities need further examination.
In unpolluted areas of coastal and lowland Britain, submontane Wales and the Pennines. BLS map here.
On soft textured siliceous or slightly base-rich rocks, in sheltered, preferably rather damp, situations, also occasionally on walls.