Radiocarbon mortar dating of newly uncovered medieval ceiling paintings at Stavanger Cathedral, Norway

Radiocarbon dating was carried out on mortar substrates beneath newly uncovered wall paintings on the vaulted ceiling of Stavanger Cathedral, one of only three such examples in Norway and the first uncovered in the last 50 years. Bulk mortar samples were retrieved from three locations closely associated with the wall paintings and radiocarbon mortar dating was conducted on both organic (wood/charcoal) and inorganic (lime lumps from the lime binder) subsamples.

Charcoal/wood dates produced a range of calibrated ages, from the early 11th to late 13th century AD. Variation in ages is consistent with wood of different ages (‘old wood’ effect) being used as fuel in lime production or as aggregate in the mortar.

Ramped pyrolysis radiocarbon dating was applied to lime lumps extracted from two of the bulk mortars (spatially separated). The 5–6 fractions of CO2 trapped at different thermal decomposition ranges produced radiocarbon ages that were in excellent internal statistical agreement for each sample and also in excellent statistical agreement between the two samples from different sampling locations. These dates are furthermore in statistical agreement with the youngest of the charcoal/wood samples dated.

The results securely date the mortar substrate of the painting's plaster to 1216–1271 cal. AD (99.7 %) and raise the likelihood that the mortar and associated paintings pre-date a fire in Stavanger in 1272, traditionally associated with a phase of rebuilding of the cathedral. As the first in-depth published scientific dating at Stavanger Cathedral and more broadly to medieval wall paintings in Norway, the results highlight the potential of ramped pyrolysis or thermal decomposition mortar dating approaches to understanding not only building archaeologies, but the cultural heritage embedded in these structures.