The distillation of vapour, from the burning of peat, occurs when the flue cools.
The majority of painted pebbles in Scotland have been found in Shetland, where they form an important collection within the Shetland Museum.
We carried out an experimental study of this collection to understand their decoration, the material used in painting the pebbles, the methods of application of the design and to consider of its survival.
The pebbles were left to dry overnight before the stability of the pigment was tested by scrubbing the stone with a coarse pot-scourer in hot water.
The marks on the pebbles remained exactly the same and did not fade with further scouring.
The external gable walls containing the unlined, stone-built flues for open hearths often show a dark, blackish-brown stain, very similar to the colour of the spots on the painted pebbles.
The staining penetrates right through the stonework.In most areas where painted pebbles have been found, peat was generally used for domestic fires and also for smelting and smithing, due to the dearth of wood resources.This was especially the case in Shetland from the early Iron Age onwards.It also highlights predominant clusters of finds within the northeast regions of the Highlands and Islands.Why they appear more frequently in Caithness and the Northern Isles remains a mystery to be resolved, and while their purpose remains enigmatic recent excavation has offered us new information about when they were in use.The process of peat burning produced considerable quantities of pitch, which could have also been used to water-proof ropes and fabric coverings With no chimneys from which to retrieve the distilled tar during the Iron Age, peat tar was probably produced by cooling back stones around an open hearth.