This autumn, specialists have been working to conserve some of the Stonehenge sarsens, the latest in a series of initiatives stretching back to the dawn of the 20th century. Carly Hilts spoke to Heather Sebire to find out more.
Visitors to Stonehenge in September may have noticed a strikingly modern addition to the Neolithic monument: a mobile scaffolding tower. This autumn has seen one of the most significant conservation projects to take place at the site in more than six decades, aimed at securing the stones against erosion as well as undoing some of the effects of well-intentioned but ultimately damaging repairs carried out more than half a century earlier.
The recent conservation works stemmed from two previous pieces of research: a laser survey of the monument, and a detailed engineer’s report. The former initiative saw high-resolution scans undertaken in 2012, during the first comprehensive laser survey of Stonehenge. This was intended to help record and assess the condition of the stones, but also revealed that prehistoric graffiti that had first been identified at the site in the 1950s was much more extensive than previously thought. Some 71 previously unknown carvings of axe-heads resembling known artefacts from the Early Bronze Age (c.1750-1500 BC) were picked up during the survey, bringing the total at the site to 115, and doubling the number of similar motifs documented in Britain to-date (see CA 273). These scans were a vital tool, too, to help English Heritage, in whose care Stonehenge lies, to identify any cracks or other erosion to the stones.
The other key piece of research was a study in 2018, led by Professor David Nash at the University of Brighton, which saw chemical analysis used to pin down where the Stonehenge sarsens were quarried. The sarsens, added to Stonehenge c.2500 BC in the third stage of the monument’s construction, make up most of Stonehenge’s key features, including all 15 stones in the inner trilithon horseshoe, 33 uprights and lintels in the outer circle, and other outlying elements. Unlike the smaller bluestones, which have been traced to the Preseli region of west Wales (CA 366, 345, and 311), and the sandstone Altar Stone, which is from east Wales, the sarsens are rather more local in origin, having been traced to West Woods in the Marlborough Downs, about 25 miles north of Stonehenge (CA 367). Nash’s research also gave English Heritage the chance to conduct a fuller examination of the stones’ state of preservation.
‘At the same time as the scaffold was up for this chemical analysis, we took the opportunity to ask Historic England engineers to do a report on the stability of the stones, especially the horizontal lintels, as they hadn’t been looked at in years,’ explained Dr Heather Sebire, Senior Property Curator at Stonehenge. ‘We wanted to check that everything was OK, particularly in the light of the more extreme weather we have been having in recent years. The good news is that there was nothing structural that needed doing – we weren’t worried the lintels were going to fall down or anything like that – but the report did highlight some problems associated with repairs from the 1950s and 1960s, when they used a very hard mortar that we wouldn’t use today.’
The historical repairs particularly affected the joints between the lintels and their supporting uprights. These towering constructions represent an awe-inspiring feat of prehistoric engineering: with each sarsen measuring up to 9m in height and weighing up to 30 tonnes, they fit together using an impressively intricate system of protruding tenons and mortice holes, while the lintels themselves slot together using tongue and groove joints reminiscent of woodworking. However, when the stones had last been restored over half a century ago, many of these joins had been packed with hard mortar that is not breathable. As the material degrades over time, it leaves the stones vulnerable to damage caused by trapped moisture freezing and expanding within cracks, causing erosion.
To counteract this, English Heritage contracted specialists from Sally Strachey Historic Conservation to remove the old mortar and repack the joints with breathable lime mortar, which allows water to escape. Two conservators used a scaffolding tower to access the tops of the stones in order to complete this work, and they have also been examining the skyward surfaces of each of the nine lintels that are still in place, checking for any naturally formed holes that might be large or deep enough to form puddles. It is hoped that these efforts, combined with regular monitoring in the future, will allow the stones to withstand the forces of wind and weather for many years to come.
This is an extract of an article that appeared in CA 380. Read on in the magazine (click here to subscribe) or on our new website, The Past, which details of all the content of the magazine. At The Past you will be able to read each article in full as well as the content of our other magazines, Current World Archaeology, Minerva, and Military History Matters.