Conservation

Date 26 September 2011 - 7 October 2011

Event ID 963743

Category Building History

Type Conservation

Permalink http://canmore.org.uk/event/963743

NC 0366 2784 Historic Assynt with the assistance of AOC Archaeology Group undertook a comprehensive survey of the parish of Assynt in the winter of 2009–10 as part of their Assynt’s Hidden Lives Project (DES 2010, 86). From this survey they identified three sites on which to focus their attentions during the Life and Death in Assynt’s Past Project of 26 September–7 October 2011: Clachtoll Broch, Glenleraig and Loch Borralan East Chambered Cairn.

Concerns had been raised previously about the condition of the broch at Clachtoll and in 2002, HS had organised an emergency intervention in the area of the entrance. Whilst this work addressed the immediate threat to the monument, the instability of the entrance area was not remedied, and the upper inner wall at first floor level was in imminent danger of collapse. In addition, the uppermost slab of the right hand guard cell had been removed, weakening the structure of the guard cell and allowing the ingress of people and debris. As part of the 2002 works, a pillar (of red Syenite) had been introduced to support a cracked lintel. Over time this had become unsightly and ineffective and was found to be hanging by its fixing from the damaged lintel, as the stones on which it had been supported had settled away from its base.

Conservation works began with the excavation of material to the left of the entrance which had slipped downhill and was eroding back into the wall fabric. This revealed the intramural gallery of the first floor wall level, now infilled largely with wind-blown sand. Iron Age pot sherds were found at the base of this infill, resting on the upper surface of the in situ deposits of the gallery floor. The gallery provided compelling evidence of the structural failure of the entrance area at some point in time. The debris slope toward the front of the outer wall and c3m left of the entrance passage, continued behind the poorly constructed wall face at this point. There is no doubt that the inclined slope of structural stones from the inner face of the outer wall continued and continue to exert pressure on the outer wall skin. To counteract this force, a new block of masonry was erected from the horizontal level to which our excavation had proceeded (roughly the level of the bottom of the entrance lintels). This was designed to withstand lateral forces from the entrance area and to buttress the outward lean of the inner wall and adjoining debris slope. All newly placed stones were marked with two 5mm modern drill holes so that on close inspection the new can be distinguished from original or authentic fabric, while at the same time the holes are not detectable from more than a few metres away.

The area above the entrance passage was stripped out down to the level of the tops of the lintels. This material was mainly backfill from the 2002 incursion and its removal revealed the bronze armature used to support the most seriously damaged of the lintels. We did not disturb this arrangement. The excavation revealed the existence of a ‘lintel pocket’, a space within the wall into which the lintels had been dropped. Stub walls had been built over the lintels’ ends the W one still intact, the E example had been removed in 2002 and the space it had occupied was rebuilt into the adjacent wall, masking its existence and function. We concluded that the stub walls were intended to carry at least a second layer of lintels, relieving the stress on the primary lintels. We had anticipated some such relieving structure but, in the absence of any record from the 2002 intervention had only the evidence of the surviving stub wall to support this interpretation. It had been our intention to provide long term relief for the passage lintels by inserting secondary concrete lintels above them. The structure revealed allowed us to dismiss this approach and to insert replacement secondary stone lintels to achieve the same end.

AOC's stone conservator replaced the stone pillar with a steel plate and stainless steel dowels under the end of the split lintel. Suitably textured and coloured epoxy resin was used to secure this support and to mask its appearance sufficiently to reduce its visual impact.

Stripping of the inner face of the upper wall adjacent to the entrance revealed the scarcement first noted by Dr MacKie, which was revealed over several metres on the W side of the entrance passage. The scarcement was constructed by the simple expedient of moving the upper wall inwards c350–450mm from the inner face of the ground floor wall. Each of the stones revealed had on it a deposit of carbonised material and many of them were broken. It is clear that they were fractured by the collapse onto them of large structural stones. Within the broch interior a mass of vacuous structural stone could be seen to underlie the current solum. This mass is interpreted as primary collapse of the broch wall in this area. A sample from the carbonised material has returned a radiocarbon date of 2025±30 years BP. Calibrated, this indicates that the wood sample dated had ceased growing at some date between 111 cal BC and 55 cal AD (with a probability of 95%; or between 53 cal BC and 22 cal AD with a probability of 66%). This date can be described as implying the cutting of this wood near the end of the 1st century BC.

The damaged wall to the E of the entrance passage at first floor level was stripped down and laser scanned at intervals to compile a continuous record. It became clear during the work that virtually every stone in this wall segment has been fractured. Using the largest surviving fragments together with new material from among the loose slabs within and around the broch, we rebuilt this wall, restoring it to a vertical profile, especially along the side of the entrance passage, which it had come to overhang dangerously.

Finally, the gap over the guard cell was closed off with stone lintels and a large stone slab. The latter was dowelled into the adjacent roofing slab of the guard cell; the only direct intervention into the in situ fabric of the monument. This in turn was buried under a further layer of large stones, to deter vandalism and soil was introduced to the wall head. This soil will be topped up and returfed in the spring of 2012, when the weather is more clement and the turves may have a hope of surviving and establishing a grassed over wall head.

Archive: RCAHMS (intended)

Funder: Historic Scotland; Highland Leader+ and Heritage Lottery Fund

AOC Archaeology Group, 2011