New harling conserves C16th Hardwick Old Hall

Falling masonry has closed the 16th century ruined Hardwick Old Hall to the public for more than three years, but in a £1.3million, year-long conservation programme of its shell by English Heritage, Donald Insall Associates has assessed the structure’s condition and proposed a stabilisation plan in the context of EH’s Sustainable Conservation and Asset Management Plan (SCAMP). A priority was to preserve plasterwork bas-reliefs on the inner walls which are being degraded by water ingress through exterior stones.

Though modern, the intervention involved traditional lime render and harling techniques, explains Donald Insall associate Tom Bromet, creating a ‘new’ weather shield that sensitively recreates its original look: ‘In 1912, the Old Hall was a major conservation project by the Society for the Protection of Ancient Buildings (SPAB) and [followed original] principles. It was the later, more heavy-handed repairs using cement, by the then Ministry of Works, that helped accelerate the decay we are rectifying now.’ 

Bromet explains that with endemic erosion of the soft sandstone structure, while a SPAB approach would have been nice, its principles prioritise significant interventions to reduce long term costs, as EH’s SCAMP advocates. Instead, Insall’s more ‘radical’ approach of re-covering the stone with new harl returned walls to their ‘original’ state on the building’s west face. ‘Our aim was to preserve as much of the historic fabric as possible. Adding the harl achieves that, even though it results in a big visual change. By reinstating a lost element of the building you are able to maintain everything else.’

The architect engaged with master plasterer Philip Gaches of Gaches Plastering, with Skillington Workshop and Historic Property Restoration to agree the best working methodology; with a JCT traditional form of contract, they had to look at ‘worst case scenarios’ to price the job, to give EH cost security.

Before the harl was applied parts of the wall needed ‘dubbing out’. Where stones had eroded into sawtooth shapes, filling in with small stones and lime mortar bridged the gaps and created a flattish surface for the harl to key into. A painted layer of lime plaster was applied to the surface to hold moisture in the core of the wall and prepare it for the harl application. The final surface is covered by three layers of limewash. Bromet explains that the make-up of the new harl was based on detailed study of the old. ‘Our analysis of the original harling informed our mix for the new. As the two interface, we wanted the same hydroscopicity, elasticity and density.’

Erosion levels affected the thickness of the harl too, which ranged from 80mm to 20mm where it met non-eroded ashlar quoins or window reveals. Applying it too thickly would add undue weight to the structure. The harl was left rough and untrowelled, maximising surface area so water can evaporate. ‘As it is softer than the stone, moisture spreads across it rather than penetrate the stone,’ says Bromet. 

While it’s very bright now – visitors think the walls have been painted – the architect says it builds a patina quickly, as the limewash lets the colour of the stone bleed through. He and the client are happy with the result. ‘The other option would have been to replace the stone but that would have been time consuming and expensive, involve loss of original stone and been as visually jarring. Despite the big visual change, the harling ticks all the conservation boxes,’ Bromet concludes.