Early conservation findings indicate that the steel ‘box’ is slowing down deterioration of Charles Rennie Mackintosh’s design masterpiece, the Hill House.
Four years ago the National Trust for Scotland erected an architect-designed, award-winning steel cage to protect the house from rain and damp, allowing buildings specialists to dry out and stabilise it.
A trial to carefully remove small areas of render from the A-listed building’s exterior has shown that the walls, which had begun to show signs of water ingress from as early as 15 years after its completion, are steadily drying and regaining strength.
The conservation charity has welcomed these results as it embarks on the next stage of its pioneering project to save the Helensburgh house for Scotland and Mackintosh enthusiasts all over the world.
Project director Liz Davidson said: ‘These works at the Hill House will help the National Trust for Scotland establish the properties of the materials used by Mackintosh and subsequent repair projects.
“This in turn will allow us to identify the most appropriate replacement materials to keep the building free of the persistent damp that has threatened its stunning interiors, such as the original stencilling by Mackintosh’s wife Margaret Macdonald.
“The protection offered by the box has allowed this drying-out and transformation in structural performance, but all of us working on this wonderful and important building are aware that the clock is ticking on now being able to embark on permanent repairs and conservation before the shelter of the Box is removed in compliance with planning rules.
“We’re delighted with the initial research findings but remain very much aware of the scale of the conservation task ahead.”
The new study saw a team of engineering and conservation specialists carefully recording the condition of the walls and overseeing the excavation of areas around the building to examine the very core of Mackintosh’s construction.
The team meticulously removed the portland cement roughcast in six locations around the building to reveal the substrate beneath, including what is thought to be the original dampproof course, red sandstone and brick walls as well as some of the repair details of the last century.
The works allowed the trust to take small samples of all the materials found – including a sample of what is believed to be Mackintosh’s original roughcast mix surviving in one area of the building.
Small quantities of all the materials exposed were sent to researchers at the University of Dundee for analysis of their composition and material behaviour.
The most significant result of the work is the finding that the walls, under the roughcast, have been steadily drying and regaining strength in the controlled environment provided by the box.
Further investigation will start on a similar exercise working from the inside face of the building, targeting rooms where historically the damp has been most prevalent, including chimney flues and areas below steep parapets.
Meanwhile the excavations themselves have been neatly sealed up with cover plates which allow specialist surveyors to revisit each site, match the data from internal recording, and steadily explore the questions of what lies beneath the cement jacket of the Hill House.
Trust chief executive Philip Long added: “The Hill House is the only original Charles Rennie Mackintosh domestic dwelling open to the public, and over the coming years, the conservation of this internationally-renowned and unique heritage asset will be a critical project for our charity.
“The work to save and conserve the Hill House is a vital and substantial task, and one we won’t be able to undertake without generous support from funding bodies, philanthropists and Mackintosh enthusiasts in Scotland and around the world, whose help we will be appealing for.’”
The Hill House was designed by Charles Rennie Mackintosh in 1902-4 for the publisher Walter Blackie and his family.
It represents his largest and most complete domestic work designed at the height of his architectural powers in a striking fusion of proto-modernism and Scots baronial styles style with the trademark art nouveau and symbolist interiors developed in partnership with his wife and artist Margaret Macdonald.
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