FALL 2018
A Historic Change at a Historic House
Climate Management at Knole
By Nigel Blades and Katy Lithgow
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Dating back to the fifteenth century, the Knole Estate, situated in west Kent, includes one of England's largest and most historic houses. Simultaneously a medieval archbishop's palace, a Jacobean Renaissance courtier's house, a repository of royal Stuart furniture, a grand eighteenth-century ducal residence—home to Earls and Dukes of Dorset and Lords Sackville—childhood home to Vita Sackville-West, and inspiration for Virginia Woolf's Orlando, Knole and its increasingly fragile antiquarian charms have drawn visitors in the thousands since the late eighteenth century.

By 1874 Reginald Mortimer, the First Lord Sackville, found that "people strayed away from their parties, broke into our rooms, tore the fringe off the chairs and couches, and did all manner of things, whereupon I felt obliged to shut up the place." Although visiting resumed at reduced levels on his death in 1888,1 and the collections were repaired, the massive costs of maintenance and taxes in the twentieth century led to the house being transferred to the National Trust for England, Wales, and Northern Ireland in 1946, with subsequent gifts of some contents in lieu of taxes.

In 1946 James Lees-Milne of the National Trust found the building and contents in poor condition, describing "piles of dust under the chairs from worm borings. The gesso furniture too is in a terrible state. All the picture labels want renewing; the silver furniture cleaning; the window mullions mending."2 Government-aided building repair tackled high-priority work in the latter half of the twentieth century but did not fully address the poor buffering of the internal environment. Knole also lacked modern heating, and electric lighting and power were minimal. Thus collections at Knole had become accustomed to a relative humidity (RH) of typically 60–90 percent—much higher than found in most English country houses.

Housekeeping was reintroduced to control light and dust, reflecting the Trust's emphasis on preventive conservation, which prioritizes dealing with cause over effects.3 A 2002–5 Leverhulme-funded research project demonstrated that the high RH bound dust to Knole's textiles, while condition surveys (most recently 2008–12) disclosed environment-related damage. For example, at least 80 of 310 paintings were affected by mold, insects, and condensation. However, conservation was limited to emergency and "little and often" treatments to maintain the status quo, as there was little point in treating objects only to return them to a poor environment.

The most recent phase of emergency building repairs began in 2012 as Phase 1 of the £20 million "Inspired by Knole" project.4 Phases 2 and 3—addressing showrooms and contents, and supported by £7.75 million from the Heritage Lottery Fund—began in 2013. Building repair provided the opportunity to improve the environment through humidity-controlled conservation heating. This technique is used successfully in most of the Trust's historic houses to stabilize RH for the care of collections.5 However, there were concerns about how objects would respond to the lower RH environment of the Trust's 40–65 percent RH control specification, to address mold and insect pests.

The feasibility of introducing conservation heating was studied from the early 2000s and then tested in Knole's Reynolds Room in 2011. Insulation to enhance the building's thermal performance and reduce energy consumed by conservation heating was considered.

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To assess the impact on the Knole collections of changing the environment from the uncontrolled 60–90 percent RH to a controlled 40–65 percent RH display environment, acoustic emission (AE) monitoring was undertaken from 2016, as a research collaboration with the Polish Academy of Sciences Jerzy Haber Institute of Catalysis and Surface Chemistry, which has expertise in using AE to measure the response of historic furniture and wooden sculpture to changing RH and temperature. Because AE monitoring is labor intensive, only two sets of "canary objects" were studied. Examples of the most sensitive and significant items were chosen—a torchère and table from a rare 1671 Parisian suite of furniture by Pierre Gole, and a torchère and table from a set of japanned furniture by Gerritt Jensen, dated 1691. AE monitoring showed that both suites were physically very stable in the uncontrolled high RH environment at Knole, and that the AE response did not increase significantly as the furniture was moved to the controlled environments with an RH upper limit of 65 percent (although woodworm activity was recorded). In the future, the set point will be reduced to 60 percent so the annual display climate will be broadly within 40–65 percent RH.

Knole ballroom

The type of heating needed careful consideration to avoid detracting from the showrooms' sense of history. Although not as environmentally sustainable as hot water heating systems using renewable energy, fixed and portable electric heaters were selected. The infrastructure for electric heating is far less invasive than the pipes, radiators, and valves needed for "wet" heating. Where there was wall space, fixed traditional pattern sectional radiators were installed. However, in most rooms heating is from black-painted oil-filled radiators powered from a dedicated conservation heating circuit. In addition, Knole's Ballroom has an electric heat mat under the carpet, which can deliver low-level radiant heat across the main floor area of the room. (This technology is not yet sufficiently resilient to footfall to withstand wider installation.) Insulation was installed chiefly in ceiling spaces and in some areas of the walls to reduce heat loss from the showrooms, and in turn to reduce heating power needed to maintain conservation conditions. Its use was constrained by the desire to limit disturbance to the historic fabric.

The first-phase showrooms opened in March 2017 with a building management system operating a conservation heating control strategy with an upper RH limit of 65 percent. AE monitoring continued until summer 2018. This technique has proved extremely useful in demonstrating that the environmental changes brought about by the introduction of heating have been safe for the house and collections. Keeping the collections at around 60 percent will prevent mold growth and wood-boring insect attack, while avoiding damage to furniture and paintings from drying and desiccation. The project is nearing the end of its final phase, with full completion in spring 2019.

As the highest-priority conservation project in the Trust concerning a building, interiors, and a collection of preeminent significance and fragility, great emphasis has been laid on research to minimize the risk of unintended consequences in the introduction of well-intentioned improvements. A clear understanding of the condition of the collection to identify causes of deterioration and rationalize the selection of control measures, real-time testing of novel as well as more familiar solutions, and research to assess the risks and benefits of changing environments and object response have been essential to strengthen confidence in conservation decision-making at Knole.

Nigel Blades is the preventive conservation adviser of the National Trust for England, Wales, and Northern Ireland. Katy Lithgow is the National Trust's head conservator.

1.Peter Mandler, The Fall and Rise of the Stately Home (London: Yale University Press,1997), 201.
2.Oxford Archaeology, Knole Kent Conservation Management Plan: Volume One (2007), 45.
3.National Trust, The National Trust Manual of Housekeeping: Care and Conservation of Collections in Historic Houses (London: National Trust, 2011).
4.Siobhan Barratt, "Inspired by Knole," in The Artifact, Its Context and Their Narrative: Multidisciplinary Conservation in Historic House Museums, A Joint Conference of ICOM-DEMHIST and Three ICOM-CC Working Groups: Sculpture, Polychromy, & Architectural Decoration; Wood, Furniture, & Lacquer; and Textiles, The Getty Research Institute, Los Angeles, November 6–9, 2012, available at Interim%20Meeting%202012/07-Barratt-DEMHIST_ICOMCC-LA_2012.pdf.
5.Linda Bullock, "Environmental Control in National Trust Properties," Journal of Architectural Conservation 15, no. 1 (2009): 83–98.