Claydon House, Buckinghamshire

Re-glazing the staircase lantern February 2017

© N

atio

nal T

rust

Imag

es/A

rnhe

l de

Serr

a

n removal of hazardous asbestos

n upgrading glazing to limit damage from UV and visual light

n replacing glazing seals to prevent further water damage

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Staircase lantern

Conservation & maintenance

Background Claydon House is a Georgian mansion, seat to the Verney family since 1620. Today, only a fragment of the great house is still standing with magnificent 18th century state rooms. It is notable for its refined Rococo interior carved decoration, by Luke Lightfoot and the wonderful plasterwork of Joseph Rose. One of the main attractions of the house is the beautiful main staircase made of mahogany, boxwood and ebony with ivory inserts, located underneath a prestigious roof lantern.

Unfortunately, the condition of the glazed roof lantern over the main staircase had begun to deteriorate leading to raised levels of UV (higher than the National Trust upper limit 75 µw/lumen) and water ingress during heavy rain. This risked damage to the fragile interior features, such as blanching, fading and staining to the high level carved decorations and staircase landing – as well as the potential risk of corrosion to the metal balustrade and washing out of mortar on to the stone floor. The rope seals used in the glazing system contained asbestos, which posed a major health risk to all members of public and staff working at the premises. The light levels were 13 times above normal limit and the UV absorbing layer in the laminate had become less effective.

Above The west front of Claydon, Buckinghamshire

© N

atio

nal T

rust

Imag

es/J

ohn

Mill

er

Project brief This project was less focus on the lantern’s conservation, as an object, and more on the preventive conservation and stabilisation of the environment around it. In accordance, the project aimed to encompass the following tasks:

n Repair the structural integrity of the lanternMake the lantern weathertight and prevent further water ingress to the building.

n Remove the harmful asbestos Replace loose asbestos seals, according to the current health & safety legislation.

n Reduce visible light and UV levels Protect the unique internal decorations from over-exposure to light, considering a pleasant visitor experience.

n Access the high level carved decorationsInvestigate and carry out remedial work to the high level carving below the dome.

Concept

2Staircase lantern | Claydon House | London & South EastDesign guide

© N

atio

nal T

rust

/Reb

ecca

Elli

son

The necessity for this project became a priority after the completion of the annual asbestos survey in March 2015. This highlighted the unstable condition of the asbestos rope seals; one of which was hanging loose from the glazing, risking fibres being released.

An earlier condition report on the main staircase revealed that the light levels on the landing greatly exceed the recommended annual lux hours. The blue wool dosimeter results from 2012 to 2015 reveal annual exposures of 1,750,000–1,850,000 lux hours, far above the 150,000 lux hour designation for the highly light sensitive floor and collections below. In addition the Polyvinyl-butyl (PVB) laminate in the glazing, which absorbs ultra-violet radiation, was deteriorating. Spot readings taken in 2015, using an Elsec 765 meter, measured over 80 µw/lumen directly below the dome, on the ground floor. UV film or laminate in good condition should reduce UV below 30 µw/lumen. The lantern was not weathertight anymore and the glazing was past its lifespan of 25 years, with the last re-glazing of the dome in 1993. These factors were causing severe damage to the sensitive historic fabric beneath.

Due to the nature of removing and replacing glazing, the ideal period for this project was during the spring–summer months, however, due to the disruption the project would cause to the visitor route and experience it was decided to run the project in early autumn when the house was still open – although with lower visitor figures.

The project started in September 2016, while the property was still open to the public for a month before the house was scheduled to close for the winter. During this time visitor routes were re-arranged and health and safety precautions were considered while an internal scaffold was raised. It was a great opportunity for the public to engage with the ongoing conservation project and witness the National Trust‘s maintenance plan for this property.

Feasibility work carried out prior to the project found the lantern’s structure to be in generally good condition with only minor issues regarding to flexibility and movement in the glazing bars. Specialist contractors produced a condition report of the lantern structure. The Metals Conservation Adviser for the National Trust and a structural engineer were consulted.

During feasibility the building surveyor insisted that the scaffolding and temporary roof structure was designed and checked by a structural engineer so as not to impact on the historic fabric. Not only was the design detailed in advance, it also meant that accurate costs could be produced on a like-for-like basis at tender stage. Scaffolding formed a major part of the works and costs

Consultees n Heritage Project Contracts Ltd. n National Trust Furniture and Metals Advisers n National Trust Park Farm Conservation Building Team

Above Claydon lantern before treatmentDesignations

n Grade I listed building

Feasibility

3Staircase lantern | Claydon House | London & South EastDesign guide

Above The Staircase Hall at Claydon, Buckinghamshire. The staircase is made of mahogany, boxwood and ebony with ivory inserts in a geometric pattern

Site issuesProtection to internal areas, Staircase Hall and staircase, as well as routes into the building were completed prior to the contracted works taking place. This was carried out by the National Trust Conservation Building Team and conducted outside of the main contract works due to the sensitivity of the interior.

The main contractor supplied and installed suitable protection to all leadwork at roof level that was likely to be exposed to foot traffic during the works and particularly the perimeter leadwork to the roof lantern. All appropriate health and safety precautions were taken with regard to removal of the asbestos rope seals from the existing glazing system.

The internal ‘birdcage’ scaffold had to be designed around a ground floor venting system and delicate metal balustrade. In order to achieve the desired level of protection at ceiling level the internal scaffold had to be positioned very close to the main staircase (15cm in places) and a project conservator was allocated on site to supervise the erection and dismantling of the scaffold in order to manage the risk of damage to the staircase and the stone floor.

The property remained open, while the internal scaffold was erected. The visitor routes had to be re-arranged and due to a pre-planned wedding event at the property, the scaffold was put in place later than expected.

Dust and debris fell onto the veneered staircase when the scaffold was dismantled, despite vacuuming of the scaffold beforehand.

The project team comprised internal and external expertise including:n Project sponsorn Clientn Project managern Contract administratorn Building surveyorn Metals advisern Furniture advisern Main contractorn Glass suppliern Mesh blindn Scaffoldingn Structural engineer

Project team

© N

atio

nal T

rust

Imag

es/A

ndre

as v

on E

insi

edel

A small number of the carved mouldings were affected, presumably as a result of fluctuations in relative humidity possibly caused by the lack of glazing. However the conservation heating controls in the house were not working effectively throughout the duration of the project making it impossible to establish the route of the problem. The addition of ply templates fitted into the space between the glazing bars once they had been removed was considered in order to minimise fluctuations in relative humidity; however, these would have excluded all daylight from the staircase hall, which was not desirable as there is no direct power to this space.

Feasibility

4Staircase lantern | Claydon House | London & South EastDesign guide

Design approach This project was a conservation project and therefore the goal was to achieve the best possible results with the least possible intervention and alteration to the original materials. The various ways of controlling light entering into the building via the lantern were appraised at the outset of the project. These included neutral density film, tinted glazing, blinds and externally fitted mesh. The solution chosen was bespoke to Claydon. The initial plan for light control was to apply a dark coloured neutral density film to the outside of the glazing to improve the light and UV control. However the shape of the individual glazing panes was such that fairly detailed individual templates had to be created and the demands from the glazing film installers for a dust-free, warm environment (+5ºC) for a period of four weeks, meant that the handling of the glass units would have been excessive and carried risk to breakages. Essentially, once the glass was hoisted onto the roof and tried in the lantern, it was better left in place and not taken back down for the film to be applied and then returned to the roof again.

The glazing film installers were not able to provide a warranty or lifespan for the product which would have been held captive by the glazing bar detail and rendered difficult to remove at a later date. The risk of disruption to the glazing seals was too great to consider given that rain penetration was one of the original defects requiring the works.

All of the above demands presented risk and additionally, extended working periods would have increased the overall cost of the project for no apparent gain.

Above Detail of the frieze beneath the glass dome in the staircase hall at Claydon House, Buckinghamshire

On receipt of this information an alternative plan was produced to reduce light levels, inspired by another Trust conservation project at Mount Stewart in Northern Ireland. The installation of mesh blinds made from Philfertex™ a vinyl coated polyester mesh, to be fitted on top of the original metal glazing bars was suggested. In order to ensure a good fitting, a two day site survey was conducted.

The disposal of the asbestos seals process was in accordance with the asbestos waste regulations with the harmful seals being removed externally. Prior to the extraction of the seals, proper protection was installed to all the openings leading to the internal areas to avoid asbestos residue getting inside the building.

Advice was sought from the National Trust Furniture Adviser and it was thought that the risks of substantial alternation in moisture content of high level timber decoration were manageable given that glazing was only absent for three weeks and carvings could be closely monitored due to scaffold access. During the last week while the glazing was being fitted, the weather conditions changed resulting in greater fluctuations in temperature and humidity. This resulted in three minor losses to the decoration, in areas of glued joints, which were reattached before the scaffold was removed. All the remaining plaster and carved decoration was also condition checked and consolidated where necessary.

A member of the main contractor’s team worked on site for 10 days, to the specification of the Trust’s Metals Adviser to remove residues found on metal glazing bars and preparing them for fitting the new Neoprene glass seals.

© N

atio

nal T

rust

Imag

es/A

ndre

as v

on E

insi

edel

Due to the unexpected detachment of carved decoration, there was the need to extend the scaffold time hire, over the Christmas period.

AccessAn internal birdcage scaffold was erected under the dome along with an external access scaffold and temporary roof over the roof lantern.

A new external ladder had to be built in order to easily access the decorative cap on the top of the lantern in the future.

Engagement and interpretationAn interactive display was produced for the public explaining the damaging effects of light on the main staircase under the dome. A model and drawings of the scaffold made by the Visitor Engagement Team as well as samples of neutral density film were available for the public to look at. All staff and volunteers on site where briefed and trained by the National Trust conservator and project manager on the works being carried out.

Design & Plan

5Staircase lantern | Claydon House | London & South EastDesign guide

Health and safetyA structural engineer provided advice on scaffold requirements and the structural integrity of the lantern.

Heritage Project Management company undertook the paint analysis of the topcoat present on the glazing metal bars prior to the conservation. The topcoat of paint on the glazing bars was found to be a white oil paint based on white lead with some added zinc oxide. This scheme is likely to have been applied after World War II. Soluble lead compounds are toxic by ingestion and inhalation, therefore it was essential for health and safety reasons that the lead containing surface was not stripped prior to repainting, but left as part of the archaeology of the building. This approach required the lead surface to be stable and not flaking.

The main risk associated with asbestos is from breathing in airborne asbestos fibres which can lead to asbestos-related lung disease.

In this case asbestos ropes were used as sealants on the lantern, during the re-glazing process in 1993. Based on the property’s asbestos monitoring report, the asbestos ropes where in unstable condition and their removal was necessary.

Below Neptune side saddle on a seahorse, part of the carved wood decoration at the base of the dome above the Staircase at Claydon, Buckinghamshire

Competitive tendering was used to procure the contract works, however, the Trust’s approved suppliers were also used to provide protection of the historic interiors, project conservator and provision of the mesh blind. The selected contractor had worked with the building surveyor in the past; therefore a good communication was already established.

Internal protectionNational Trust Park Farm Conservation Building Team Park Farm, Chapel Lane, High Wycombe HP12 4BN 01494 440513

Mesh blind supplierStorrar Marine181-3 Coast Road. Newcastle-upon-Tyne NE7 7RR01912661037www.storrarmarine.co.uk

Main contractorHeritage Project Contracts Ltd.Unit 1, Ainsdale Drive, Harlescott Industrial Estate, ShrewsburySY1 3TL 01743460035www.heritageprojectcontracts.com

Scaffold firmNiall Scaffolding Hartwell Depot, Oxford Road, Hartwell, Aylesbury, Bucks HP17 8QG01296427171www.niallscaffolders.com

Products and services

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Design & Plan Procurement

6Staircase lantern | Claydon House | London & South EastDesign guide

Conservation workRemoving old glazing The lantern was last re-glazed in 1993, using laminated (PVB) float glass with average thickness between 3-4mm per glass unit. Once the old glazing was removed it was noted that a copper glazing detail was applied to the iron glazing bars – potentially as a later alteration to the lantern. Taking the glass out provided a good opportunity to check for tightness and stability all fixings. Lantern decorationThe top of the lantern has a central ring, which is leaded over the top of the glazing bars with a decorative top copper cap at the head of the lantern. This was designed to incorporate air flow via a screw thread device, opening and closing the dome. This device had been altered recently as two galvanised threaded bars were added to assist the suspension of the centrally mounted motif. The dome device was taken apart and thoroughly cleaned as part of the contract works, to the specification of the Metals Adviser. On the top of the timber panelled support frame for the motif was noted a lead weight attached onto a deteriorating plastic carton. The plastic carton was removed and counterweighted.

Above Claydon dome without glazing

Left Lantern from interior prior to mesh being fitted

© N

atio

nal T

rust

/Reb

ecca

Elli

son

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Implementation

7Staircase lantern | Claydon House | London & South EastDesign guide

Asbestos ropes Based on both visual observation and on the 1993 re-glazing drawing of the lantern, the sealing system was made up of a decorative capping section bedded on asbestos containing rope. This section is compressed into place with metal nuts fixed to threaded bars located centrally and to the front face of the main glazing bar.

The old contaminated seals were removed with care externally. Prior to the removal process the dome was sealed internally so harmful dust and particles would not be released into the interior.

The new seals used were neoprene seals. E-shaped sections were used to bed the glass onto and D-shaped seals were set within the capping. The exact sizing of the seals differed from unit to unit to ensure a good seal to each one. The seal to the top of the glass units was improved by a silicon sealant prior to reinstating the lead cover flashing, which could then be kept to the same size as the existing.

Top left 1. Rear of paint particles indicating the paint layers were attached to a corroded iron surface (50x magnification); 2. Paint stromatography analysis (50x magnification); 3. A small number of paint fragments indicated two different white paint schemes with an earlier colour being more buff in colour (50x magnification); 4. Both white schemes tested positively for lead (200x magnification)

Bottom left Drawings from the last re-glazed in 1993 evidence of asbestos rope seals were utilised to form the main glazing seal into the metal roof light glazing bars and onto weathering supports

Glazing barsThe lantern is made up of cladded iron glazing bars, the cladding also forms the integral drainage channel that runs down to the lead covered upstand and falls just short of the lower shoe. Once the glazing was removed the metal bars were inspected and found to be stable enough to withstand the re-placing of the glazing.

The bars were degreased and primed within ten days and prepared to be painted. In order to identify the appropriate colour for the bars, paint analysis was completed in order to determine paint strata, colour and type of existing paint. The sample was taken internally in the form of very small paint fragments due to the corroded nature of the glazing bars (surface corrosion). Samples were collected from the underside of the lantern – they were all taken by scalpel, identified and badged for analysis.

The samples were set in a clear coat polyester resin and then cut and polished to enable them to be inspected under a microscope. The resin holds the paint layers together and allows a polished flat plane for viewing at high magnification. In addition, a proportion of the samples were physically tested for lead content. Samples were tested with a solution of Sodium Sulphide in which the presence of lead turns a dark brown colour.

The samples taken from the glazing bars indicated the paint layers were attached to a corroded iron surface. It was possible to determine a number of different paint layers that were present however no single sample indicated the presence of all identified layers.

Based on the Farrow & Ball paint chart a similar paint colour to the later white schemes would be an ‘All White’ gloss paint.

© H

erita

ge P

roje

ct M

anag

emen

t

Implementation

8Staircase lantern | Claydon House | London & South EastDesign guide

Above Mesh fabricated access ladder

Mesh blind Whilst the project was underway, measurements were taken for a custom-made mesh blind to be installed on top of the metal glazing bars. The blind was made from 100% Woven Vinyl-coated Polyester mesh, with yarn diameter 0.025 inch, fabric diameter 0.033 inch, and openness 35%. It was installed in a reversible way, using epoxy resin bars (painted to match the glazing bars), attached on top of the original glazing bars using spots of mastic.

As the mesh blinds are translucent, the visitor experience should not be affected and the view from below the dome still conveys a sense of the variation of sky conditions with time of day, weather and seasons. However the annual lux hours on the top landing of the staircase have reduced by more than half: from 2,600,000 lux hours in 2016-17 to 1,000,000 lux hours in 2017-18. Although the annual light dose has halved, the levels are still way over the recommended dose for highly light-sensitive objects and further light reduction methods are being considered. Re-installing glazing / weather-tightnessA ply template was taken from each existing pane of glass as it was removed. Each pane and template was numbered consecutively. These were used to provide the exact size of the new glass. The new glass panes were hoisted up the external scaffold tower, in their packaging. The glass was installed temporarily to ensure it fitted and to assess the correct thickness of neoprene seals to use. It was then removed and repacked and stored until ready to fit. The glazing chosen was Pilkington Optilam 7.5 mm laminated and sand blasted on the interior face. A silicone seal was added to the outside of the glass beneath the lead flashing to improve water resistance. The glass was water-tested after fitting.

Consolidation of carved decorations A conservator checked and consolidated all carved high-level decoration once the scaffold was in place with the agreement of the main contractor. The adhesive used on the ceiling was cold set collagen adhesive, fish glue HiTack Fish Glue. Apart from the three small areas which became dislocated, there were no more loose carved decorations once the glazing was fitted in place.

Associated repairsThere was an opportunity to investigate and carry out remedial work to the high level carving below the roof light whilst the scaffold was on site. Permanent roof access to the dome was created using a special ladder to provide better access to the lantern for future conservation work.

High level lead gutter repairs were carried out replacing sheets where previous water ingress had been an issue.

The interior decorative surface of the high level frieze had its water marks toned down to improve the visual appearance.

Project timelinen Design approval: 29 July 2016 n Scaffold erected: September 2016 n Scaffold taken dismantled: January 2017 n Project completion: February 2017

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Implementation

9Staircase lantern | Claydon House | London & South EastDesign guide

Project costs The project was funded by the National Trust. n Total: £61,123

Right Mesh fixing

PerformanceOverall this project was a success and all the original aims and objectives were met by stabilising the weather performance of the roof lantern and therefore securing the protection of the collection underneath. The original conservation plan was altered during the first months of the project, since the weather conditions did not allow the application of neutral density film to the exterior of the glass. In addition there was unanticipated detachment of a small number of localised pieces of carved decoration. Both these factors delayed the removal of the scaffold by one month and the completion of the project by two months.

The decision to re-glaze the lantern and add mesh blinds proved to be an excellent option. The light levels reduced by half immediately after the mesh blinds were fitted, without any negative effect on the visitor experience. The new glazing system also prevented further water ingress.

The mesh was inspected by the main conservator after six months of the completion of the project and is performing well.

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Implementation Handover Review

Management and maintenanceA visual inspection was recommended every six months and every three years the blind is to be removed and sent to the manufacturer for cleaning and checking for repairs to the stitching.

10Staircase lantern | Claydon House | London & South EastDesign guide

Best practicesThe mesh blinds proved to be an excellent protective measure from light exposure, providing additional protection when fitted properly on domes, without interfering with the visitor experience.

The communication between the main contractor and the project team was paramount to the success of this project. The site supervisor was always available, allowing work to run smoothly and meant that problems were dealt with in a timely fashion.

Overall, the quality of the works carried out both on site and in the studio met the original expectations.

Below Glazing specification. The glazing chosen was Pilkington Optilam 7.5 mm

Lessons learntThere are restrictions when applying neutral density film to exterior during cold temperatures. Adequate attachment of the film with the glazing in temperatures below 5°C cannot be guaranteed. This could have been overcome if the project had run in the summer months rather than the autumn. The film needed one month to cure, leading to additional complications regarding contamination with dust, as well as issues regarding handling and manoeuvring the glass. The lack of supplier warranty and difficulties with access for future maintenance also influenced the decisions taken.

The sensitivity of the carved decorations to fluctuations in environment over the period of the contract works led to some decorations becoming detached at areas of glued joints.

The option of installing temporary or false glazing or plywood could have helped to buffer environmental change to avoid change in moisture content of the carved mouldings. However this should be balanced against the additional time it would take to apply and remove this each day to enable works to be carried out to the glazing bars.

An environmental monitoring system could have been placed in the area of the carved mouldings, to monitor the sudden changes in environment.

FeedbackVisitorsThe conservation in action tours and activities proved very popular among visitors and volunteers.

Staff Staff members were positive about the results of the project.

© N

atio

nal T

rust

/Reb

ecca

Elli

son

Review

11Staircase lantern | Claydon House | London & South EastDesign guide

Left Mesh fitted externally

Further information

Middle Claydon, near Buckingham, Buckinghamshire, MK18 2EY www.nationaltrust.org.uk/claydon

Products and services: Use of products and services is not necessarily an endorsement by the National Trust.Copyright: The National Trust retains copyright for this document.This is an internal document; please do not reproduce/photocopy without prior permission.

© The National Trust 2018. Registered charity no. 205846.

© N

atio

nal T

rust

/Reb

ecca

Elli

son

If you require this information in an alternative format, please email DesignGuide@nationaltrust.org.uk

12Staircase lantern | Claydon House | London & South EastDesign guide