Historic Churches 2015

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

ND ANNUAL EDITION

c50mm and gaps at the top and bottom

of each light allow a constant air flow

between the protective glass and the

historic stained glass, ensuring that both

surfaces of the medieval glass are kept

dry. Although the efficacy of this kind

of protective glazing has been tested in

projects all over Europe (Bernardi et al,

see Recommended Reading), the design

of the east window installation was also

informed by a year of environmental

monitoring undertaken by Ivo Rauch.

A major development has been the

decision to use Glashütte Lambert’s

Restauro UV glass for the exterior

glazing, a mouth-blown cylinder glass

that incorporates effective UV protection

up to 405nm. Restauro UV handles

like any other mouth-blown antique

glass but also protects light-sensitive

materials, notably the epoxy resins used

in edge-bonding, against light-induced

alteration or discolouration. York Minster

is the first building in the UK to use this

extraordinary new glass.

The new protective glazing system,

Nick Teed and surveyor of the fabric

Andrew Arrol, has eschewed the ferrous

materials which can cause damage

to stone, working with compatible

bronze alloys that minimise the risk

of bi-metallic corrosion. McKechnie

Brass’s bronze composition Alloy 300

is being used for the manufacture

of frames for the medieval glass, the

integral support bars, T-bars and the

rods from which the glazing pins are

made. New T-bars will support both

the exterior glazing and the medieval

panels, each nearly one metre square.

The exterior glazing is seated

on a T-bar and is mortared into the

stone in the traditional manner. Each

medieval panel will sit on an inner

row of T-bars, and will be held firmly

in place on the ledge of the bar with

bronze pins. For safety, exhibition or

future conservation, panels can therefore

be removed easily and safely from an

interior scaffold, leaving the exterior

glazing in place as an effective weather-

shield. The medieval panels are framed

in bronze Alloy 300 U-channel.

YGT’s Nick Teed has developed a new

framing method which has moved away

from the hot-working methods commonly

used to connect components of the panel

frames. Only traditional tin solder can be

worked at a sufficiently low temperature

to be used in proximity to medieval glass

but YGT was seeking a stronger bond. The

simple screw-fixed frame that Teed has

developed can be assembled around the

panel of historic glass, allowing maximum

precision, greatly increased strength and

an entirely reversible and recyclable use of

expensive metal components.

STONEWORK

The masonry repair project began with

a meticulous stone-by-stone assessment

of the fabric leading to the compilation

of detailed proposals for repair,

replacement and conservation which had

to be submitted to the Cathedrals Fabric

Commission for England for approval.

From the outset the project was seen

as a holistic design exercise, incorporating

design considerations for the protective

glazing and the integration of the conserved

glass panels into the masonry work.

A very careful assessment was made

of the overall geometry of the east front

and in particular the distorted geometry

of the window masonry, which leans out

at an angle of approximately 2° from the

vertical in an easterly direction. North-

south movement (across the window)

has also resulted in a 35mm wide crack at

the head of the window. This, however, is

relatively small, particularly compared to

the overall outward tilt of the east front,

which would have amounted to almost

1 metre had the upper parts of the facade

not been reconstructed during the 18th

and 19th centuries.

The four principal challenges arising

from the repair and conservation of the

east window masonry were as follows:

• to devise an appropriate methodology

for the introduction and indenting of

new tracery elements into the window

• to carry out extensive plastic repair,

pinning and conservation to those

tracery elements which were being

retained

• to conserve the intrados sculpture, which

was all in poor condition as a result of

19th-century applications of linseed oil

• to design and carve a new seated

St Peter figure, which required a great

deal of consultation and discussion.

STONE SELECTION

The basic building stone of the Minster

is magnesian limestone which, over the

years, has been sourced from several

different local quarries. The original

quarry source was Thevesdale, not

far from Tadcaster where the current

quarry source for the majority of Minster

stone is located. Other historic quarry

sources of magnesian limestone were the

Huddleston group of quarries and the

Bramham group of quarries. Stone is also

taken from Warmsworth and has in the

past also been supplied from Cadeby.

After detailed testing of previous

medieval stone types and an assessment

of contemporary quarry sources, the

stone from Tadcaster has been selected as

the preferred replacement stone with the

best match to medieval stone.

Inner face of protective glazing

Any condensation will form on this surface

Outer face of stained glass

This surface remains free of moisture due

to constant airflow, maintaining an even

temperature with the inner face of glass

Inner face of stained glass

This surface remains free of moisture and

at the temperature maintained within the

building

The mounting of the stained glass with an

equal space at the top and base of the lancet

admits uninterrupted airflow from inside the

building

A lead condensation tray at the sill will allow

any condensation to run away to the exterior

without damage to the stone

Small-gauge gravel within the upstand of

the lead tray allows moisture to run off but

resists ingress of air from the exterior

PROTECTIVE GLAZING PRINCIPLES

Interior Ventilation System

NB:

This illustration shows a common solution to an effective ventilated protective glazing system

(often known as isothermal glazing). Variations in treatment and positioning of the glass and

the technical details of fixing are possible, depending on the space available and the nature of

the building. It is important to consider the needs of each glazing scheme in context and on a

case by case basis.