Historic Churches 2017

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

TH ANNUAL EDITION

to areas of pigment. This relentless decay

mechanism is particularly damaging

to the fine limestones widely used for

architectural detail and monuments

as they are vulnerable to damage by

sulphates, chloride and nitrate salts.

Deterioration of historic paint

layers, biological growth and surface-

staining of church monuments can be

associated with the microclimate and

repetitive cyclical changes in temperature

and humidity. Condensation can form

on surfaces which are colder than the

general environment, contributing

to ongoing salt activity, staining and

deterioration of paint layers. The

problem of spring condensation in

churches – when warm, humid air

enters the building and condenses on

interior fabric – is well-documented.

MANAGING THE ENVIRONMENT

Because critical decay mechanisms such

as the corrosion of iron fixings and salt

activity are associated with different

forms of moisture, it is essential to reduce

the levels of moisture within the fabric.

A detailed inspection of roof coverings,

rainwater goods and drainage is necessary

to identify any defects. This may not

be as straightforward as it sounds.

Historic churches often have complicated

roof structures and drainage systems

which have been subject to numerous

adjustments. The ground level around

churches can gradually become elevated

as drainage channels are dug out and

other debris builds up. Moisture readings

should always be taken on the internal

fabric surrounding the monument as

well as on the monument itself to assist

in understanding how the building

distributes moisture.

The external walling must also be

inspected. Decay and fracturing to the

stonework with hairline cracking and

failure to the pointing will dramatically

increase the levels of moisture

immediately behind and around the

monument. The presence and extent

of cement mortars should be recorded

because they reduce the surface area

through which moisture can evaporate

significantly. Cement mortars also cause

ongoing damage to the surrounding

historic stonework.

The condition of the external

fabric needs to be addressed before a

monument conservation programme can

begin and this can present a significant

and sometimes unexpected cost to the

local community. Improvements to the

drainage system can be complicated by

raised ground levels, the presence of

underground water systems and more

The monument to the Hon Robert Dormer

(d1726), Church of the Holy Cross, Quainton,

Buckinghamshire, showing the location of the iron

fixings and the significant movement through the

joints caused by the corroding iron fixings

The canopy to the monument to Sir Edward and

Lady Francis Rodney during dismantling showing

corroded iron fixings

of historic decoration, including pigment

and sometimes gilding. All these materials

are subject to decay mechanisms

associated with moisture, either as a

liquid or a vapour, which can compromise

the structural integrity of the monument

and cause surfaces to deteriorate, leading

to a loss of detail and decoration.

Typically, the cause of structural

movement is the deterioration of the

system of iron fixings used to construct

the monument. Iron is susceptible to

corrosion in the presence of moisture

and air. This causes the fixings to expand,

exerting pressure on the sections of

stone and often resulting in dramatic

movement through the joints, cracking

and buckling. Rust ‘jacking’ has the

power to lift substantial free-standing

elements, destabilising them. An example

of this can be seen on the monument to

Robert Dormer (illustrated above left),

which is attributed to the great Flemish-

born sculptor John Michael Rysbrack.

Here, two large sculptures have been

lifted out of position through pressure

exerted by the corroded iron fixings.

Once the pointing mortar falls out, this

type of deterioration accelerates due to

the greater exposure of the iron to both

moisture and air.

The critical factor which draws the

attention of the PCC and architect to a

church monument is usually evidence of

movement through the joints, sections of

the monument shifting out of alignment

and associated iron-staining on the

surface. At this stage a monument may

pose a significant threat to public safety

and demands investigation.

Surface deterioration often indicates

high levels of moisture in the building

envelope and the internal environment.

Historic fabric will always retain moisture

and soluble salts and the management

of salt activity is one of the biggest

challenges in the treatment and care of

church monuments. Cyclical changes

in temperature and relative humidity

in the microclimate can trigger an

acceleration in the movement of soluble

salts through porous material towards the

surface, where they move out of solution

and crystallise either on the surface

(efflorescence) or just behind the surface

(cryptoflorescence).

Some salts are hygroscopic and

draw moisture in from the surrounding

microclimate, causing them to dissolve

and recrystallise in the pores of the

material in increasing concentrations.

The force of crystal growth, like rust

expansion, can be substantial, breaking

down the integrity of the pore structure

and leading to blistering, spalling and loss

Salt crystallisation damage at a high level caused by

warm, moist air condensing on cold fabric