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18

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

23

RD ANNUAL EDITION

EARTH MORTARS

Tom Morton, Nigel Copsey and Rebecca Little

I

N RECENT years, increasing

recognition of the prevalence and

importance of earth mortars in

traditional construction has informed the

development of appropriate techniques

for the repair of ruined and standing

structures. Churches form an important

part of this heritage. Most medieval

churches were constructed using earth

mortars, with use declining through the

18th and 19th centuries.

From prehistory until the 19th

century, earth was used in a variety of

ways to construct buildings of every

type across Britain and Ireland, as it was

in virtually every culture and climate.

The ubiquity of earth as a construction

material was due to its wide availability

at low cost and the ease with which it can

be worked into a range of materials to

construct durable buildings.

Traditional earth mortars could

be very varied in quality, reflecting

local subsoil geology. Some vernacular

buildings have mortars of natural earth

which has very poor technical qualities,

while other earth mortars are either

naturally robust or have been altered to

enhance their qualities. In this context,

churches, like other high-status buildings,

tend to present mortars of higher quality.

The conservation community is only

now beginning to recognise the extent to

which earth mortars were traditionally

used. However, a number of organisations

including Historic England, Historic

Environment Scotland and Earth Building

UK & Ireland (see Useful Contacts,

page 45), are working to better document

their distribution and characterisation.

TYPICAL PERFORMANCE

Earth mortars were typically used as

the bulk mortar to construct the wall,

with the face pointed or rendered in

lime to create a more durable external

finish. Thus, earth mortars are typically

only recognised when building fabric

deteriorates or during alterations. As

churches tend to be well-maintained and

rarely altered, it can be expected that

many churches contain unrecorded earth

mortars. Increasing awareness of the

distribution of these mortars and their

characteristic performance should foster

a better assessment of standing structures

and inform appropriate repairs.

While earth mortars typically sit

happily hidden within a wall in their

original state, this can change and

significantly affect the condition of the

structure in ways that are different from

lime mortars.

Earth mortars typically have good

compressive strength, but negligible

tensile strength. If a structure is subject

to movement, for example through

settlement of foundations, changes in load

patterns or the decay of timber elements,

earth mortars provide little restraint and

movement will tend to be more directly

apparent than in lime-mortared masonry.

St Mary’s Priory Church, Old Malton, Yorkshire (Photo: Nicholas Fletcher) and, below, earth mortars visible in

its 12th-century vaults (Photo: Nigel Copsey)

This is also true for the cohesion through

a wall, where earth-mortared cores

provide little restraint to the separation

of masonry faces subjected to stress

where there is poor stone-to-stone

bonding. The quality of the masonry

build becomes more important to the

strength of the wall where it relies less

on the binding strength of its mortar.

So local and pronounced movement of