Historic Churches 2017

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

TH ANNUAL EDITION

A NANOLIME CASE STUDY

The City of London Cemetery entrance screen

Ben Newman

NE OF the core principles of

conservation is to save and repair

as much surviving material as

possible. However, repairs often effect

a degree of change, posing ethical

issues as well as technical challenges

for the conservator, and few aspects of

conservation are quite as challenging as

consolidation. For example, where a fine

medieval timber boss has largely been

destroyed by woodworm, consolidating

its remains in resin raises questions about

originality and significance in much the

same way that its replacement with a new

timber carving would.

From a technical perspective, the

aim is to find a consolidant which

binds the original material together

effectively, but without harming it

or the rest of the structure. Where

the fabric to be consolidated is on

the exterior of a building, this can be

particularly challenging, and there is

much ongoing research into the ability

of a new lime-based material, nanolime,

to consolidate decayed limestone and

plaster. In particular, the EC-funded

Stonecore project carried out extensive

research into the use of the material

from 2008-11 and its findings were

disseminated across Europe through a

series of public meetings. More recently,

research carried out at the University

of Bath for Historic England has led to

the publication of guidance by Historic

England (see Further Information).

This article will explore nanolime’s

benefits and limitations through a case

study of the work that London Stone

Conservation undertook on the entrance

screen to The City of London Cemetery

in spring 2016.

WHAT IS NANOLIME?

Nanolime consists of artificially made,

nano-sized particles of calcium hydroxide

suspended in alcohol. There are two

commercially available brands: Calosil

was introduced to the market in 2006

followed by Nanorestore in 2008. The

alcohol can be ethanol, isopropanol or

n-propanol.

A nanometre (nm) is a unit of measure

equivalent to one millionth of a millimetre

and visible only under the most powerful

microscopes. Nanolime particles are

typically 150nm and since the average size

of stone pore structure is around 1,000nm

there is a very good potential to fill some of

this space, thereby consolidating decayed

stone and reducing further loss.

For the conservation of limestones

and other lime-based materials,

nanolime seems particularly appropriate

because it is based on a well-established

traditional material, lime, as opposed to

the organic consolidants widely used in

the last century, such as alkoxysilanes.

These organic consolidants have been

controversial because they change the

stone, both chemically and visually (see

Further Information, Hull). Using lime

is preferable because after carbonation

the calcium hydroxide is converted

into calcium carbonate which is the

main constituent of limestone – in

essence a ‘like-for-like’ replacement.

One downside is the inconsistency

in depth of penetration, which is

dependent on varying environmental

and stone characteristics.

The consolidation of stone with

nanolime follows the same principles

as ‘lime watering’, which is part of the

‘lime technique’ pioneered by Professor

Baker on Wells Cathedral in the 1970s,

but has the advantage that the lime is

carried by the fluid in a suspension,

rather than in a solution like lime water.

Consequently, a significantly larger

amount of lime can be introduced into

the stone with each application; lime

water typically contains 1.7g of lime

per litre compared to 50g/l available in

nanolime. The alcohol of the nanolime

will evaporate leaving behind the lime

to carbonate and bond with the stone,

whereas lime water requires more than

40 applications to be effective. The large

amount of water that is absorbed by the

stone may mobilise salts in the masonry,

accelerating the decay process which the

treatment is attempting to slow down.

This is a particular problem where salt

crystallisation is already contributing

to the decay, as is often the case.

The City of London Cemetery screen before the project: the use of impermeable cement repairs had increased

the rate of decay to the point that the stonework had become unsafe. The central shield in particular was

perilously close to becoming detached.