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T W E N T Y T H I R D E D I T I O N

T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 6

1 4 1

SERV I CES & TREATMENT :

PROTEC T I ON & REMED I AL TREATMENT

4.1

ABRASIVE CLEANING

METHODS FOR MASONRY

JAMIE FAIRCHILD

S

UCCESSFUL LARGE-SCALE

cleaning or coating removal is

usually achieved by combining

techniques, either in sequence or

area by area. Since a technique may

offer a wide range of adjustment,

devising the best methodology can

appear to be a complex procedure.

However, it is made much easier if

we first understand the virtues and

shortcomings of the techniques. In

practice these shortcomings will

normally eliminate beforehand all

but a few of the initial options.

GENERAL CONSIDERATIONS

The cleaning of a listed building

may be deemed an alteration, in

which case it would require listed

building consent or a faculty before

commencement. This protection

applies to the inside and outside

of a building, the removal of paint

and the undertaking of trials.

An acceptable alteration is one

that maintains or enhances the

significance of the building, so it

would be necessary to justify the

intervention and to first consider

schemes which retain in situ all

or part of any compatible existing

coatings. This extends to layers

considered to be part of the

historic fabric and, if they are to

be removed, they should first be

recorded and samples archived.

The principal current guidance

document for masonry cleaning is

BS8221:

Code of practice for cleaning and surface

repair of buildings – Part 1: Cleaning of

natural stones, brick, terracotta and concrete

.

Revised in 2012,

BS8221

addresses the

relevant factors and considerations together

with a categorisation and description of the

principal cleaning methods. The broad groups

of substrate are defined and their individual

characteristics described as they relate to

cleaning. While the document is punctuated

by precautions and caveats, it recommends

which methods are generally appropriate to

each substrate and soiling type – although

this should itself be treated with caution. The

document is a guide and not a specification,

nor can a specification be written by reference

to it alone. The cleaning operation and its

outcome must also comply with statutory

legislation with regard to health and safety

and the environment.

The litmus test (no pun intended) will be

to trial cleaning methods in situ. Producing

test panels is a normal requirement of the

consent process. They should be positioned

discreetly but must be representative of the

substrate, soiling and detail and produced

at the earliest opportunity, if possible

long before commencement of the main

project. The trials can be used not only

to demonstrate the cleaning or coating

removal but also to assess the condition of

the underlying masonry, its type and colour,

and the pointing and repairs required.

Before starting the trials it is important to

establish what outcome is required, so that the

parameters of the process can be determined.

The test panels should demonstrate whether

the methods will actually work and, if so,

how quickly and whether they are practical

in the context of the specific project and site.

Approved panels are retained as exemplars

and are a prerequisite for setting

the standard and monitoring

of all subsequent cleaning

work. However, unsatisfactory

results should also be noted

as these too will inform the

decision-making process.

To minimise the range of

methods to be trialled, it will be

necessary to identify the substrate

and soiling types and the nature of

the coating/deposition. This starts

with a visual examination, perhaps

with the aid of a 5-10x magnifier

or USB microscope. However,

sampling and off-site analysis can

help to predict the reaction of the

substrate to the cleaning process

and may be essential where toxic

coatings or residues are suspected.

The assessment will help to

differentiate between that which

is natural and benign, and that

which is disfiguring or harmful.

We might describe the former as

patina and perhaps intrinsic to

the substrate, and the latter as

a contaminant

and extraneous.

This judgement can be highly

subjective but for cleaning, one

indicator might be the relative

ease with which unwanted matter

can be removed without visible

loss of sound substrate.

TERMINOLOGY

In society at large the word

cleaning has a positive connotation but in

conservation it is regarded with caution,

especially if prefixed by ‘chemical’ and

particularly by ‘abrasive’. ‘Sand-blasting’ is

frequently a disparaging term which implies

a violent or disruptive process and is often

used by the uninitiated to describe any form

of air-abrasive cleaning. It is important to

understand that this method is not necessarily

harmful, but it does require appropriate

testing, specification and skill.

While the method categories set out in

BS8221

are a little idiosyncratic, we might

simplify them as chemical, water-based,

mechanical (both hand and powered)

and laser or radiation methods. Abrasive

techniques fall within the description

‘mechanical’. (In reality there is a considerable

overlap between the categories since, for

example, chemical treatments and abrasive

methods frequently use water.)

Dry micro-abrasive system fitted with a 1mm aperture nozzle