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
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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