This article provides an introduction to the techniques and pitfalls of the principal masonry cleaning methods used on historic brick and stone.
The need to
clean a building is commonly perceived as being critical for its
well being. Such a physical act is driven by aesthetic considerations
often without considering the after-effects or consequences properly.
The decision is also frequently made on townscape or streetscape
grounds regardless of the physical impact on the actual face of
which are listed or in a conservation area are concerned, Scotland
differs from the rest of the country. Here, acknowledging that
physical change can occur, stone cleaning has been deemed an 'alteration'
since 1992. As a result, all proposals to clean listed buildings
now require Listed Building Consent or, in the case of unlisted
buildings within a conservation area, planning permission. Such
an approach has been determined necessary to ensure that the risk
to the fabric is considered and that appropriate damage-limitation
measures are adopted.
of the most difficult problems that can arise as a result
of deciding to clean a structure is deciding where to stop.
By cleaning the upper part of this bridge only the impression
has been created that a totally different stone was used for
the treated part. In reality the material has been bleached
and discoloured by the chemical used.
|Understanding the soiling of buildings is critical if cleaning is
proposed. Sandstones are particularly difficult to comprehend as soiling
can often occur from within the stone, resulting in a variegated surface.
|Where the depth of soiling only extends to a few microns in thickness
on the surface, the use of high pressure grit blast cleaning, whether
wet or dry, can bite deeply into the historic surface below, destroying
the quality of the original banker mason's workmanship.
recently, masonry cleaning methods generally lacked proper specification
and site control. This has resulted in a wide variety of techniques
being offered by contractors without due regard to the full consequences
of their effect, and usually little or no detailed consideration
was shown by manufacturers or suppliers to this need. In attempting
to deal with all types of dirt and surface coatings in one treatment,
contractors and specifiers catered for worst-case scenarios, and
over-treatment was the established norm.
physical methods os stone cleaning, such as the use of rotating discs, were
extremely difficult to control. As a result innumerable buildings
now display an exceptional degree of surface damage. Viewed at close
quarters, the loss of arrisses, irregular shadow effects, disrupted
moulding runs and undulating faces typify the consequences. In many
cases it is impossible to recreate the quality of the original finish
and detail due to the amount of material that has been lost.
|In much the same way that heavy-handed techniques can remove the surface
of stone, considerable quantities of the surface of period brickwork
can also be removed. Here, deep scour marks run from brick to brick
along the mouldings and there is a general loss of sculpted quality.
a sedimentary rock, limestone which has been cleaned often mirrors
the defects created on cleaned sandstone. A general opening up of
the natural stratification can produce a blurred and uneven surface,
disturbing the once clean lines of fine mouldings. The textured surface
is liable to hold a greater degree of water and airborne particles
and assists more rapid resoiling.
wide variations that occur in the mineral content of the different
sandstones are often ignored when cleaning is proposed. It frequently
comes as a surprise to discover that these factors then manifest themselves
after chemical cleaning has been carried out. Not infrequently, this
is compounded by efflorescence from the cleaning residue, seen here
as wavy white lines under the pediment mouldings.
|It has been repeatedly shown that cleaned buildings can attract a
high level of attention from graffiti artists, tempted by the fresh
exposed surface. With the use of highly penetrating aerosol paints,
the applied colours can permeate deeply into the stones structure.
Where attempts have been made to mask the effect by painting over
it, the consequences can be even more complex to remedy.
insufficient consideration was given to the wide range of natural
materials being dealt with and their relative susceptibility to
deteriorate as a result. No building is homogeneous in its construction
or detail. Materials such as sandstone, limestone, granite, brick
and terracotta are liable to be bound by lime mortar. Some may
be used in combination, and other factors such as variations in
colour, texture, tooling and form are likely to be met. In many,
the composition will vary, and different combinations can lead
to the interaction of materials, one with the other. Decay may
also be present and different patch repairs, with different substances
at different times, may further complicate the issue.
A basic difficulty
is deciding where to stop. This can lead to a form of facadism,
with only the principal elevation being treated. However this
approach has one advantageous side-effect: any change that subsequently
reveals itself can be compared against the untreated return faces
of the same stones at the extremity of the cleaned area.
The type of
soiling also needs to be taken into account. In the cleaning debate,
soiling is often pre-supposed only to be an external agent, with
particulate deposition and reaction resulting from either wet
or dry conditions. Damaging crust-formation can be evident on
the surface limestone, but the prospect of benign mineral movement
occurring from within the body of sandstone is rarely considered.
surface soiling is equally complex, with bacteria, algae, fungi
and lichens each seeking out the appropriate colonisation conditions
within which they will flourish. Influencing factors in their
growth can include atmospheric and micro-climatic conditions,
fluid movement and concentrations, surface roughness and physical
Once the building
has been cleaned, incidents of resoiling, iron mobilisation, efflorescence,
vandalism and graffiti further complicate matters.
THE PRINCIPAL CLEANING METHODS USED
methods of cleaning buildings have habitually been adopted, although
a number of specialised techniques have become available in recent
methods: these include brushing and rubbing, washing and steaming,
and dry abrasives(commonly referred to as , or surface redressing.
methods: applied as liquids or poultices,
these may employ the use of alkaline treatments, acidic treatments
or organic solvents, singly or in combination.
techniques may promote the use of impregnated sponge,
laser technology, ultrasonic equipment, heat lances,
gypsum inversion, bacteria, poultices or gels, and surfactants
(degreasants or soaps).
Not all treatments
are appropriate for all materials, and extreme care needs to be
exercised when deciding which system to adopt. Due to the need
to minimise the risk of damage, the 'do not clean option' should
also be borne in mind.
MEASURES TO BE ADOPTED TO MINIMISE RISK WHERE STONE CLEANING IS ACCEPTABLE IN PRINCIPLE
the actual testing methodology, and the validity of trial area
results, can be an elaborate process if it is to be carried out
effectively and meaningfully. Topics that should be considered
include colour measurements, depth profiling, surface roughness
tests, and the use of scanning electron microscopy, determining
the petrology (the geological structure of the stone) and pH values,
porosity and permeability measurements, before and after test
cleaning. Care needs to be exercised when contemplating the consequences
of washing chemicals into underlying masonry during rinse-off
stages, and the question of how to stop 'wash-in' occurring on
porous stone surfaces when 'washing off' chemicals should also
and recording the consequences of tests should lead to a full
analysis of results before writing a relevant specification and
obtaining statutory consents (if required). Risks may also be
reduced by carefully selecting an experienced and suitable contractor;
by determining the training and experience of site operatives;
by ensuring effective site controls and health and safety precautions;
and by the maintenance of adequate site progress records.
practitioners in this involved area of work, a number of detailed
reports, literature reviews and Technical Advice Notes have recently
been published by Historic Scotland. Based on intensive research,
these publications deal in depth with the cleaning of sandstone
and granite buildings, and the application of biocide to sandstone
facades. Other available material deals with a broader range of
surfaces. At the same time, development work has progressed at
BSI, where the current British Standard (BS 6270) has been under
responded to this published research material, and developments
have occurred with the launch and promotion of a variety of new
of the need to use more environmentally friendly and delicate
techniques, a number of non-aggressive processes have been released.
As the principal areas of concern and risk are at the point of
operation, most of the new systems have been designed to make
sure that operatives have total control over what is happening.
Working with very low water volumes or air pressures, and with
a range of nozzles, a variety of new agents are offered. These
include spherical nodules of calcium carbonate for use on limestones,
and fine particle aluminum silicate for use on sandstones and
granites. Calcium carbonate, olivine and calcium silicate abrasives
are also available.
operative training packages are promoted by suppliers, and professionals
are also encouraged to attend the courses to ensure that they fully
understand the correct application of the systems.
SCOTTISH/NATIONAL VOCATIONAL QUALIFICATIONS
A CITB Occupational
Working Group is currently focusing on stone cleaning at Level
3 with the intention of producing a Scottish/National Vocational
Qualification (S/NVQ) in Facade Maintenance. Although a Level
2 S/NVQ has already been validated, its uptake is poor. The Level
3 S/NVQ is being developed to take into account the need to recognise
the operatives' skills in a hands-on capacity, whilst providing
some chargehand responsibility for the supervision of others.
This initiative is a serious attempt to improve the quality of
operators in the field, but it can only work if practitioners
demand the use of operatives that are appropriately qualified.
cleaned sandstone buildings the visual effect of subsequent
surface growths can be interesting. In this example, in
addition to the surface damage caused by the cleaning regime
itself, algal colonisation can be seen to follow the architectural
patterns of the building's design. Water-holding and run-off
surfaces are particularly prone, as are the localised splash
zones in the vicinity of projecting details.
use of hydrofluoric acid to clean a sandstone superstructure on top
of an inadequately protected granite base can produce some interesting
side-effects as feldspars are converted to clays. In effect this makes
the granite sufficiently porous to enable biological colonisation.
A series of decay contours can be created on the vertical face of
the ashlar blocks, and an interaction with the mortar joints and beds
can also be found.
is a complex issue. In the past it has been undertaken without
sufficient consideration being given to the consequences. As a
result, an incalculable amount of permanent damage has been caused
to the country's building stock. How well this point is accepted
greatly depends upon the perceived value of carrying out cleaning
in the first place. For those committed to broader planning and
social benefits, it is possible that no amount of evidence will
shake the belief that cleaning is for the overall benefit of a
building and its surroundings.
the health of a building is not determined by an appearance that
might be viewed from a distance, or across the street, but by
what is happening on the actual surface itself. The appropriate
course of action must be driven by establishing what can be accepted
at this level, bearing in mind the long-term effects on the appearance
of the building and its surroundings, not just the short-term
benefits. The adoption of a damage-limitation approach should
also influence the final choice of mechanism or technique.
With a greater
awareness of the issues now to hand, there should be no excuse
for inarticulate decision-making or specifying. There can be no
standard answer offered as to what the most relevant technique
might be. All involved must share in the responsibility of getting
- Nichola Ashurst, Cleaning Historic Buildings (2 vols),
Donhead, Shaftesbury, 1994
Commission Investigation: Biological Growths, Biocide
Treatment, Soiling and Decay of Sandstone Buildings and
Monuments in Scotland (Report and Literature Review,
Masonry Conservation Research Group, The Robert Gordon University).
Historic Scotland, Edinburgh, 1995
Commission Investigation: Cleaning of Granite Buildings (Report and Literature Review, Masonry Conservation Research
Group, The Robert Gordon University), Historic Scotland,
Cleaning: A Guide for Practitioners, Historic Scotland,
Cleaning in Scotland (5 vols), Historic Scotland, Edinburgh,
Advice Note 9: Stone Cleaning Granite Buildings,
Historic Scotland, Edinburgh, 1997
Advice Note 10: Biological Growths on Sandstone Buildings:
Control and Treatment, Historic Scotland, Edinburgh,
- RGM Webster
(ed), Stone Cleaning and the Nature, Soiling and Decay
Mechanisms of Stone, (Proceedings of the International
Conference, Edinburgh 14-16 April 1992), Donhead, Shaftesbury,
This article is reproduced from The Building Conservation Directory 1998
MAXWELL DA(Dun) RIBA FRIAS FSAScot was, at the time of writing, Director of the Technical Conservation,
Research and Education Division of Historic Scotland. He
is involved with a wide range of conservation groups and bodies and
is currently a member of the RIAS Conservation Working Group and the
ICOMOS UK Executive Committee.
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