Abrasive Cleaning of Stone and Brickwork
Ashurst introduces one of the most widely used techniques for the
cleaning of masonry
|Paint is being removed
from this glazed terracotta head using very fine calcium carbonate
abrasive, low abrasive/air/water flow rates, at a pump pressure
of 5 psi and a working distance of 375mm. It is presented to demonstrate
the versatility and sensitivity of some abrasive cleaning systems.
(This is not a general recommendation for the use of abrasive
cleaning of terracotta which is now known to be extremely sensitive
to damage by abrasive cleaning.)
decision to clean an historic building is not one which should be
made lightly, as cleaning can have significant physical and visual
results. A period of detailed investigation must be undertaken to
determine whether cleaning should be undertaken and, if so, the details
of how this should be done. The nature and condition of all substrates
must be understood, not forgetting pointing materials, as must the
soiling to be removed. The latter may include atmospheric soiling,
paint, limewash, metallic staining, anti-pigeon gel and graffiti.
Each can require a different cleaning approach or at least modifications
to the system selected for use elsewhere.
cleaning system can be used correctly or incorrectly. Poor cleaning
should not be blamed on poor application alone as it is often the
result of incorrect selection of a process. Glossy trade literature
is no guarantee of correct selection. The design of a cleaning regime
for an historic building is often deceptively complex, requiring specialist
The purpose of cleaning is to remove soiling, often a source of long-term
deterioration to masonry, while causing little or no disruption of
the masonry beneath. This can be difficult to achieve due to the intimate
relationship between the stone and its soiling, as the soiling can
be embedded deeply in between the surface particles.
published sources now exist which outline the basic constituents of
various historic masonry materials and the susceptibilities of these
to selective cleaning procedures. Previous experience must also come
into play in assessing surface conditions and characteristics particular
to the job at hand. The basic principles of any cleaning process must
be determined if it is to be considered for use. Works should be undertaken
by skilled supervisors and operatives from specialist masonry firms
experienced in the cleaning of historic masonry.
abrasive cleaning systems are usually considered when soiling is not
water-soluble and when, for reasons of site logistics or material
incompatibility, chemical processes are inappropriate or less preferable.
wide range of air abrasive techniques is currently available. These
include a variety of machines, nozzles and abrasives from Hodge Clemco,
Neokleen, Liquabrade, JOS and the suppliers of pencil abrasive techniques.
Some larger scale equipment can be used in a very versatile and sensitive
air abrasive techniques operate by directing particles of abrasive
onto the soiled masonry in a stream of compressed air. Cleaning is
accomplished by impingement of the particles which dislodge or pulverise
the surface layer of the masonry. This may be the layer of soiling
or the stonework or brickwork to which it is attached. Most systems
also involve the use of water, either additional to the air/abrasive
stream or combined as a slurry with the abrasive. The main effect
of the introduction of water is to reduce dust (both dry and wet abrasive
systems clean in a similar manner), although the mist produced is
still a health hazard.
abrasive cleaning techniques are most successful on surfaces of even
profile and consistent surface texture and hardness. An air abrasive
stream cannot on its own differentiate between the removal of soiling
and the removal of masonry. Nor can it distinguish portions of masonry
which are closer to the nozzle from those further away or areas of
masonry which are softer. Damage to the masonry can only be avoided
through the skill and ability of the operator to make the necessary
adjustments in technique.
abrasive cleaning is usually most successful on plain stone surfaces
of even hardness. Careful use can enable the technique to be employed
on moulded and some carved stone surfaces. However it is difficult
to successfully clean brickwork by abrasive means without any damage,
due to the many variations in surface texture and hardness that are
often present and due to the intolerance of many bricks to its impact.
The removal of hard, traditional paints can rarely be achieved successfully
from any masonry surface using air abrasives.
the normal use of abrasive cleaning, two factors are of utmost importance;
the velocity and the concentration of the particles which impact on
the surfaces. These parameters are controlled by the pressure and
volume of the air flow and the concentration of abrasive feed into
the line. It is therefore not adequate to specify pressure alone.
Important parameters will also include the size of the abrasive particle,
its shape and its hardness. Commonly available abrasives for facade
cleaning include aluminium silicate, calcium silicate, olivine and
calcium carbonate. More specialist materials are also available, particularly
for pencil abrasive equipment used by conservators.
shape, nozzle size, rate of water flow and working distance must also
is usually best to determine the many parameters relating to abrasive
cleaning on site when all soiling types, the degree of soiling and
masonry conditions can be properly assessed. Specific advice such
as recommended pressures and abrasive types cannot be given here as
they are only a few of the many variables which must be determined,
as already described. However, the following general principles can
Smaller particles of the same abrasive type can be less damaging
than larger ones, used in the same manner.
Harder abrasives can be more damaging than softer abrasives of the
same size, used in the same manner.
A higher concentration of abrasive particles can be more damaging
than a lower concentration, all other factors being equal.
Higher air pressure and volume can be more damaging than lower air
pressure and volume, all other factors being equal.
A closer working distance between the end of the nozzle and
the masonry can be more damaging than a greater one, all other factors
Depending on how they are used, some small scale abrasive systems
can be as or more damaging than larger scale systems.
Differences in technique will be required for plain and carved
surfaces, sound and deteriorated conditions.
recommendations cannot be made in relation to air abrasive cleaning,
any more than with any other cleaning approach. Pre-contract on-site
trials are always recommended for the cleaning of historic masonry.
These should be overseen by an experienced professional who can observe
and assess the effects of each procedure and produce a detailed specification
for the works.
- C Andrew et al, Stone Cleaning: A Guide for Practitioners, Historic Scotland, 1994
- N Ashurst, Cleaning Historic Buildings, Volumes 1 and 2, Donhead, London, 1994
- British Standards Institution BS 6270: Code of practice for cleaning and
surface repair of buildings, Part 1, BSI, London, 1982
- Technical Pamphlet
4: Cleaning Brick and Stone, SPAB (Society
for the Protection of Ancient Buildings), London, 1994
- ME Weaver and FG Matero, Conserving Buildings: A Guide to Techniques
and Materials, John Wiley, New York, 1993
- RGM Webster, Stone Cleaning and the Nature, Soiling and Decay Mechanisms
of Stone, Donhead, London, 1992
This article is reproduced from The Building Conservation Directory, 1996
as a conservation architect in Sydney and Rome. Following five years with
the Research and Technical Advisory Service of English Heritage, she has
operated her own consultancy, Adriel Consultancy, specialising in the
repair, conservation and cleaning of traditional external masonry materials.
Her most recent technical writing is the two-volume publication, Cleaning
Historic Buildings (Donhead, 1994). Nicola Ashurst acts as a consultant
to the Architectural Conservation Branch of English Heritage
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