Page 148 - The Building Conservation Directory 2013

1 4 8 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 3 T w e n t i e t h a N N i v e r s a r y e d i t i o n
4.1
Services & Treatment : Protection & Remedial TreatmentSteamCleaning
DAVID ODGERS
The cleaning of historic buildings
has been and continues to be a subject
that attracts considerable debate; on
the one extreme there are the exponents who
believe that cleaning is a necessary part of any
conservation process and, at the other end
are those that hold that cleaning only causes
damage and should be avoided. Of course
the answer lies not in any pre-determined
prejudices but rather in a proper assessment of
the building and a well thought out conclusion
as to what is in its best interest.
Principles
Although it is sometimes thought that the
cleaning of external elevations of buildings
will slowly become unnecessary due to
reduction in sulphur dioxide levels, there is
a corresponding increase in nitrous oxides
which can act as a catalyst for other reactions
including the deposition of sulphates. It
is inevitable therefore that resoiling will
continue and correspondingly unlikely that
the debate on cleaning buildings will cease.
What is clear is that methodology for
cleaning masonry and brickwork can have a
lasting effect on the ongoing behaviour – and
indeed survival – of the substrate. In some
cases wholly inappropriate methods (such as
sand blasting) have been used with obvious
damage to the stone; such interventions are at
the heart of the continued reluctance to clean
sandstone buildings in Scotland.
Even well intentioned cleaning
programmes can have a long-term effect that
might not be foreseen. Schaffer (writing in
1932 – see Recommended Reading, below)
reports on the benefit of regular water
washing with water and cites the example
of Goldsmiths Hall in Gresham Street,
then being washed twice a year: ‘the clean
appearance and good condition of the
Portland stone’ he states, ‘are unquestionable’.
And yet recent assessment of some of the
public monuments in London (including the
Cenotaph, illustrated on the next page) has
shown that regular maintenance cleaning has
led to the excavation of the surface pores and
colonisation by microbiological growths that
are very difficult if not impossible to remove.
A decision to clean on aesthetic grounds
is rarely sufficient. In all cases, before any
decision on cleaning is made, a thorough
assessment should take place to:
• identify the substrate, its condition,
and its vulnerability to cleaning
• understand the nature of the soiling
remembering that this will differ
according to location, orientation
and local environment
• decide whether the soiling is
superficial or ingrained
• establish whether the soiling is
causing damage to the substrate
• ascertain whether cleaning is necessary
to allow other treatments.
Once these issues are understood, further
consideration must be given to what the
advantages and disadvantages of the removal
of soiling might be, including the likelihood
and rates of re-soiling. Even after all this,
it is also essential that small scale, well
documented cleaning trials take place to
identify the appropriate methods and to allow
all parties to understand what result can be
achieved without damage to the substrate.
Since the rather uncontrolled cleaning
blitz of the 1960s and 1970s where it was
customary to see thousands of gallons of
water being poured indiscriminately down
the elevations of buildings, there have been
considerable developments in the techniques
and materials available for cleaning masonry.
The options
The cleaning techniques broadly fall into four
categories; mechanical, water-based, chemical
and laser radiation. In practice a combination
of techniques is often useful, and most
buildings would require more than one method
to deal with the different types of soiling.
Mechanical cleaning includes simple
brushing and vacuuming but principally refers
to specialised forms of abrasive cleaning. The
most commonly used types are micro-air
abrasive, dry air abrasive or wet air abrasive.
All of these rely on the use of compressed air
and aggregate; the latter also includes water.
The parameters of all these constituents
can be varied (for example air pressure,
size and nature of aggregate) so the method
is sufficiently versatile to deal with many
different types of soiling. In practice, it is
used mostly for brittle soiling and coatings
on limestone, sandstone, brick, concrete and
granite but it is generally not advisable for
polished surfaces.
A small industrial unit being used to clean carved stonework. The run-off is being collected using a sponge.
In enclosed spaces the increase in humidity may become an issue. (Photo: Humphries & Jones)