Page 150 - The Building Conservation Directory 2013

1 5 0 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 Treatment
collected usually by means of a sponge held
beneath the nozzle.
There are now small machines that
generate ‘dry steam’. These heat water up to
higher temperatures (180°C) which under
pressure means that water has effectively
become a gas which is invisible as it exits
the nozzle. Although there will be some
conversion to vapour and a small amount of
condensation on the surface being cleaned, the
heat of the dry steam is sufficient to convert
that liquid to vapour; as a result, there is very
little run-off.
Large unit systems
The larger steam cleaning machines have been
designed for site use and operate using an
electric pump to pressurise the water and a
diesel-fired heat exchanger to heat the water.
The resulting combination of superheated
water and steam has a temperature typically
between 120° and 150°C and a flow in the range
of 3 to 10 litres per minute, with a nozzle
pressure of 30 to 150 bar. Although this is
similar to hot water pressure washers, the use
of an atomizing nozzle that diffuses the jet of
steam can result in a very low pressure at the
surface being cleaned. In general, the wider
the spray angle, the lower the pressure at the
substrate and a spray angle of 40° is standard
(see illustration above). A narrower angle can
result in greater pressure that can be sufficient
to cause damage to soft or decayed limestone
and sandstone.
Nozzle selection plays an important part
in the way in which the steam cleaner works.
A range of spray shape and angle is available;
commonly used might be solid cone (suitable
for carved surfaces) and fan shape (suitable
for cleaning larger areas of flat ashlar). But
these nozzles also have different properties
in terms of the diffusion temperature of the
jet of steam; for example a standard nozzle
might lose sharpness at temperatures above
140°. This variable can be used to control the
precision and effectiveness of the jet.
Although some steam cleaners come with
the option of adding chemicals or detergents,
control of the amounts is difficult and in most
cases chemicals require a certain dwell time
to work and this is not provided by including
them in a jet of steam.
Relative merits
Steam cleaning is simple and safe as long
as appropriate precautions are taken. The
advantage of steam is its heat; it can be
used generally for flexible materials such as
microbiological growth (algae for example)
and paint coatings. It also has the advantage
over hot water washers that less water is used
and therefore the process is easier to control.
However, it is not generally suitable for brittle
soiling such as calcium sulphate which, in any
case, is less soluble in hot water than in cold.
There are some drawbacks to the use of
steam cleaners. Anybody who has tried to
take a picture of a steam cleaner in use will
realise that the steam generated can also
make it difficult for the operator to see the
result of their work. In closed spaces, the
increase in humidity may be an issue so good
ventilation is normally essential. However,
there have recently been developments in the
use of vacuum heads that both deliver the
steam to the surface and collect the residue
(see illustration below left); this enables use
in sensitive environments and allows the
operator to see what is happening.
The larger machines will also
generate fumes from the diesel so this
too needs to be taken into consideration
when choosing a system.
The way in which a steam cleaner is used
by the operator can make all the difference
to its effectiveness. In many cases, the need
for speed can lead to the nozzle being held
too close to the surface; this can result in
damage to the surface and uneven cleaning.
In many cases, the best cleaning is achieved
using a double-pass technique. The first
pass is at lower pressure and will soften the
soiling; after a period (which might be up
to a few hours) to allow this softening to
happen, a second pass will allow the soiling
to be removed more completely and without
the need for the nozzle to be held close to
the surface. In all cases, the operator should
carry out trials to ascertain the optimum
parameters (such as pressure, temperature
and nozzle type) that best suit the condition of
substrate and the type of soiling.
Steam cleaning is an important element
in the range of options that are available for
cleaning masonry and brickwork. Even as
the machinery gets more refined, there still
remains the fundamental need to understand
the likely short and long-term effects of the
cleaning on the substrate. Each case must be
treated on its merits and always there must be
an underlying criteria to ‘do no harm’.
Recommended Reading
English Heritage, Practical Building
Conservation: Stone, Ashgate, 2012
Nicola Ashurst, Cleaning Historic Buildings,
Donhead 1994
RJ Schaffer, Weathering of Natural Building
Stones, 1932 (Donhead reprint, 2004)
David Odgers trained at Wells Cathedral
under Professor Robert Baker. He was a
founder of Nimbus Conservation Ltd in 1984
and its managing director from 1991 to
2005, during which time he was responsible
for repair and conservation works to many
important historic buildings, monuments
and sites. Since 2005 he has been an
independent consultant on all aspects of
conservation works to stone and plaster.
He was the editor for the Stone volume
of English Heritage’s Practical Building
Conservation series. He is senior tutor for the
Building Conservation Diploma at West Dean
and is an accredited conservator.
A large steam cleaner being used to clean ashlar with a 45° fan nozzle (Photo: OCC)
The use of a vacuum head which collects the steam
and the dirt; this is very useful for large areas of
flat ashlar particularly inside buildings. (Photo:
Restorative Techniques)