Page 149 - The Building Conservation Directory 2013

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 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 1 4 9
4.1
Services & Treatment : Protection & Remedial Treatment Water based cleaning includes steam
cleaning, sponging, intermittent nebula sprays
(which create a fine mist to slowly soften the
dirt layer), water/clay poultices, rinsing and
pressure washing. Water is an effective solvent
and can be used hot or cold, and as a liquid or
vapour. It is suitable, in limited quantities, for
most substrates, and it is particularly useful
for removing sulphate crusts from limestone,
for some coatings, for superficial deposits and
surface biological growths. The simplicity of
water-based cleaning can be appealing but the
use of too much water can lead to substantial
risks of residual staining, mobilisation and
re-crystallisation of salts, and corrosion of
hidden metal cramps.
Chemical cleaning agents include acids,
alkalis, solvents, chelating agents, biocides
and detergents. They can be delivered to the
surface either as liquids, gels or poultices,
the advantage of the latter being that there is
a longer contact time. All chemicals rely on
breaking down the bonds within the soiling
or between the substrate and the soiling.
There has been considerable investment
in developing targeted combinations of
chemicals that deal with specific types of
soiling on particular substrates. All of these
must be used with care, and most of them
require neutralisation or rinsing with water
afterwards: this factor must be taken into
account when specifying their use. Their
effect on adjacent materials (for example glass,
metals and timber) must also be considered.
Laser cleaning is beginning to have
a wider impact in the UK although the
machinery remains expensive. The method
works on the principle that the dirt absorbs
enough energy from the beam to lose cohesion
and vaporise; so laser cleaning is most
effective when there is a contrast between the
dark soiling and pale substrate.
No matter how benign the technique and
methodology chosen, it is always necessary to
carry out a thorough initial assessment and
trials. As with all other areas of conservation,
it is of course not the machinery or the
materials that are the ultimate reason for
successful cleaning but rather the skills of the
person using them.
Steam cleaning methods
Of the methods mentioned above perhaps
the most accessible and most widely used is
steam cleaning. Steam cleaners have been in
use since the early part of the 20th century.
Shaffer refers to the use of steam cleaners to
clean a ‘blackened frontage’ and goes on to
say “…the steam process is unlikely to cause
any more damage than washing with water
or scrubbing with stiff brushes”. In truth, it
is now recognised that steam cleaners cause
much less damage than those methods.
There are however many different types
of steam cleaner available and they should be
distinguished from hot water washers. Hot
water has a lower surface tension than cold
and thus is more likely to clean more deeply
and faster.
This principle is at the heart of hot
water washers which have diesel fuelled
boilers and a pump that delivers water at
temperatures up to 90°C through a restrictive
nozzle which increases the velocity of the
water. This results in pressures of between
60 and 150 bar and water-use of between
5 and 20 litres per minute. These can be
used in conjunction with detergents or other
chemicals but in reality, this is rare for historic
buildings. More often than not, hot water
washers are the method of choice for rinsing
after chemical cleaning and for removing
algae and other materials from paving.
Some of the machines used in hot water
washing can result in quite aggressive cleaning
because of the high water pressure and volume
they can deliver. Apart from these, other
parameters which can provide some control
include the design of the nozzle, the angle of
spray to the surface being treated, distance
of the spray to the surface and the duration
of contact. All of these can be manipulated
by the operator so it is possible to carry
out careful cleaning using lower pressures,
keeping the nozzle at a greater distance from
the substrate and ensuring the nozzle spray
angle is above 35°.
Steam cleaners can broadly be divided
into small industrial/domestic units and
the larger machines (such as Doff and
ThermaTech) that are commonly encountered
in building conservation.
Small unit systems
The domestic steam cleaner units that are
available at the local DIY store come with a
variety of attachments (including brushes and
nozzles). These however have been devised
mostly for upholstery cleaning and tend
not to be sufficiently robust or to develop a
consistent temperature of steam. They do
The results of steam cleaning on a typically soiled
section of Bath stone; the steam has removed surface
dirt and the flexible microbiological materials but the
more brittle sulphation layers remain. (Photo: OCC)
A trial area of paint removal from faience; steam
cleaning was used to soften and remove most of the
paint from the substrate. This was followed up with
a paint softener (on the right hand side of the panel)
and a final rinse with steam. (Photo: Restorative
Techniques)
As a nationally important monument, the Cenotaph
is regularly maintained. The grey appearance at
the top is not surface dirt but dead microbiological
material that now lies deep within surface pores
that have been excavated by repeated cleaning with
pressurised water. (Photo: Odgers Conservation
Consultants (OCC))
however have some similarities with the small
industrial steam units (see title illustration)
that are used in conservation; these emit
very small quantities of water (typically 3 to
4 litres per hour) at a pressure of 4 to 6 bar
through handheld nozzles. They are used for
cleaning intricate carved detail, sculpture and
monuments. They are effective on marble but
can dull the surface; they should not be used
on alabaster.
Most of these machines produce wet
steam in which there are also droplets of hot
water. The pressure comes from the steam
generation process itself. As the vapour is
generated, the pressure inside the vessel builds
up; steam at 160°C remains a liquid as long as
the pressure in the container is above about 7
bar. When the pressure is released on opening
the nozzle, the liquid water will vaporize into
steam and cool to the boiling point of water at
atmospheric pressure (100°C). In doing so, it
will expand by about 1.5 times; this expansion
occurs in the nozzle and helps to provide
the pressure of the steam. The temperature
of steam will tend to drop quickly after the
vapour exits the nozzle and some of it will
condense into water droplets. Steam cleaning
using these small machines is effectively a
combination of mostly steam but including
some droplets of hot water; there tends to
be some water run-off that will need to be