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)
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.
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.
|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))
|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.
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 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.
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 quickly.
|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
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 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 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
|A large steam cleaner being used to clean ashlar with a 45° fan nozzle (Photo: OCC)|
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.
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.
|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)|
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’.
English Heritage, Practical Building Conservation: Stone, Ashgate, London, 2012
N Ashurst, Cleaning Historic Buildings, Donhead, Shaftesbury, 1994
RJ Schaffer, Weathering of Natural Building Stones, HMSO, London, 1932 (Donhead reprint, 2004)