20 Apr Stone Cleaning
Spencer Hall ACR
Stone cleaning is an important aspect of works in the conservation world. Cleaning doesn’t simply help to improve overall appearance, but more importantly can help remove potentially harmful atmospheric pollutants which can damage stonework irreversibly. Cleaning can also help to expose further potential damage and identify masonry requiring repair or replacement. Biological growth can actually harbour moisture and in turn lead to accelerated decay and so regular maintenance can be important.
Consideration should however always be given to the level of clean that is to be achieved. It is important that we do not over clean and strip stone of all its chronology, and so a judicious approach should always be undertaken. This can be difficult to define sometimes, as the ‘correct’ level can be somewhat subjective. As always, good communication is the key to understanding client expectation, the context of the work and what is best for the object (whether building, sculpture or monument).
Wherever possible, an approach of minimal intervention will be strictly adhered to when working with historic fabric.
- Wherever possible, like for like repairs will always be carried out and any “foreign” materials introduced, kept to an absolute minimum.
- New repairs should not disturb the aesthetics of the architecture, but under scrutiny from a specialist be notable.
- All replacements/repairs should only be carried out with the assistance of documentary evidence; no speculative works should be undertaken.
- It is imperative that only craftspeople who are experienced with traditional materials should ever undertake a repair in a building of historical significance.
- All work should be extensively documented in both photographic and annotated forms; before, during and after any intervention.
Before cleaning is undertaken, a biocide may be applied to help remove the growth. Moss, algae and lichens attach themselves to stonework, can encourage a damp environment and exacerbate potential breakdown.
Following cleaning, a bio-inhibitor can also be applied again to help discourage the regrowth of similar.
Doff/Torick/Hot Box/Thermatech – The Doff is the most commonly used brand name for building conservation specific superheated pressure washers. It is NOT a pressure washer as one might use on say a patio etc. The advantages of the Doff as a piece of technical machinery is that both pressure and temperature are fully controllable – allowing the use of cold water at minimal pressure to be used all the way up to a constant steam temperature of 150 degrees at pressures of up to 90 bar. The flexibility of this plant allows (following trials) for targeted areas to be cleaned in close detail. Care should be taken using the Doff, due to the fact it is operated by hand and so a systematic approach should be taken – cleaning from say top left-hand side, across to the bottom right hand side in smooth sweeping movements.
Jos/Torc are all variants of the same machine and work very differently to the Doff (described above). The Jos has been a cleaning tool for over 20 years and was developed by German engineers. The Torc is an English alternative, but works in the same way. They all operate on the principle of water under pressure being applied through a nozzle to the face of masonry in a vortex. This swirling vortex sweeps across the surface and very gently washes/abrades the surface. A range of aggregates can be added to this vortex to increase the effectiveness of the clean. These aggregates are rated on the Mohs scale (which measures hardness) – a ‘hard’ material being something like corundum powder (9 Mohs) through to a ‘soft’ material like talcum powder (1 Mohs). The Mohs scale is a purely ordinal scale and as such, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The ‘normal’ material however is Calcite powder which is rated 3 on the Mohs scale. As with all methods here, care must be taken not to overclean and remove the face of the masonry and this is very much down to the condition of the substrate, the choice of aggregate used and most importantly, the skill of the operator.
Nebulus water cleaning is perhaps the most natural and subtle method of cleaning, as the process replicates rainwater running over the surface of the building which gradually washes away surface dirt. The method by which this is completed relies on a set of nozzles being set up across a proportion of the façade (working from top down) leaving them to run continuously for a period (which can be anything from 10 minutes to several days, depending on level of soiling) and then repeating the same process again on the next area. Whilst this will give a good overall basic clean it does not target problem areas and so it often used in conjunction with one of the other methods listed.
Not unlike the Jos (described above), micro air-abrasion systems work by forcing air through a compression chamber into which abrasive particles are placed. The air carries said aggregates down the hose and out via a nozzle onto the area in question. Type of aggregate, pressure and the quantity of abrasive used are the 3 main factors which control the effectiveness of this equipment. Unlike the Jos, no water is utilised. I have used this to greatest effect on small delicate carved sculpture in a studio environment, although I do understand larger scale equipment is available on the market for building facades. In the wrong hands, this could be as damaging as sandblasting and so only a practitioner who understands the fundamentals of its use should be tasked with its deployment. Its use is perhaps most appropriate in a conservation studio, although I have also used it in the field.
Cleaning with chemicals is a wide field that can incorporate everything from mild detergents through to strong acids and alkalis. Acid and alkali cleaning has fallen out of favour in recent years and for good reason. High strength acids will react adversely with Calcium Carbonate (CaCO3 – limestone) and also have the potential to damage both brickwork and some sandstones. Strong alkaline solutions can conversely be linked to the formation of salts on both stone and brick. In our opinion, the use of either on natural stone is probably best avoided. The cleaning of granite, brickwork and terracotta may sometimes be appropriate, but great care and consideration will need to be imposed and a suitable number of trials completed to establish the best approach. As with all aspects of cleaning, low strength percentages should be employed first and increased only if deemed absolutely necessary. Ammonium carbonate is mild alkaline cleaner and is often used held in a clay medium (the process of which is described in greater detail below) to soften black encrusted clinker. It works by reacting with Calcium Sulphate (CaSO4) on the blackened surface and forms both Calcium Carbonate and soluble Ammonium Sulphate (NH4)2SO4 which can be rinsed away easily with the addition of water (H20) and light scrubbing.
Both ‘clean’ water and chemical poultices can be used to good effect in cleaning and lifting staining. Poultices work by drawing out the stain using capillary action. The most common poultice medium used is Sepiolite (a clay powder), although paper pulp is often added at 50:50 to produce a highly absorbent and malleable mixture that doesn’t dry out too quickly. Sepiolite is a relatively inert Magnesium Silicate (Mg4Si6O15(OH)2·6H2O) and is often favoured as a medium in poulticing by conservators on stone sculpture. Another type of poultice popular in conservation cleaning to stonework is EDTA (Ethylene Diamine Tetra-acetic Acid). The common form of this poultice is where the product is bound in a naturally produced latex (commonly containing small amounts of ammonia). The ammonia acts as a useful cleaning agent in its own right and triggers the curing of the rubber once exposed to air. This cured latex film can easily be removed mechanically, without the use of water and as both surface based dust and imbued dirt lifts with it, the removal results in a cleaned surface. This method was used to great effect to the interior of St Pauls during the major restoration completed a few years ago.
The word L.A.S.E.R. is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser can be used to clean carbonation/sulphation from a variety of substrates (limestone, sandstone, marble, terracotta etc) in a very controlled manner. It is most suited to working on delicate material in a workshop environment, but can be used successfully on site. There are however limitations, due to scale of operation and in turn cost. Trials should always be encouraged to establish suitability and are influenced by nature of substrate, the level of soiling and other environmental factors. Hand held units now make it far more portable. All units however run off a 240V supply (110V version is not currently available, but special dispensation can normally be gained for use as a ‘specialist’ piece of equipment – an RCD breaker must of course be employed) Safety issues are paramount. Laser can damage retinas from a considerable distance away, so light shielding around the scaffold is essential. All adjacent areas MUST also be well signed to avoid people unexpectedly chancing across it in use. Goggles must be worn and these are specific to the wave length of laser being used and so guidance must be sought. The control offered by laser cleaning enables the conservator to remove unwanted layers without over-cleaning the valuable surface of the artwork; patina, fine surface detail, tool markings and important surface coatings can be preserved. Laser cleaning systems provide the conservator with an extremely gentle method of cleaning which can be used to remove dirt from very fragile surfaces. Click here to see the case study on St Georges Cathedral, Southwark
Further to stone cleaning it is often necessary to reassess condition. If losses to joint material have occurred or friable material is removed, then these areas should be reassessed before reinstatement is considered.
To recap, when considering cleaning it is important to try and achieve a consistent finish that meets expectation but does not strip the object fully of its integrity. A balance should ideally be sought between removal of dirt and detritus and retention of some of the accumulated patina which is in keeping with the age of a piece. Over-cleaning can be viewed as ‘damage’ by many in the conservation world and so trials should be completed in advance to an area where they can be evaluated without having impact on the overall finish.
For further help or information on any aspect of the services or advice that PAYE Conservation can provide please in the first instance call Spencer Hall on 020 8857 9111